The cast crown, also known as a full gold crown or full coverage crown, is a type of dental restoration that is used to fully encase a damaged or decayed tooth. This type of crown is typically made of gold alloy or a combination of gold and other metals, and is used to restore the function and appearance of a damaged tooth.
The process of getting a cast crown typically begins with a consultation with a dentist. During this appointment, the dentist will examine the tooth in question and take X-rays to determine the extent of the damage. If a cast crown is determined to be the best option for restoring the tooth, the dentist will then prepare the tooth by removing any decay or damage and shaping the tooth to make room for the crown.
Once the tooth has been prepared, an impression of the tooth will be taken. This impression will be used to create a model of the tooth, which will be used to create the final crown. The crown will be custom-made to fit the tooth perfectly, ensuring a comfortable and secure fit.
The cast crown is typically made of gold alloy, which is a strong and durable material that can withstand the forces of biting and chewing. Gold alloy is also biocompatible, meaning that it is not likely to cause an allergic reaction or irritation in the mouth. The gold alloy is mixed with other metals such as palladium, and copper to create a stronger material. The color of the alloy can be customized by adding different metals to give the desired color.
The cast crown is then cemented onto the prepared tooth, and the patient will leave the office with a fully restored tooth. The patient will be instructed to avoid eating or drinking anything that could stain the crown, such as tea, coffee, and tobacco products, for at least 48 hours after the procedure.
Although it is an unaesthetic crown, mostly recommended in the lateral area, but with very good endurance that restores the masticatory function efficiently, this crown is currently accessible to a broad category of patients.
Most of the time, the indication for a metal crown (full-thickness or directed) is given by the financial means of the patient.
- Definition: Full shell crowns are prosthetic devices extrinsically bonded by cementation to the ground surface of the dental crown microproteins of the type “shell crowns” may cover all or only part of the tooth surface, hence the specification “full shell crowns” or “partial crowns”.
Types of Cast Crowns: The side walls can have different thicknesses, if they are uneven in thickness it is known as a) “full thickness crown”, and if the walls are equally sized it is called b) “directed thickness crown”.
2. Guidelines: General directions have a dual purpose:
3. a) for morpho-functional and prophylactic purposes:
- on teeth with caries accompanied by large losses of tooth tissue, when restorations with fillings or inlays are no longer possible;
- on teeth with extensive or multiple fillings, which are prone to fracture of the dental crown, where veneering crowns are used to reinforce the mechanical strength of the teeth;
- in loss of tooth substance through trauma, when reconstruction by other means is no longer possible (e.g., in some forms of fractures of incisal edges and angles of front teeth) or in case of cusp or tooth wall fractures on the lateral teeth;
- in pathological abrasions when the occlusal relief can be reconstructed by applying crowns;
- on teeth with changes in shape, volume, position and color, when these can be corrected by applying veneer crowns; in young people, for physiognomic reasons, in the elderly, especially for periodontitis prophylaxis purposes. For color correction, in the frontal area, the physiognomic veneer crown is utilized, while, in the lateral areas, when the resistance of the structures is low, a micro-metal veneer is applied to provide mechanical protection to the dental tissues;
- for the restoration of contact points, when the interdental space is less than 2mm, it can be closed by a single crown; more than 2mm apart, two crowns are required on adjacent teeth, while in spaces exceeding 4mm, it is necessary to insert a replacement element between the two crowns.
- for prophylactic reasons, the application of veneer crowns is indicated in secondary and dental cervix and multiple cavities, as well as in neuromuscular disorders with bruxism, to prevent rapid and extensive lesions of the dental substance;
- prophylactic application, to prevent tooth wear due to friction exerted by the retaining elements of removable dentures. It should be noted that a selective attitude on the part of practitioners is required in order to avoid the inevitable use of protective micro prostheses in all cases;
- in situations where the teeth show a large difference between the maximum coronal and cervical cross-sectional diameters, cases which would require too extensive resection of dental tissue, with the risk of damaging the dental pulp and reducing or compromising the mechanical strength of the bridge;
- full-thickness cast crowns are particularly indicated for lateral teeth with limited cervical-occlusal dimensions;
- directed-thickness cast crowns are indicated on the crowns of lateral teeth with a large cervical-occlusal dimension.
- b) for prosthetic purposes:
- for the construction of bonding elements, to which bridge fixtures are attached in restorations of partial dentures Interlayers;
- for the anchorage of removable partial dentures, in the case of crowns provided with convexities for clasps, with pockets for occlusal support, with skeletal denture, etc.;
- to immobilize teeth in periodontal disease; by fixing a series of crowns together, a rigid system is formed, acting as a containment seal;
- in the treatment of occlusal dysfunction, to restore the occlusal surface and the occlusal plane, when, following the need for levelling, teeth are ground down and subsequently require crowns;
- for the restoration of support areas, when crowns can be used to restore normal biostatic conditions between the arches in order to prevent changes in intermaxillary ratios resulting from coronal destruction in the lateral areas.
The application of crown caps is contraindicated in the following situations:
- in apical pathological processes (osteitis, granulomas, cysts), which have not been previously treated surgically or conservatively;
- in incorrect or incomplete endodontic treatment (false canals or incomplete fillings); prosthetic restoration is applied only after endodontic resolution of the case;
- on teeth with inflammation of the marginal periodontium, until the inflammatory processes have been corrected by the periodontal treatments instituted;
- on teeth with gingival and bone pouches; in these cases, a surgical periodontal treatment must be performed in advance;
- on teeth with advanced, irrecoverable tooth mobility requiring extraction;
- on teeth with alveolar resorption, when they exceed the apical root crest, a situation that is assessed in relation to the degree of atrophy of the alveolar margin;
- on teeth inclined more than 30° to the occlusal field, subject to permanent, non-functional stress, even when integrated into a bridge;
- on teeth without antagonists, with some exceptions, for example a molar without antagonist, forming a second post in a normally stressed bridge;
- in large losses of tooth substance, which no longer provide the necessary retention of a crown. In such cases, pre-prosthetic crown reinforcement preparations are made, either by inlay-mod if the clinical situation allows, or by reinforced restorations, over which a veneer crown can be applied;
- in youngsters under 16 years of age, who have wide dentinal canals, enlarged pulp chambers, incomplete root development and incomplete closure of the apex; in such cases only a temporary coating can be made by means of a protective cap.
THE CLINICAL-TECHNICAL STAGES OF MAKING A CAST METAL CROWN
These two types of full-thickness and directed-thickness crowns are made in identical clinical-technical phases. The only difference between the two types of crowns is the techniques used in the modeling phase.
The clinical stages consist of:
- examining the patient for the purpose of dental surgery;
- diagnosis and therapeutic recommendation;
- tooth preparation;
- try-on and adjustment of crowns;
- crown cementation.
The technical steps consist of:
- creating the prototype;
- making the mold;
- melting – casting;
- crown processing.
1. Preparation of the tooth crown
The objective is to obtain a non-receptive geometric figure in the cervical-occlusal direction. The crown of the tooth prepared with the characteristic shape of each tooth is called ” dental cervix “. The configuration of the transversal section at any level fits into that of the dental cervix area. The truncated cone (close to the cylinder) with a large, cervical oriented base is the non-retaining shape. The occlusal face of the crown of the tooth is reduced by approximately 1.5 mm, respecting the morphology. The lateral faces are oriented by grinding slightly converging the occlusal plane. The maximum diameter of the preparation is gingivally located.
Techniques for dental crown preparation
1) Classical technique – uses abrasive instruments diversified in terms of shapes and sizes
The equipment is represented by:
- flat, concave, convex discs, active on one side, on two sides or on the edge;
- mill-stone or carborundum (Heatless) or diamond-shaped stones of 10mm diameter, fixed to the right-hand piece;
- cylindrical or conical diamond stones mounted on the straight or contra-angle piece.
2) Contemporary technique – uses abrasive tools diversified in terms of shapes and sizes.
The tooling is represented by:
- cylindrical and cylindrical-conical diamond cutters
Preparation of a molar
Phase I Grinding of proximal facets.
The proximal facets are ground to become parallel to the tooth axis and also tangent to the dental cervix.
The instrument used:
- grinding discs, fixed to the right piece, which can be flat, active on one side only when there are neighboring teeth to be maintained or active on both sides, used when the contact points are dismantled by slicing. The discs can be impregnated with carborundum powder, diamond stones or Horico type;
- special grinding discs, abraded, concave, active on their medial side or abraded, convex, active on their distal side. The concave discs are used to grind the distal faces of the lateral teeth and the convex discs are used to grind the medial sides.
Two known techniques for grinding proximal facets:
1) When there is a contact point.
In this case the following methods are used:
- use the active disc on one side (Horico), which is inserted at the contact point, from the occlusal to the interdental space and with rotary movements, moving towards the dental cervix, seek to overcome the contact point progressively. The danger of this method is the possibility of the disc slipping with injury to the soft tissue. This accident is avoided by a strong, adequate support of the operator’s hand on the dental arch and possibly with an auxiliary aid that removes and protects the lips, cheeks, tongue and plank – with a dental mirror or special guard. It is necessary to know the possible directions of slippage in relation to the hemiarch being worked on. After a minimum gap has been created by overcoming the contact point, the gap can be widened with a thicker disc, such as a carborundum disc;
- another way of suppressing the contact point is the slice-cut method, contraindicated by Körber, because of some of its risks, but used by other authors. The instruments used are diamond discs abrasive on the edge or carborundum discs abrasive on both sides. It is applied close to the edge of the occlusal surface, in a plane parallel to the axis of the tooth, so that the entire thickness of the instrument penetrates from the occlusal to the inside of the tooth perimeter, but tangential to the dental cervix. The correct position is obtained by a corresponding inclination in the direction of action of the disc.
A portion of the tooth is sectioned which includes the maximum convexity of the contact point with the neighboring teeth. Care must be taken not to fall inside the root perimeter, as in this case the threshold created is almost impossible to correct. The grinding is perfected by vertical movements, from the socket to the occlusal, associated with rotational movements, around the sagittal axis of the piece, towards the vestibular and towards the oral, in order to avoid the creation of a threshold at the socket and to smooth the lateral-proximal edges.
Difficulties in suppressing contact points:
In most cases, maintaining the disc parallel to the tooth axis is difficult, especially in the lower molars due to the mesial or distal inclination of these teeth. Possible remedies:
- a contra-angle piece with a short mandrel can be used, but it is difficult and unsafe to handle;
- concave or convex discs can be used when available;
- a modified straight piece can be used on the mandible, with a slight end angulation which allows much better interdental access in accordance with the axis of the teeth to be prepared;
2) when there are no contact points.
When neighboring teeth are missing or at some distance, grinding is much easier; the proximal faces being easily accessible, they are abraded with suitable discs or cylindrical stones.
The grinding of the proximal teeth can be done at high speed with a thin conical diamond or flame diamond, starting from the lingual and from the vestibular towards the contact point, which is then loosened with feathering movements, taking care not to injure the neighboring teeth.
According to Johnston, the objectives of proximal grinding are:
To parallel the mesial and distal facets with the insertion axis of the veneer crown to aid in its retention;
To create a space large enough to ensure the strength of the micro prosthesis, giving it adequate thickness;
Unrestricted interdental access for rounding of the edges and for preparation of any grooves, cassettes, etc.;
The danger of grinding by the modern method is the exaggerated coincidence of the preparation with the loss of retention and damage to adjacent teeth, which can also happen if not enough attention is paid to grinding by the traditional methods.
Special features in grinding proximal teeth.
Their preparation may include the following aspects:
In the case of short teeth, the proximal faces are ground as parallel as possible to each other, in order to increase crown retention;
In the case of tall teeth, they can be prepared slightly converging towards the occlusal;
If an accidental threshold has been created, a proximo-occlusal amalgam filling is attempted, using a Horico disc placed subgingival to the threshold, by grinding in a cervico-occlusal direction.
If the margin cannot be corrected with the disc, it will be ground with a diamond flame shaped stone, which will be moved along the margin until it is levelled. It is desirable not to have such accidents, because often the correction of the edge is extremely difficult or even impossible.
The control of the grinding of the proximal facets is done in this way:
with the side of the probe, from the dental cervix to the occlusal, to check for lack of retention;
the profile of the abutment is examined with a mirror to check the shape of the proximal walls;
check the proximal facets to ascertain that the neighboring teeth are not affected by caries.
Phase II Grinding of the occlusal surface
From the occlusal surfaces a uniform layer of tooth substance is removed, equal in thickness to that of the crown cap.
Körber gives the following values for the thickness of the coronal occlusal surfaces:
in the case of a dynamic occlusal relief, it is necessary to remove 0.5 mm;
when a static occlusal surface is to be functional by grinding in the oral cavity, a thickness of 0.8 – 1.5 mm is required;
a thickness of 0.8 mm is required for porcelain and 1.5 – 2 mm for acrylic.
It is recommended that preparations for micro prostheses with molded or fully molded caps should have a gap to the antagonists of about 2mm to allow for possible occlusal equilibration in the oral cavity and to have sufficient resistance to abrasion.
In general, grinding is done following the occlusal relief and not on a flat surface. In both vital pulp teeth and devital teeth, grinding should result in the removal of an equally even layer of tooth substance from the entire surface, which will thus maintain a configuration similar to the original one (Costa).
According to Jüde, a pyriform grinding stone should be used which by its shape corresponds to the angle between cusps and fissures.
In a vital tooth this grinding method is justified by the protection of the pulp organ, which must maintain a protective parapulpal wall of equal thickness to the pulp at all points. In the case of a devital tooth, the justification is not to ablate too much substance and at the same time to obtain a flat occlusal surface of equal thickness in all its areas (Shilling – Burg).
With a 10 mm diameter carborundum (Heatless) or diamond mill wheel stone, fixed to the straight piece, or better with diamond cylindrical mounted stones, the slopes of the occlusal surfaces are worked at conventional speeds, following the grinding objectives mentioned above.
Despite the fact that in the classical procedure occlusal grinding is started from the occlusal side, for the reason of reducing the height of the proximal and vestibulo-oral faces, Körber recommends to start with the separation, justifying that after removing the contact points the occlusal preparation is carried out more easily, avoiding the danger of touching the neighboring teeth. Besides, there is no absolute indication on the sequence of the two times – separation / reduction of the occlusal surface – this can be done according to the preferences of the respective school or the practical experience of the specialist.
The authors of the American school recommend that on the occlusal surface to be ground, high speeds and rounded conical diamond or conical fissure grooves are made to guide the grinding in depth. The differences in level resulting from the drawing of these marks are smoothed, while maintaining the most important cuspidal and fissural contours.
According to Johnston, the fissures and trenches are grinded first and the occlusal surface is then completely reduced. In order to provide the necessary space to the antagonists the functional contact areas must be marked during kinematics. In the case of teeth that are in version and do not enter into full occlusal relations, grinding is done only in the places that come into contact or that are only 1 mm away from the opposing tooth.
Today, with the use of the turbine, any tooth surface can be quickly prepared, with the precautions described above.
Characteristics of occlusal surface grinding.
The orientation of the grinding, in the case of premolars and molars without antagonists, is usually done in relation to the height at which the neighboring teeth are located. When there are pronounced extrusions, it is advisable to correct the unevenness of the occlusal plane at the same time as grinding; consequently, the preparation of such teeth requires prior grinding to reduce the height of the abutments as much as necessary in relation to the level of the neighboring teeth and also to ensure the space corresponding to the occlusal thickness of the crown.
The control of occlusal surface grinding is carried out according to the following guidelines:
for indication of the depth of the preparation, radiological checks are carried out to assess the contour and size of the pulp chamber;
the control of the grinding in terms of the distance from the antagonists can be done with blue paper, 0.25 mm, which, folded in 3, 4 or even 6 and interposed between the dental arches, marks the raised points that still need to be ground;
a heated transparent wax pad can also be used, which is squeezed between the teeth, in maximum intercuspation occlusion. After cooling and removal of the plate, areas where the wax layer is thinner can be seen through the transparency, indicating insufficient space at that level;
grinding control must be carried out not only during maximum intercuspation but also during lateral and propulsive movements, in order to ensure sufficient space between the ground surface and the antagonist teeth during movements.
Phase III Grinding of vestibular and oral facets
Using conventional instruments ang speeds, the preparation is carried out according to the following sequence of maneuvers:
Grind the buccal and oral surfaces with wheel stones, Heatless or diamond, 1-1.5 cm in diameter, fixed to the right piece.
with the wheel stones only, the large convexities located at the equator of the tooth are removed;
next, the vestibulo-oral faces are ground with stones of cylindrical shape, vertically and parallel to the axis of the tooth in question;
grinding with cylindrical carborundum and diamond stones starts with large shapes and continues with stones of smaller and smaller diameters, in order to have access as close as possible to the neighboring teeth and to the dental cervix;
vestibulo-oral faces can also be polished with stones in the shape of a reverse cone, active both on the base and on the lateral faces.
using a small, base-active reverse cone stone, fixed to the contra-angle piece and resting on the buccal face, it is possible to extend the grinding up to the gingival margin and even 0.5 mm into the gingio-periodontal sulcus, without causing damage to the marginal periodontium;
the sanded vestibulo-oral faces must be finally parallel to the tooth axis.
For the reduction of large vestibular and oral convexities, the classical method described above can be used initially, followed by further grinding with a turbine, with specific high-speed instruments. Körber recommends that when working with the turbine, cylindrical stones with an inactive base should be used, under cooling with water, and that the direction of grinding should be in the opposite direction to the rotation of the instrument, which should only touch the gingiva, very gently, by the inactive polished base. If there is free access to the tooth, or when the tooth is an isolated bridge post, phase III can be combined with phase I.
Shillingburg proposes the drafting of an axial leading trench, on the larger surface, as in the mandible the buccal wall, and in the maxilla the oral wall; when the crown is to be the aggregating element in a larger bridge, the author makes two trenches (on the vestibular and oral surfaces) to prevent the tendency of a mesiodistal dislocation of the crown.
Johnston suggests that the oral concave faces should be ground with small round stones in order to achieve a uniform depth, and the occlusal half should have a shape similar to the normal contour after preparation.
Phase IV Rounding and finishing the edges
In this phase, smoothing and rounding of the ridges resulting from the transition between the proximal and vestibulo-oral faces is done. Use reverse conical stones active on the base or side or abrasive discs on the convex or concave face with which to act on the M-V, M-O, D-V, D-O edges, for the M-V and M-O edges reverse conical stone or convex disc, active on the base, for the D-V and D-O edges reverse conical stones or concave discs active on the lateral surface.
Uses conventional gears, reverse taper stones, convex or concave discs. Note that depending on the direction of insertion of the instruments into the oral cavity, the base active reverse cone is used for mesial crossing areas and the lateral surface-active reverse cone for distal crossing areas. Johnston further explains the need for rounding the edges with burrs, diamond stones or paper discs, so that the thickness of the casting is equal throughout and can be adapted to the configuration of the gingival margin.
According to Johnston the rounding of the angles can be done at first with conventional abrasive tools and speeds, to be completed at high speeds, with cylindrical-conical stones rounded at the tip, at the counter-angle piece. Most of the time this phase can be carried out from the beginning with a turbine. It is important that the diamond stones are small enough in diameter to penetrate the interdental spaces and long enough to reach the cervical level without being impeded by the occlusal surface.
Phase V Grinding at the level of the dental cervix
This is the critical moment in the preparation of the abutment. The configuration of the “preparation boundary” will be chosen according to the type of micro prosthesis to be applied.
Preparation possibilities at the cervical margin.
According to Johnston, cervical preparation can present itself in the following aspects:
it can be with an undefined subgingival termination, in the form of a “wedge” as in the case of tangential preparation after Körber;
it may have a “sharp-angled chisel” shape which is often performed both lingually and proximally, with satisfactory results;
it can be a “chisel in the crossbite”, used where a deeper grinding is required (for example, in the case of caries);
the preparation can also be carried out with a “threshold” – right-angled, pointed or obtuse – simple or bevelled. The threshold can be sketched all around or only partially – vestibular – or vestibular and oral, depending on the type of micro prosthesis and the clinical situation of the case;
it can also have a “hollow” (western) shape, called “en congé”.
Shillingburg is an believer in a precisely verifiable casting limit both in the impression and on the model, which are prerequisites for obtaining a casting that accurately fits the level of the dental cervix. In the author’s opinion, threshold preparation complicates impression, fit and micro protection insertion. Consequently, he favors the “en cougé” method, particularly for gold crowns.
Shillingburg also points out that for gold crowns the so-called tangential preparation is not suitable because it requires the shaping of thin, knife-edged crown margins which, not being sufficiently resistant, often deform under the action of masticatory forces.
Consequently, a number of authors recommend that the limits of the preparation should as far as possible allow the micro prosthetic margin to be embedded in the depth of the enamel, which seems to be achieved satisfactorily by the preparation en congé.
Threshold preparations can be carried out with different types of stones or milling cutters, at conventional or turbine speeds. The final shape is perfected at low speed and finishing is recommended to be done with hand tools.
Location of the preparation limit.
The cervical preparation margin may be located sub gingivally or at the gingival free edge. It is generally accepted that subgingival placement of the preparation satisfies the periodontal-prophylactic principle, less so the caries-prophylactic principle and not at all the physiognomic principle. In opposition to the past tendency for the edges of veneer crowns to penetrate as far as possible subgingival, current concepts are in search of diversified techniques and methods which, in relation to the priority of the objectives pursued, also aim to achieve the shape of the preparation.
European dental schools use, on a case-by-case basis, subgingival or supragingival placement of the preparation limit.
There should be no preconceived ideas when choosing the level of placement of the micro protection margin, as this is done on an individual basis, depending on the patient’s age, the state of his periodontal health and his physiognomic needs.
In order to achieve a subgingival penetration of the coronal margin, the enamel edge at the level of the gingival festoon – resulting from grinding the lateral faces – is smoothed.
Also, in relation to the depth of the bottom of the gingival sac, sand 0.5-1mm in depth, all around under a strong jet of water, to detach the gingival festoon. The abrasive instrument must be moved parallel to the tooth axis – in continuous movement – so that no vertical grooves are formed; any subgingival gaps are also removed at this stage.
Often, before grinding, elastic rings, subgingival threads or felt rings are applied as a preparatory measure to remove the gum margin. In connection with the preparation of the dental cervix, it has been recommended to make not only a demarcation line but even a surface of 1-1.5 mm width, which makes the cylindrical-conical base of the mouthpiece more retentive (Costa). Remember that the prepared abutment must have the same shape as the section of the tooth in question at the level of the dental cervix, knowing that each has its own shape, oval, circular or trapezoidal.
As regards the subgingival penetration of crown margins, Richter, in a clinical study carried out over a longer period of time on a larger number of patients, could not find a significant difference between subgingival and supragingival placement of microprosthetic margins. According to the author, the adjustment and finishing of the crown margins is more important than their limit in relation to the gingival festoon. In general, for periodontoprophylactic purposes, it is recommended that the microprosthetic margins be supragingivally located. When the aim is to achieve retention and stability of the crowns, it is recommended to extend them subgingivally in order to increase the friction surface between the abutment walls and the microprosthesis.
The subgingival location may be conditioned by the presence of cariogenic field or older, pre- existing restorations.
In the literature, there is a tendency to give the most important role to the perfect finish of the coronal margins and to a lesser extent to the level at which they are placed.
In the general context of current concepts, we believe that rather than making unsuitable threshold shapes, where it is not particularly necessary, with unsuitable instruments, it is more important to make a correct tangential grinding with circular smoothing of the enamel ridge, as indicated by Ene Körber, or a grinding “en congé”. It is also important to avoid irritating spurs, with their possible harmful influence on the marginal periodontium, which can be avoided by obtaining a perfect impression of the crown margins with a proper shaping and finishing.
Preparation at the dental cervix in gingival retractions.
In the preparation phase by grinding at the level of the dental cervix, depressions may be evident with the appearance of the interradicular bifurcation, which forces the creation of a trench-like extension that continues the bifurcation on the respective face of the coronal abutment.
Failure to follow this technique can result in a crown being made that passes over the interradicular depression like a rope, causing the micro protection to misfit to the tooth surface, exposing the abutment to decay and loosening of the prosthetic element. To remedy such a situation, a groove is ground with a cylindrical-conical stone, placed with the tip in the bifurcation, with vertical movements in the insertion axis of the crowns.
As has been seen, such trenches can also be made in the absence of root depressions, if it is considered that this increases the retentivity and stability of the microprosthesis.
Auxiliary grooves can be made in the posterior teeth or straight or conical fissures, they must be parallel to the insertion axis and end close to the cervical line.
Phase VI Finishing the abutment
Proceed to the finishing of the polished facets, the removal of the secondary coronal ridges and those between the occlusal and lateral surfaces.
Finishing is done with small, cylindrical, diamond-shaped, very fine-grained stones and paper discs. Secondary ridges, irregularities and roughness are smoothed with moistened paper discs under water cooling.
The use of polishing pastes is contraindicated, as their penetration into the dentinal canaliculi makes it impossible to obtain a good degreasing of the crown abutment at the time of cementation.
Non-standard polishing for special veneering crowns.
In the case of short teeth, the occlusal-lateral angles should be maintained, as opposed to the usual preparations where rounding is required;
For short teeth under 5 mm, it is recommended that, in addition to parallel faces, a box-shaped occlusal cavity is prepared, into which a crown veneer block fits;
For long, globular teeth and teeth with pronounced periodontal recession, crowns are recommended with edges that stop at the centric point of the tooth;
For telescopic crowns, the teeth should be ground more sharply – conically, in order to create the necessary space both for the cap which is cemented to the abutment, and for the second crown which, being solid with the bridge, slides over the cap.
Control of the preparation of the finished abutment – is done with the side of the probe along all the facets following the following objectives:
– The overlap should be 0.5mm, barely perceptible to the naked eye;
– The subgingival circumference should follow the contour of the root cross-section;
– Vertical facets should be flat;
– Vertical facets to be even;
– The occlusal perimeter should correspond roughly to the path of the root cross-section;
– The occlusal perimeter must be inside the cervical perimeter;
– The interocclusal space should be large enough for the thickness of the crown.
De-restriction controlling – the perimeter is taken with a 0.2 mm diameter malleable Remanium rod/hard round wire which is wrapped around the dental cervix and if it can be lifted easily, without encountering obstacles and deformations, it is judged that the shape of the preparation is correct, free of retentions. In special cases, an impression is taken and the model is poured, thereby checking a number of aspects related to the correct preparation of the abutment.
The abutment is exclusively turbine sanded.
According to Körber, tangential preparation takes place in two phases:
1. supragingival phase.
In this phase the tooth is prepared up to the gingival festoon with long, slightly tapered diamond stones.
The supragingival preparation is carried out with a three-stage turbine:
separation: when preparing a tooth that has a neighbor with a contact point, the tooth is attacked from the vestibular and oral side by inserting the grinding stone into the interdental space, of course grinding the tooth being prepared without touching the neighbor. If two adjacent teeth are being prepared at the same time, pass the diamond stone between them;
occlusal reduction: use truncated-cone or cylindrical diamond grinding stones, or lenticular stones, with which, by back-and-forth movements, alternately towards the vestibular and oral cavity, using the methods already described, enough dental tissue is removed to obtain the necessary distance from the antagonists;
preparation of the lateral faces: by grinding all around, all the lateral faces of the dental abutment are prepared tangent to the dental cervix, taking into account the degree of convergence that they must have in the end.
2. subgingival phase.
With a fine, extremely thin, diamond-shaped, tapered stone, with an active part of only 2-3 mm, the subgingival area is penetrated, circumscribing the parcel, under the action of a powerful jet of water which removes the gingival festoon. The phase is performed in the case of subgingivally penetrating crowns. In situations of caries resistance and a certain degree of periodontal insufficiency, it is possible to grind only up to the level of the gingival margin according to the preparation of the excavated type, presented previously.
Mistakes, accidents and complications in tooth grinding.
mistakes with consequences on the shape of the grinding
excessively tapered bluntness shapes, with consequences on crown retention and pulp integrity;
occlusal perimeter of the dental cervix larger than the cervical perimeter;
retentive areas at the level of the dental cervix – consequence of an incorrect preparation and finishing of the abutment;
vertical grooves on the surface of the abutment and at the dental cervix indicating that the grinding tool was not handled continuously;
insufficiently deep preparation of the crowns with subgingival penetration (not to be confused with the excavated preparation where the grinding of the border is deliberately done supragingivally);
retentive undermining (accidental deep thresholds), the consequence of a wrong technique in the orientation of the grinding instruments, resulting in an incomplete and incorrect insertion of the crown;
insufficient interocclusal space through too little reduction of tooth substance at this level.
Possible accidents during grinding:
– pain, which depends on individual sensitivity, in case of vital teeth anesthesia is used;
– severing of the lip, tongue, plank or jugal mucosa, due to the lack of a firm support point for the operator’s hand, which can cause the grinding instrument to slip;
– depending on the severity of the injury, measures such as H₂O₂. /Hydrogen peroxide bandages, compression for hemostasis or surgical suturing may be taken;
– opening of the pulp chamber, by misalignment of the pulp organ dimensions;
– trauma to the marginal paradontium following grinding may result in injury to the subepithelial fibers, leading to scar retraction or the appearance of gingival pockets;
– inadvertent or incorrect grinding of the adjacent tooth – starting point for the development of caries;
– coronal fracture of the neighboring or antagonist tooth through carelessness or incorrect handling of grinding instruments.
Possible complications after grinding:
pulpitis accompanied by pain that occurs shortly after grinding. In principle, treatment consists of vital removal of the pulp, or in some cases treatment with drugs such as corticosteroids can be used.
Pain sedation does not eliminate the danger of damage to the pulp, especially by cementing, so it is recommended in such situations to apply temporary protective crowns with Calxil for a period of several weeks, or in the case of single crowns to cement with a neutral cement. In addition to the attempt of drug therapy, vital extirpation is indicated, especially when the bridge is one of the pillars of a larger bridge;
transient pain, which is caused by thermal and chemical agents and which subsides following the application of protective crowns;
late pulp necrosis and pulp gangrene – complications of grinding – with consequences on the integrity of the periapical tissues (applied, chronic and acute periodontitis);
fracture of the coronal bridge, as a consequence of the lack of mechanical consolidation, especially of the depulped monoradicular teeth.
The prosthetic field is a basic notion, frequently encountered in the field and defines the totality of the morphological elements with which the crown coating presents contact relationships. The impression transfers the prepared tooth and the rest of the elements of the prosthetic field from the oral cavity to the dental laboratory in the negative.
The following elements are recorded in the impression:
– the dental abutment
– neighboring teeth
– antagonist teeth
– dental occlusion in the position of maximum intercuspation.
The impression is made using the following materials:
– thermoplastic materials (Stents and Kerr)
– elastic materials – elastomers.
The following nomenclature has been established in the literature for the classification of impression techniques (Witz 1977).
- Uni-maxillary impression:
- Ordinary or one-time imprint;
- Two-step imprints;
- Wash imprints (two-stage);
- Double-mixture imprints;
- Composite (sandwich) imprints.
- Single imprints, with thermoplastic or elastic masses (synthetic elastomers) in a copper tube (ring).
Bi-maxillary imprints, with thermoplastic mass and synthetic elastomers or with synthetic elastomers putty and fluid in special spoon.
- Uni-maxillary impression:
- Ordinary impression.
The material chosen for the impression is deposited in a universal spoon, which is placed on the prosthetic field. This impression is made to record antagonist teeth. The material used is generally irreversible hydrocolloids. For all impressions with synthetic elastomers the gingival socket is prepared.
- Two-stage impression.
In the first stage, the fluid elastomer is injected onto the prepared tooth and into the bottom of the gingival sac.
In the second time, the solid silicone prepared with the reagent, deposited in a universal spoon, is placed on the prosthetic field to incorporate the fluid silicone. The injection molding technique can be introduced in this category.
Wash technique impression.
The impression is obtained in two phases:
In the first phase
the solid elastomer (putty) is prepared, deposited in the universal spoon equipped with an efficient retention system (2mm holes) and placed on the prosthetic field. After the material is set, the impression is removed from the prosthetic field. The solid elastomer impression is prepared as follows:
washed the surface under water jet;
dried with the air jet from the dental unit;
cut the lags (material that has penetrated into the proximal dental spaces) using scissors;
cut triangular grooves (delimited by 2 walls) on the buccal and oral sides of the indentations (tooth impressions) which continue on the buccal and palatal or lingual (lower) side. The role of the grooves is to allow the excess silicone fluid to flow back when the impression is placed on the prosthetic field. The presence of grooves prevents the first impression from deforming by overpressure.
The variant to this technique is to shrink the spoon during the impression when the material has plasticity. The direction of movement of the spoon is horizontally, forwards, backwards and sideways left, right. The movement of the spoon creates space for the fluid impression material.
In phase two
the fluid elastomer is prepared and inserted into a syringe, to be deposited in the gingival- dental socket, on the surface of the bridge, on the neighboring teeth and in the impressions of the impression made in phase one from the chitosan elastomer. The spoon containing the impression and the elastomer fluid is reintroduced into the oral cavity on the prosthetic field in the same position.
After the bite, 2-4 min, the impression is removed. It is washed and dried.
Examination of the impression:
- fluid material, different color than the chitosan, shows the borders of the abutment preparation and the bag bottoms.
Each indentation is well defined by an annular protuberance that extends 0.5-1 mm beyond the surface of the impression as far as it has penetrated the gingio-dental fossa.
- between the dental bridge impression and the mesial and distal neighboring teeth, a thick wall appears, showing the space created for the future micro prothesis wall;
- in the delicate parts of the impressions appear the negative reliefs of the occlusal faces;
the impression to include a number of neighboring teeth, mesial and distal;
- both materials are solid. Fluid material for detail registration may detach from the solid one if the surface of the first impression has not been very well dried – possible defect requiring repetition of the impression in phase II.
This technique works like this:
- Gingival saline is prepared according to a method known and preferred by the specialist;
- in the first phase, the elastomeric putty material is prepared and deposited in the universal spoon, placed on the arch where the dental bridge is located. Several movements are made in the horizontal plane. Enlarged and deformed impressions of the teeth are obtained, the spoon together with the impression material is immediately removed from the oral cavity;
- the removed putty material, in a plastic state, is dried. The fluid elastomer, prepared by the nurse, drawn into syringes, is deposited in the gingival fossa, on the surface of the abutment and on the surface of the putty material in the spoon, which is also in the plastic phase;
- the spoon with the two materials is reintroduced into the oral cavity and placed on the dental arch.
- No pressure is exerted during the processing of the materials.
- After insertion, the impression is removed from the oral cavity, washed and all elements are systematically examined.
5. Composite (sandwich) impression – a variant of the technique described above.
The technique consists of the following:
- the consistent, putty elastomer is prepared and deposited in the preferred universal spoon. At the same time the fluid elastomer is prepared and deposited on the surface of the spoon to cover it in a uniformly thick layer.
- the spoon with both materials is placed in the necessary position for making the impression, on the dental arch. After polymerization of the materials, which takes place at the same time, the impression is removed from the oral cavity, washed and dried.
The characteristics of the technique are the simultaneous setting of the materials, the absence of internal tensions and the possibility of deformation. An assistant is required to prepare the fluid material.
- Unit impression
This is the impression that records a single tooth prepared for restoration, filling or substitution.
The unitary impression for veneering has two characteristic elements:
* The shape of the dental abutment with all possible aspects (truncated cone closer to the cylinder or closer to the cone);
* The size of the dental abutment on the lateral faces, between the gingio-dental socket and the occlusal face or incisal edge;
Also, the dimensions of the occlusal face for the lateral teeth and the incisal edge for the frontal teeth.
The gingival boundary is always very accurately recorded.
Techniques for unitary impressions
Two techniques are known: the classical technique and the modern technique. Both techniques use a copper tube (ring or cylinder) as the microcarrier. The copper tube can be obtained in the dental laboratory using a simple technology made of 0.2 mm thick sheet metal strip. It can be prefabricated and sold in different sizes (diameters).
Prefabricated rings are sorted by size and number. The selection is made using the perimeter recorded with 0.2 mm malleable Remanium hard round rod wire at the level of the gingio-dental groove. A graduated conical instrument, known as a “ring picker cone” is used to obtain the ring corresponding to the perimeter. The selected ring is disinfected by clinical (spray) or physical procedures, heated to red (flamed), after which several procedures known as adaptation follow.
Direct impression techniques in one time (without unitary impression)
It is a modification of the two-step impression method. It is an occlusion impression using silicone materials of different consistencies. In this type of impression, it is necessary to use a special metal spoon as an impression holder (Ketenbach).
Occlusion impression with flowable elastomers – in a pre-impression with thermoplastic masses (stent).
The technique is characterized by the following elements:
- does not use a spoon for elastomer deposition;
- it does not use vascular elastomers (putty
- it uses stent and wax-like thermoplastic materials;
- uses a gauze band of variable length and 2 cm width.
The impression technique is performed as follows:
- on presentation of the patient for teeth grinding before the procedure begins.
- the prosthetic field is stent-printed, the material is wet heat-plasticized (water at 70 degrees), it is shaped into a 5mm thick parallelepiped.
- the faces are covered with wet gauze.
- the plasticized mass is placed on the area of the arch where the dental bridge will be prepared.
- the mandible is placed in the position of maximum intercuspation. The position is controlled at the level of the opposite hemiarch.
- the stent is cooled with the dental unit spray.
- the stent preamp (conformer) is air-jet dried.
- the prepared fluid elastomer is loaded into the syringe to be deposited in the gingival sac on the bridge and on both sides of the stent preamp impressions.
- the elastomer layer is recommended to be thin and uniformly thick.
- the elastomer preamp in which the elastomer is placed is repositioned on the prosthetic field in the same position;
- the patient has been instructed to achieve the maximum intercuspation position.
Removal of the impression from the prosthetic field and the oral cavity is achieved after the polymerization reaction. This type of impression is used with maximum frequency for prosthetic work represented by micro prostheses and metal and metal-acrylic dental bridges.
Try-in and fitting of metal veneer crowns
– Insertion of the crown on the abutment.
The crown should fit completely, by gentle friction, under the action of a light force. Difficulties in insertion may be due to surpluses within the micro prosthesis, inconsistencies between the shape of the clinical preparation and its model, due to deficiencies in the impression or in the design of the model.
Excessively shaped proximal contacts may also often prevent complete crown insertion. If, after removal of a layer of proximal convexity, the crown does not fit completely and if there is a presumption of internal pluses, the abutment is impressed with a copper ring and the model is poured on which the preparation boundary is drawn. Blacken the inside of the crown with carbon black (by burning a wad of cotton wool dipped in petroleum jelly) and place it on the model’s rim up to the point where it stops advancing. Remove the crown from the block and look inside for the area that locates the point to be treated. Make the necessary retouches until the edge of the crown reaches the limit of the preparation.
The same method of retouching can be carried out directly in the oral cavity, if the prepared abutment does not show exaggerated sensitivity after grinding.
In general, major corrections are not allowed. A mismatch that cannot be easily and quickly remedied should lead to re-crowning (if it is only a technical error) or requires some correction of the preparation, followed by a new impression, if clinical errors have occurred.
– Cervical adaptation of the crown.
The crown may be “shortened” at the crown due to inadequate processing by the technician or incorrect impression (the impression material does not penetrate deeply because the gingio-periodontal fossa has not been widened beforehand, or secretions have not been removed from that level). The crown may be “long” due to exaggerated etching and over-shaping of the margins by the technician. If the crown has been fully inserted, check that the marginal fit is correct.
In crowns with subgingival penetration, it is difficult to check the relationship of the crown margin to the cervical area, as the investigation can only be done by probing. Careful insertion of the probe tip into the gingio-periodontal fossa and palpation all around can give information on whether there is a smooth transition between the tooth and the crown margin or whether a flared crown is felt. The latter is evident by grasping the instrument at that level. In addition to checking the transverse fit, the vertical fit is also checked to see whether or not there is a concordant relationship between the edge of the crown, the bottom of the gingio-periodontal sulcus and parallelism with the gingival festoon.
– Occlusion check.
After the marginal fitting of the crown, the occlusion is checked with the help of joint paper or a wax sheet.
In case of a crown that is too high, the marked areas are sanded until the engagement relationship is obtained at the initial vertical occlusal dimension. The way the cuspid-cavity contact is made is also controlled. Of course, support cusps are not ground under any circumstances.
For correct removal from the functional occlusion field, interocclusal contacts are marked with different colors: with one color the premature contacts in the intercuspidal occlusion and with other colors the interferences in the trajectory of laterality and propulsion movements.
Technological variants of metal shell crowns.
For the realization of this micro prothesis, the abutment can be ground without threshold, by the tangential or hollow preparation method (“en congé”), as well as with circular threshold. Cast crowns can be used as micro prostheses for protection, support, anchorage and, in particular, as aggregation elements in plural fixed prostheses. From a technical point of view, cast crowns can be made with or without a controlled thickness. The undirected-thickness cast crown is applied intimately around the entire circumference of the bridge, giving it a high retention value and resistance to masticatory forces.
- the possibility of transmitting temperature variations and irritation of the pulp, due to the intimate contact with the crown abutment and the very thin layer of dental cement;
- the amount of alloy often too high when casting from gold alloys.
Prefabricated wax or plastic coronal matrices can be used to rationalize modelling, achieving a directed thickness of the crown walls. The melting of the margin of about 2-3 mm, carried out during the waxing phase, guarantees a correct closure and adaptation, resulting in a crown that is not applied completely around the circumference of the abutment, but only at cervical level.
The thickness-directed crown is considered to be the most suitable for the following reasons:
- it can accurately reproduce the proportions and anatomical details of natural teeth;
- it provides a good adaptation to the crown and to the entire surface of the bridge, and it also saves on noble alloys when made of such materials.
Cast crowns are currently made in our country from:
- gold alloys:
– 916 ‰ (22 carats);
– 833 ‰ (20 carats);
- palladium-silver alloys:
– Palliag, Palidor;
- chromium-cobalt alloys:
- Cu, Al and Ni-based alloys:
Cementation can be final (definitive).
The cast metal crown does not have a provisional cementation because it can be swallowed.
Cementation is carried out for long periods of time expressed in a greater or lesser number of years.
Final cementation is achieved if a cement manufactured and marketed for this purpose is used.
The following cements are used for final setting:
- zinc oxyphosphate;
- zinc polycarboxylate;
- diacrylic cements;
- glass ionomer cements
This clinical phase completes the clinical-technical work carried out over varying periods of time to obtain prosthetic work.
The value of this phase is determined by the effects that follow; they are generally positive. Only a moment of carelessness or superficiality completely cancels the hard work.
The patient is comfortably seated in the chair, head resting on the headrest with different positions for the mandible and jaw.
The phase is carried out successively as follows:
- Prepare the necessary instruments represented by:
- dental consultation kit (mirror, tweezers, probe);
- the saliva aspirator and training the patient in its use;
- sufficient rolls of cotton wool;
- the cement bottle (powder and liquid);
- glass plate;
- spatula for preparing the mixture.
- Prepare the prosthesis physically, chemically and hygienically;
- physically, any residues of polishing paste or packing materials are removed from the inside. In general, the interior is cleaned in the laboratory to adapt the micro protection to the model;
- chemically, degreased substances like rubbing alcohol are used, with the use of cotton wool pads to dab the faces of the micro protection;
- hygienically, disinfection with alcohol or antiseptic sprays.
III. Prepare the prosthetic field in the dental abutment by:
- Removal of temporary cement or food debris (dental plaque);
- dressing with chloroform, alcohol, or only warm water if vital and sensitive, painful;
- isolation with vacuum cleaner or cotton wool rollers;
- air-drying, cautious for sensitive and painful mouthpieces, and for congested periodontium. If there is assistance, the cement paste is prepared at the same time as the preparation of the prosthetic field by the specialist.
1. Preparation of cement paste:
- The powder is deposited on the glass in two to three portions;
- the liquid, zinc oxyphosphate cement required for a crown is 2-3 drops;
- incorporation of the powder into the liquid is progressive. With the help of a spatula, the mixture is homogenized to obtain a fluid, viscous paste that tends to flow;
- for glass ionomer cement, two drops of liquid and two teaspoons of powder are needed for one crown;
- the spatulation is carried out by precise movements in a very short time.
2. Insertion of the prosthetic work on the dental abutment:
- the paste, immediately after preparation, is applied directly with a spatula strictly on the inner walls. It is contraindicated to fill the entire cavity with cement.
- the cement paste is applied to the bridge with a spatula or, more correctly, with a brush;
- the micro protector held between the fingers is applied in the appropriate position on the dental bridge, on the occlusal face, digital pressure is exerted;
- when inserting the micro protector, make sure that no gauze or cotton threads enter between the dental bridge and the micro protector;
- the saliva aspirator is placed in the buccal cavity;
- the mandible is guided to position the lower arch in relation to the upper arch in maximum intercuspation;
- cement setting occurs after 5-10 minutes depending on the type of cement;
- the excess in the gingival festoon area is removed after the extraction by grasping with a scaling tool. The presence of debris is irritating to the marginal periodontium;
- examination of the occlusal relationships after cementing with the help of articulating paper is mandatory because the interposed cement film may partially alter the contacts with the antagonist teeth. The marks on the paper are reduced by sanding, followed by polishing.
« Technical steps »
- Making the model.
The model represents the positive copy of the prosthetic field rendered with the highest accuracy. It is made by casting plaster paste into the impression or by electroplating.
Technological recommendations for obtaining the characteristics
- the water-powder ratio should always be rigorous, according to the manufacturer’s instructions. Any variation in the proportion will affect the surface appearance, strength and expansion;
- mechanical (preferably vacuum) or manual spatulation will give maximum results if carried out in an optimal time. If the spattering is done longer, the setting reaction is brutal, accompanied by an increase in expansion;
- the plaster putty is poured and the impression is permanently vibrated, but tempered;
- particle size, setting time, expansion and mechanical strength are important physical factors that some manufacturers mention on the packaging label;
- deformation of the impression can occur if it is turned over after being filled with gypsum and pressed hard against another amount of gypsum on the worktable;
- impregnation of the impression with water, glycerin or oil has been found to reduce the tear strength (crushing) of the plaster pattern that has been cast.
Techniques for obtaining patterns
- Molding with mobilizable stumps after silicone impression.
- The model with a movable stump after impression with thermoplastic mass in a copper ring.
- The model with fixed abutment.
2. Manufacture the model.
3. Model for full-thickness crown.
The model of the future crown shows a close contact with the dental abutment model on all its surfaces.
Techniques for obtaining the model
1. Graded cooling technique;
- Machining technique;
- Technique for obtaining the coping by pressing a plastic disc;
- Technique of obtaining the model by addition according to the concept of the gnathological school;
- Wax foil and dripping.
1. Model of the crown with directed thickness
The model of this crown has the following features: the side walls are of equal dimensions of 0.30 mm; the contact with the model is only in the area of the 2 mm high crown (the rest of the side walls are at a distance) and at the occlusal face.
– Techniques for obtaining the model –
1.a) Techniques using fabricated elements;
2.b) Technique using wax foil;
3.c) Technique using a duplicate model from the casting mold.
4. Mold making.
The mold is a cavity enclosed by walls of refractory material. It appears as an intermediate stage in the technological process of making the model and the future prosthesis. The impression is obtained by two operations:
- By covering the mold with dry socket paste, after 20-30 min., the phenomenon of setting occurs (it becomes a solid body);
- By burning the mold (wax or plastic mass).
4. Melting – casting.
This is the technological phase in which the metal prosthetic part materializes.
This phase includes the following operations:
- qualitative and quantitative choice of the alloy;
- its melting;
- the introduction into the mold.
It is marked by the highlighting of the cast crowns that are common to the casting rods and the casting cone.
The packing mass fragments and detaches from the metal with the introduction into the water. The total removal is done physically by the action of a spatula in the place where it persists or by a jet of sand (primer particles) which is projected onto the surface of the micro protection, being conveyed by compressed air.
5. Crown processing.
After the rods are sectioned, the micro protection is processed, finished and polished. Prosthetic restorations fixed in the oral cavity will always have extremely smooth surfaces, sufficiently polished to prevent saliva and food debris from adhering, forming deposits, staining and becoming unsightly.
TECHNICAL STAGES IN THE PRODUCTION OF CAST METAL CROWNS
1. The model.
The model represents the positive copy of the prosthetic field rendered with the highest accuracy. It is made by casting plaster paste into the impression or by electroplating.
Characteristics of model casts.
Special plasters (hard and extra-hard) have the following characteristics:
- minimal expansion during the day;
- maximum surface hardness (compared to other plasters);
- very smooth surfaces.
Techniques for obtaining patterns.
1) The model with a removable silicone impression.
This technique uses metal rods which are industrially produced and marketed for this purpose.
The rods are of two types, simple and compound.
Simple rods have the following characteristics:
- dimensions: length 25-30 mm, thickness 2 mm;
- structure: bronze alloy;
- shape: the crown part is a cylinder with retaining asperities, the root part is cylindrical-conical with a flat facet for guidance.
Composite rods have the following characteristics:
- the root part for the guide has two rods, one cylindrical with a diameter of 0.8-1mm and the other cylindrical-conical with a diameter of 2mm;
- the two rods in the transverse plane are located at a distance of 2 mm from each other;
- composite rods ensure more precise positioning of the movable abutment in the model base than simple rods.
Hard plaster paste is inserted into the dental impressions of the impression, 7-8 mm from the edge of the coping. Before the socket, the retentive end of the metal rods is inserted into the center of the dental impression coping.
For neighboring teeth, serrated end rings made of aluminum or steel are placed. The surface of the hard plaster, corresponding to the dental bridge, is smoothed. After the hard plaster has been inserted, a plaster is inserted into the impression to make the socket and the distal extension on which the occlusal key is cut. In the total impression, plaster paste is poured for the antagonist teeth model and for the distal extension where the occlusal key is.
After dismantling the impression, with a saw having a very thin blade, the model is sectioned in continuation of the proximal faces of the dental bridge. The dental bridge model is separated from the rest of the model, together with a fragment corresponding to the alveolar process.
The mobilization operation is obtained by pushing on the end of the metal rod on the lower face of the socket. Repositioning on the model is possible, after mobilization, due to the rod and the flat surface it presents for guidance purposes.
The repositioning of movable abutments is more correctly achieved if double metal rods are used. This type of model offers advantages for the layout phase.
2) Model with movable abutment, after impression with thermoplastic mass in copper ring. It has two alternatives:
- first alternative: a 10 mm high calibrated wax loop is wrapped around the cervical edge of the copper ring, to which it is soldered.
Into the indentation inside the ring is inserted hard plaster paste by permanent vibration. Before the fitting, the retaining end of the metal rod is inserted centrally.
b) second alternative: the wax band for the conformer is wider, being 20mm. A hard plaster paste is deposited in the impression with this conformer, using known techniques, to obtain a coronal abutment with a cylindrical root extension 20 mm long. This root extension is processed by cutting and grinding to obtain a non-retaining (cylindrical-conical) shape. A flat facet is created in correspondence of the vestibular face, being the guide element of the movable bridge in the model base.
The copper ring with the cast abutment is placed in the plaster overlay, which is reconstituted and bonded to the outer face by wax bonding.
The overlay is isolated for 10 minutes in water with detergent or soap. The prepared plaster paste is poured progressively, the superstructure being vibrated to create the pattern of the neighboring teeth.
On the upper surface of the distal extension of the socket the T, Y or V-shaped depression is cut to obtain the occlusal key. The pattern of the antagonist teeth is cast differently, depending on the type of impression (global or total).
If the impression was total, the antagonist model is cast separately. After demolding, the occlusal wax is placed in the position of maximum intercuspation, as recorded in the office.
The abutment is detached from the model base by pushing on the rod end or by hammering on the root end, when it has no metal rod.
Processing of the movable abutment.
The abutment is mobilized from the base to be machined in the cervical area. A spherical bur with a diameter of 1.5-2mm is used, mounted on the handpiece, of the suspended dental technique motor. Machining consists of creating a deep groove around the perimeter of the abutment, beyond the cervical limit. The edge of the groove facing the coronal abutment is evident and is the landmark that guides the technician in modelling the model in that area. The fixed abutment in the socket is not possible to be machined cervically.
3) Fixed abutment model.
This model can be used for all types of impressions. The hard plaster paste is progressively deposited in the highest area of the impression. Through vibration it flows and glides over the surfaces of the impression, penetrating into all the recorded details. Finally, the base is made with a distal extension on which depressions are created in known shapes. The model extension is isolated, after which the model of the antagonist teeth is poured into the impressions of the antagonist teeth and simultaneously over the distal extension.
After removal of the impression the two parts of the model are obtained, which can be placed in occlusion with maximum intercuspation due to the occlusal key. In this model the dental bridge cannot be detached from the socket.
1) Replica for full-thickness crown.
The model of the future crown has a close contact with the dental abutment model on all surfaces.
– Techniques for obtaining the model –
a) The graded cooling technique is characterized by:
- Isolation of the crown (in aqueous, detergent, soapy solutions);
- Blue wax for inlay is melted in a single laboratory spoon or in a thermostat vessel (equipped laboratories);
- The crown part is immersed in melted wax for a few seconds;
- The immersion time depends on the physical-chemical qualities of the wax and the temperature of the wax;
- A solidified layer of wax with a thickness of 0.30-0.40 mm is formed around the block;
- The first layer is a cape with uniformly thick walls;
- The final shape of the model is obtained by putting the wax, drop by drop, so that on the lateral facets the specific convexity of each facet is achieved, as well as the points of contact with neighboring teeth. On the occlusal face the occlusal relief and the contact with the antagonist teeth are dripped;
- The spatula modelling, by scraping, aims to smooth the facets and creates the image of uniformity of modelling. This technique can only be used if the abutment is mobilized from the socket.
b) The drip technique is characterized by:
- Isolation of the abutment at the crown part following the usual isolation techniques;
- The deposition of the fluid wax, drop by drop, progressively on all sides of the abutment using a spatula, until a slightly larger volume than that of a dental crown is reached;
- Shaping of the facets is carried out by scraping with a spatula from close to closer.
c) Technique of obtaining a coping by molding a plastic disc.
Dental material manufacturers sell a kit consisting of: plastic discs 0.15-0.20 mm thick, a special clamp and a silicone sink.
The technique is characterized by:
Making the cape:
The clamped disc is held over a flame for lamination; in this state it is placed over the sink and the pattern is printed. The disc, made of pressed plastic, has intimate contact with the pattern facets. The plastic that goes beyond the area of the shell is cut with scissors.
The cervical adaptation and the final shape of the model are made with melted wax, which is dripped onto the occlusal facet and lateral facets.
d) Technique of obtaining the model by addition according to the concept of the gnathology school.
This technique is characterized by the following aspects:
- The models are mounted in the articulator
- The model starts with the occlusal face. Wax is dripped to locate the cusp tips and marginal ridges, thus outlining the occlusal face of the model, followed by the insertion of sagittal and essential ridges, with the delimitation of the grooves and fissures.
- The lateral facets are modelled in relation to the morphology of the neighboring teeth, fitting into the general configuration of the arch. According to this design, the relief of the occlusal face is cusped with maximum efficiency in mastication.
e) Waxing and dripping.
Control of the model.
This control is mandatory before starting the preparation for packaging, following:
- The adaptation and appearance of the cervical margin which will be in intimate contact with the abutment in that area, smooth, continuous and thin to the limit of preparation;
- Points of contact with neighboring teeth;
- Convexities and discharge grooves on the buccal and oral surfaces;
- The inscription in the morphology of neighboring teeth and in the curvature of the arch;
- Occlusal relief, functional modeling, realization of contact points with antagonist teeth;
- The general appearance of the model shows the degree of finishing, as it is known that, after a finished model, the cast piece obtained is very little processed;
- The way in which it is applied to the sides of the abutment and removed;
- The thickness of the occlusal face (1mm is ideal, it lasts over time).
Full thickness cast crowns have the following characteristics:
- The inner crown facets are in contact with the dental abutment;
- The sidewalls have large uneven dimensions;
- Frictional force occurs between the inner surface of the crown and the dental bridge surface, which gives the most effective stability;
- Temperature variations in the oral cavity are transmitted entirely to the dental abutment;
- Removal from the dental abutment is performed with difficulty (with much effort for the dentist and trauma for the patient) by cutting the occlusal and the vestibular surface;
- The manufacturing is obtained by high material consumption (the cost price is high if cast from noble alloys);
- The major indication is for lateral teeth with reduced dimensions in the cervical-occlusal direction.
2) Thickness-controlled crown model
The model of this crown has the following features: the side walls are 0.30 mm equal in size; the contact with the model is only in the area of the 2 mm high crown (the rest of the side walls are at a distance) and at the occlusal face.
Techniques for obtaining the model
Techniques using industrially manufactured elements are produced from wax or thermoplastics, sets of models, characterized by various shapes and sizes.
According to the model, the manufactured model is selected and adapted to the workpiece. The final occlusal and cervical adaptation is obtained by some addition of dripped wax. The crown model is made in a very short time. The technician does not need to be talented and skilled to make it.
Technique using calibrated wax foil.
A rectangular strip is cut from the 0.30 mm calibrated wax foil and wrapped around the bridge. The ends are glued together, forming a cylinder (in dental technique the cylinder is called a ring). In the cervical area, it is fitted intimately to the abutment by shaping and dripping wax.
The characteristic shape of the lateral facets (vestibular, oral, mesial and distal) is obtained by modelling the walls of the ring as follows: the end of a screw pushes the wall from the inside outwards to create the equator (convexities) and contact points, the blade of the spatula is applied externally to the occlusal half on the vestibular face to model the discharge groove.
The occlusal end of the ring is gathered. A washer made of the same calibrated wax is placed on this end and glued to it.
The morphology of the occlusal face is achieved by two operations:
- wax is dripped onto the disc attached to the ring;
- after solidification, it is modelled by cutting (cuspidal fissures and slopes) or by close-up grinding;
- the model of the lid is obtained by dripping. The heated wax is dripped directly onto the occlusal face of the cap.
Technique using a duplicate pattern from the packing table.
A model is molded on the abutment from special wax (containing gutta-percha), which has the following characteristics: it is fixed on the model, the cervical edge is 2 mm away from the preparation border and is in all directions (occlusal, vestibular, oral and proximal) 0.30 mm smaller.
This operation of shaping the model is carried out in order to obtain a change in the shape of the abutment and is called “remodeling”. After pre-shaping, the entire model is impressed, using reversible hydrocolloids to duplicate it.
Into the impression, the paste from the packing mass is poured and the duplicated model is obtained. The crown model is modelled on the base of this model by the wax dripping and shaving technique. The mock-up is not removed from the model for packaging. The duplicate model is part of the pattern.
Crowns made using this technique are very precise (cervical and occlusal adaptation) because the mock-up is no longer removed from the model for packaging, an operation which is a risk factor because it can often be accompanied by deformation. The first two techniques are commonly used as they are expedient.
Duplicate pattern and wax foil
In the past, there were concerns about making thick-directed molded crowns using heavy-duty techniques that have remained known under the following names: the technique using lead foil, the technique using waxed thread, the technique using packing or molding mass.
Today, these techniques have only historical value, being cited to illustrate the efforts of the forebears to make thin-walled and uniformly sized molded wreaths.
The thick-directed crown has the following characteristics:
- the side walls are of equal size (thickness) (0.3 mm);
- the side walls are at a distance from the facets of the dental abutment;
- the crown is in contact with the dental abutment on the occlusal front and in the cervical area for a distance of 2 mm;
- between the inner surface of the crown and the sides of the dental abutment there is a space occupied by the cement;
- partially transmits to the dental abutment the temperature variations in the oral cavity;
- removal from the dental abutment is carried out by cutting, but with less effort than with full-thickness ones. The operation is less traumatic for the wearer;
- a minimum amount of alloy is consumed for fabrication, which results in a lower cost price than the first ones;
- they are indicated on the crowns of lateral teeth with a large cervical-occlusal dimension.
The objective of all casting techniques is to obtain from the wax pattern a porosity-free prosthetic piece with a homogeneous structure, precisely adapted to the tooth preparation. The process of producing a small, precise and homogeneous cast part, such as micro prostheses, is carried out according to certain rules and principles. It is known that the finished casting can never be better than the wax model, which leads to the conclusion that “special attention should be paid to the model”, materialized by: highly accurate modelling, with maximum adaptation to the tooth preparation and excellent finishing.
The model before packaging is prepared as follows:
The surface is cleaned by brushing with a soft brush to remove wax residues and degreased.
Molding rods are fixed. Plastic, wax or metal rods can be used. The operation of fixing the rods to the model is particularly important for the success of the casting. Many micro projections have defects because this operation has not been carried out correctly.
The purpose of the rod is to create the channel through which the fluid alloy flows into the mold cavity, to create a homogeneous density casting on the surface and inside.
The rod fulfils its role through the following factors: cross-sectional diameter, length and positioning.
The rod diameter is dependent on the volume of the mold. The minimum diameter is 1.7 mm, the optimum size is 2.5 mm. The appropriate rod size reduces the possibility of porosity in the casting.
The metal rod should not be overheated as it melts or distorts the adjacent surface, resulting in changes in the shape and volume of the casting. It is correct to add a drop of wax to the place where the casting rod will be attached. Rods are prepared cylindrical, tubular, because they store less heat. If the rods are made of metal, they will be stainless.
It is a mistake to use small-diameter (thin) rods, as this results in castings with porosities or small holes. In order to exclude this cause of porosity, the casting rod must be of adequate size and must represent a reservoir of fluid alloy.
The positioning of the casting rods is important to obtain complete (fully) castings. The rod is placed on the surface of the model in the thickest place, but not in an area where the morpho physiology is changed, in order to reduce the sources of porosity. In crown models that are aggregating elements, the rods are fixed on the proximal facets towards the edentulous space, close to the occlusal facete.
In the case of integrated arch crown models, for the mandible they are placed on the lingual side of a lingual cuspid and for the maxilla on the buccal side of a palatal cuspid.
The positioning of the rods in relation to the surface of the model also contributes to obtaining a complete casting:
– the casting rod is oriented in such a way that the fluid alloy can flow in the direction of the centrifugal force pushing it. The positioning of the casting rods at an angle to this direction of the casting force requires a change in the direction of flow of the alloy, which will cause an incomplete casting;
– the orientation of the casting rod should be such that the alloy does not directly hit the sharp projections of the packaging which may fracture and cause obstructions;
– the molten alloy should enter the mold cavity by the shortest route;
– a casting funnel shall be created for all castings, especially when die-cast;
– the casting rod shall not be placed at a point where the part is more stressed.
The casting rod is fixed at an angle greater than 90° to the surface of the model to reduce the possibility of “swirling” of the fluid alloy.
The exhaust gas channels in the mold are placed as follows:
* one end near the cervical margin, towards the vestibular
* the other end on the conformer cone to locate the opening in the upper portion of the casting mold.
The hot gases inside the mold cannot escape fast enough through the pores in the walls of the packing mass when the fluid alloy flows through the molding channel. Turbulence and backpressure result inside the mold because these gases do not escape quickly. If the pressure of the compressed gases exceeds the pressure of the fluid alloy, then the gas will work its way through the metal, creating porosity.
Recommendations to avoid porosity caused by back pressure
Gas backpressure porosities occur mainly in cast crowns, for which gas exhaust rods are necessary and mandatory.
It is essential that the diameter of the casting rod exceeds the dimensions of the largest cross-section of the wax pattern. This eases the flow of the alloy and prevents it from solidifying before the entire mold cavity is filled.
Creating a fluid alloy reservoir will help eliminate this kind of porosity.
The distance between the end of the wax pattern and the outside of the package should be 10 mm. larger dimensions of the mold cavity wall do not favor the evacuation of a quantity of gas through its pores.
The temperature of the heating furnace (calcination) maintained at the maximum possible values for the packaging mass mold contributes to the elimination of porosity, because at high temperature the cooling rate of the fluid alloy will be slower, so that more of it will be able to leak from the reservoir into the mold cavity.
The increased number of revolutions of the centrifuge, or the increased pressure of those pushing the alloy through compressed air, plays an important role in porosity removal.
Performing the casting process in vacuum is very suitable for obtaining porosity-free castings.
Use of a larger quantity of metal than necessary to fill the mold cavity, so that a well-sized casting cone (button) remains.
Slow calcination (heating) favors the creation of pores in the mold walls through which a quantity of gas can escape.
Expansion and compensation theory
The fabrication of micro prostheses is not a very precise science, if we think of associating the different variables represented by the clinical-technical phases (tooth preparation, impression, model materials, wax pattern, packaging, casting) but, at this time, we are in possession of some knowledge about the properties of the materials used and their effects in the technological process.
In the melting – casting process, due to high temperatures, some phenomena occur which cause changes in the volume of the alloy, materialized in the finished piece. The exact coefficient of alloy shrinkage during solidification is not known; values ranging from 1.1 to 1.5% are given for noble and semi-noble alloys (Johnston, Philips and Dykema). To compensate for this alloy shrinkage, the mold cavity is required to be a larger volume, which is achieved by expanding the packing masses. The three types of packing mass expansion are used:
– the plug expansion, produced in the phase of transition from the plastic to the solid state;
– hydroscopic expansion occurs if the packaging comes into contact with water;
– thermal expansion occurs if the packaging is heated.
The wax model increases its volume when the packaging mass releases temperature during the expansion process. This expansion is in the range 0.3 – 0.5. and does not occur if the packing mass paste is introduced into a closed cavity, delimited by rigid, metallic walls (sink).
It is obtained by the following technological procedures:
– the sink with the packing mass, in the setting phase, is introduced into a vessel with water having a temperature of 35 – 40°C where it is kept for 3 – 5 minutes;
– controlled addition of water, only to the surface of the packing mass in the setting phase, water is added in a certain quantity (Asar, Mahler and Pejton). They added the specific amount of water to the packing ring, using a syringe, instead of placing the packing ring in a water bath, from which it absorbed a maximum amount of water. The operator has the ability to control the exact amount of expansion that results. The mechanism of this type of expansion is the same as for normal tooth expansion, the growth of gypsum crystals;
– wetting of the asbestos paper lining the entire inner surface of the ring (sink); this causes hydroscopic expansion.
Hydroscopic expansion of the packing was first used in 1932 by Carl Scheu. In 1943 Hollenback simplified the technique for definitive use.
The importance of the inner lining of the metal cylinder with asbestos paper:
– Asbestos is compressible under the action of pressure applied to the inner wall, allowing the packing mass to expand. The rigid walls of the cylinder would stop this expansion;
– Wet asbestos is a favorable environment in which hydroscopic expansion takes place
– Asbestos being resistant to high temperatures and compactable, it allows the ring (sink) to shrink after it has been removed from the furnace; which cools faster, while the alloy used for casting is melted;
– Asbestos favors the removal of contents after casting.
Two-stage single-material packaging technique
This technique was devised in order to achieve an optimum expansion of the mold cavity, both in the setting and thermal phases, which fully compensates for the shrinkage of the alloy during solidification. The indication has no limits, it can be used for all types of micro prostheses as well as for one-piece bridges.
The technological process is as follows:
– Mix a small amount of packing mass powder with distilled water (at room temperature). The proportion should be only as written on the product instructions. The smearing is carried out manually, mechanically or by vacuum.
– Cover the wax pattern by brushing and vibrating, carefully not to cause any air bubbles to form on the surface. The layer is very thin.
– With a brush (soft bristle), spray the dried packing powder over the deposited paste; vibrate with a notched tool to absorb all the powder. Repeat 3-4 times, alternating the deposition of the paste with the deposition of the powder. The model will be enclosed in an oval packing-table mold. The wax model, thus packed, is left to set for 5-10 min.
– Prepare another mixture of the same product (of packing table), but in twofold quantities.
– The paste is poured into the metal cylinder. Wax model, packed before (core). The outer surface is moistened by immediately dipping in and out of water. The core is inserted into the packing paste in the cylinder.
– The package is left to set for 45 min. The known procedures of removing the core, rod and pre-heating are continued.
Removing the casting rod
– the molding rod is removed 40-50 min. after packaging according to the following procedure:
– the rubber cover of the conformer that created the molding rod is removed;
– the packing material particles are removed from around the rod with a brush so that they do not penetrate the rod after removal;
– the end of the rod is heated by a Bunsen burner and pulled progressively downwards out of the packaging with the clamping pliers. A clear passage is left to the wax pattern, after which the edge of the flow channel is cleaned, unsubstantiated particles being removed from around the opening which might penetrate into the interior of the mold.
In laboratories that are equipped with a vacuum cleaner for wax in the molds, the sink is immersed 2-3 min. in boiling water, the soaked wax is removed by vacuum.
After removal of the wax, the sink is placed in a cold furnace, with the hatch down, which is then connected to the heat source.
The mold will be heated to a fairly high temperature (750 – 800°C) over a long period of time to remove the wax completely. When the wax is removed, some of it is absorbed by the packing mass, which requires a longer time to be burnt off; so that the pores in the walls are not clogged with carbon.
The purpose of burning at high temperatures is to remove all the wax and residual carbon, to achieve a print cavity that is a duplicate of the wax model. Thermal expansion is also achieved. Overheating of the mold is harmful and is always avoided because the binder in the packing mass, gypsum, which is chemically calcium sulphate, decomposes slowly at temperatures above 800°C and releases Sulphur or Sulphur compounds. Sulphur combines with certain elements in molten alloys, especially silver and copper, resulting in a sulphide film on the surface of the casting that produces a specific, hard-to-remove coloration. Sulphur corrodes the metal rings (conformers) and its heating element, causing damage.
It is advisable to establish a burning cycle in a laboratory and make it a habit.
Melting – casting of noble and semi-noble alloys
The heat source is required to act on the entire quantity of alloy. If the oxy-methane flame is used as the heat source, it will be very well tuned to highlight its three conical zones. The light blue central cone is the area of almost complete combustion, being the hottest part of the flame and slightly reducing. This part of the flame must reach the alloy and cover it as completely as possible, so that melting takes place in a short time and also protects it against oxidation. After the alloy has liquefied, there is a risk of oxidation.
This phenomenon occurs, in particular, in dental gold alloys of the “hard” type which contain elements that oxidize easily during melting. When the alloy solidifies, the oxygen that has dissolved in its mass is expelled, and in its place micro-spaces appear, which are distributed on the surface or in depth, known as porosities.
Some oxidation occurs during melting, even if the burner has been properly adjusted and operated. It is very important that the alloy is well protected against oxidation by using a deoxidizer. The deoxidizer forms a protective coating (crust) and at the same time reduces oxides, with clean metal forms, preserving the alloy composition and physicochemical properties.
The deoxidizer is added in two steps:
- when the alloy starts to liquefy
- before pouring (triggering the thrust force)
Overheating of the alloy, high temperatures produce excessive oxidation and gas absorption. Overheating results in castings with rough (large grained) surfaces that are less resistant to breakage (even brittle). Porosities and discolorations appear on the surface. If a matt film appears on the surface of the molten gold alloy, it is evidence of oxidation; the shiny, mirror-like surface shows reduced (clean) alloy.
Porosities are very serious failures because they have the following consequences:
– the porous surface provides shelter for oral fluid, food debris and microorganisms;
– the porous surface cannot be processed to obtain the characteristic shine of the alloy
– the hardness, resistance to breakage and ductility, the physical properties that characterize the alloy and for which it was prepared, are reduced, being unsuitable for the intended purpose.
It is marked by the evidence of cast crowns which are common to the casting rods and the casting cone. The operations of cutting the casting rods and processing are well known.
For gold alloys the cooling immediately after casting will produce changes in the structure of the alloy; thus, it becomes very soft and malleable. Slow cooling (in the mold or in the furnace) after casting increases the hardness, melt strength and brittleness of the alloys (the alloys become harder).
Heat treatment for softening or hardening, can be carried out effectively by adjusting the time the casting is left to cool in the packing table after casting. For softening, the sink is placed in water for a minute or two immediately after casting. For hardening, the sink is allowed to cool slowly for 3-6 min. before placing in water. Unpacking is marked by the cast crowns, which are integral with the casting rods and the casting cone.
Processing, finishing and polishing of the cast metal crown
The processing of metal materials can be achieved by two processes:
Mechanical – chipping with abrasive materials,
Mechanical machining – which achieves volume reductions, shape changes by consuming material, metal. The less processing, the less material loss. Metal processing is achieved by using micro-motors for mechanical machining that gradually remove some of the excess on the faces of the prosthesis. The horizontal motor to which a disc is attached is used for large machining operations and for cutting rods on extra-hard alloys. The cutting of the rods that have emerged from the casting grooves in all noble alloys is done with a cutting tool without sawdust.
Abrasive materials – stones containing silicon dioxide and aluminium trioxide. They are also used in sandblasting. All processing starts with sandblasting.
Sandblasting uses the sand jet formed by compressed gold and material particles in the form of granules of varying sizes depending on the purpose.
Blasting is carried out for the following purposes:
- Removal of packing foils
- Removal of oxides (during the cooling of the part the alloy oxidizes).
Machining tools: cylindrical and wheel, they operate circularly. These stones are either fixed or mobile; they have different colors (white, brown, dark grey), the color shows the hardness. Today are on the market – carbide cutters – hard alloy for work aimed at removing a minimum amount – machining for smoothing faces. These cutters come in different shapes and sizes.
Electro-chemical processing uses:
– electrolytic bath – is sometimes used for pattern making but the electrolytic solution is other than copper sulfate and the part is placed at the anode. Direct current flows from the anode to the cathode (“pulls off” the prosthetic part which appears polished).
– the ultrasonic bath consists of:
- a source that produces ultrasound that has a higher frequency than the usual sounds that equate to aluminium.
Processing is carried out in rooms other than those where mechanical processing is carried out; rooms without dust in the atmosphere. These machining operations are of high precision, require special materials and are only available in equipped laboratories.
Parts polishing. This is a mechanical or electrochemical process that produces surfaces with an appearance imitating that of polished glass or mirrors. A macroscopically polished surface does not show micro depressions but is continuous.
The purpose for which polishing is achieved:
Soft parts (tongue, cheek)
- are not damaged
- a degree of comfort is ensured
- does not favor the retention of food debris
- they slide on very easily.
The harder the alloy, the more perfect and long-lasting the shine. The shine is modified in alloys by corrosion. Corrosion matures the work.
The polishing of metal prosthetic parts is achieved by attaching brushes, fluffs, felt and gums to the horizontal engine. Chromium oxide used, has a special action in restoring the shine.
MATERIALS USED IN CAST CROWN TECHNOLOGY
The metallic materials are represented by alloys of the following metals: gold, platinum, palladium, silver, copper, chromium, nickel, cobalt, molybdenum, iron (for wire) and aluminum (for bronzes).
Metals are chemical elements that have the following common physical and chemical characteristics:
- Solid bodies at room temperature, with the exception of mercury which is fluid;
- The structure is polycrystalline (4,5) cubic or (15) hexagonal. The crystal is a form of the solid state of matter;
- Color is gray-silver with two exceptions represented by copper and gold. In the fine powder phase, all are gray, because light rays are completely absorbed;
- Opacity due to free electrons that pass from one orbit to another;
- Own luster because it reflects light rays and a part of electromagnetic radiation;
- High specific thermo-electric conductivity. Conductivity is reduced when there are impurities in the structure accompanied by irregularities of the crystalline structure. The metals with the best conductivity are: Cu, Ag and Au.
- Specific weight is variable which led to classification into light and heavy metals (specific weight over 5). It is expressed in g/cm³. It is an important characteristic for dental prosthetic work.
- The melting temperature has extremely varied values: from -39°C for mercury to 3400°C for tungsten.
- Mechanical resistance represents the property of opposing external forces that tend to deform them plastically, elastically or to fracture them. In general, metals have differentiated elasticity and plasticity, specific to each group. Platinum, silver, copper, tin are easily deformable due to plasticity. Plasticity does not modify the integrity of the metallic body. Mechanical resistance recommends metals for use for certain purposes. In dentistry, they are used for the realization of prosthetic works, differentially.
- Chemically, in contact with acids, they form salts.
- They ionize in acid solutions by releasing particles charged with positive electrical charges (positive ions).
Alloys are, in the solid or liquid phase, homogeneous mixtures composed of two or more metals.
The behavior of metals in the liquid phase is variable, which creates the following types of alloys:
- Both metals, in the liquid phase, are soluble in each other in any proportion, forming a homogeneous solution that is maintained in the solid phase. The distribution of crystals in the structure of alloys is balanced (e.g., Au-Cu alloys, Au-Ag or Au-Ag-Cu) in these types of alloys are included metals with affinity between them.
- Both metals are soluble with each other in the liquid phase but in the solid crystallization phase they are separated. The alloy in the solid phase is made of pure crystals of each element that has entered the composition. These are easily fusible alloys called Melot and Spent. The melting point is lower than the elements that form the structure. These alloys are known as “eutectic alloys”.
- Both metals are only soluble with each other in certain proportions. If these are exceeded, the alloy becomes brittle, up to brittle.
- Both metals form chemical combinations between them known as “intermetallic compounds”. The presence of these compounds determines a new behavior, the alloy becomes brittle.
- Metals that cannot be mixed in any proportion. Alloys cannot be obtained.
General physical and chemical characteristics of alloys:
- Hardness is greater than that of each component.
- Resistance to plastic-elastic deformation and breakage is greater than that of the component elements.
- Thermal and electrical conductivity is lower than that of each component.
- Specific weight has an average value, between that of the metal with the lowest weight and that of the one with the highest weight.
- Melting point is lower than that of each metal in the composition.
Corrosion represents the deterioration of the surface of metals and alloys.
Metals and alloys in contact with factors from the environment (gases or liquids) form corrosion products. These corrosion products can have a differential behavior, accelerating, delaying or not influencing the deterioration of the surface.
Alloys used for prosthetic work in the oral environment are subjected, due to variations in pH and temperature, to the phenomenon of corrosion. Resistance to corrosion for dental alloys is an essential condition.
Surface corrosion is anticipated by the appearance of maturing and the formation of spots.
Corrosion is due to several causes: chemical, electro-chemical.
Forms of presentation of alloys recommended for dental prosthetics
Dental prostheses have very different sizes and shapes and are made through various technological processes. Many types of materials are needed to obtain them.
Metal products are being sold in the following forms:
- grains, tablets, spheres, parallelepipeds with a square, rectangular or hexagonal section, cylinder, on which figures, letters or symbols of the companies are inscribed. They are used to be melted and cast;
- laminated sheet with a thickness of 0.25-0.30 mm for caps, 0.1-0.2 mm for orthodontic rings and 1 mm for melting-casting;
- wire with different diameters of rods or Cr-Co alloys. From wire with a diameter of 0.2 mm various ligatures are made. Wire with a diameter of 0.6-0.8 mm is used for dental splints of partial dentures, wire of 1.2-1.5 mm for making splints used for the containment of maxillary bone fragments;
- elements manufactured for splints, lingual bars, palatal bars necessary for obtaining skeleton prostheses.
Techniques for processing alloys
Alloys are processed at different temperatures as follows:
Processing at a temperature of 20°C, known as cold processing, which consists of rolling, bending, drawing, upsetting, stamping, pressing.
Processing at a temperature of over 100°C known as hot processing, which consists of melting-casting, melting-gluing, heat treatments.
These alloys have noble elements in various percentages in their composition. Noble metals are presented by: Au, Pt, Pa, Ir, Radium, Ruthenium, Osmium.
- yellow color, characteristic;
- specific weight: 19.3 g/cm³;
- low hardness: 43 kg/cm³ (Brinell); very soft;
- very good ductility and malleability: it can be drawn into very thin sheets;
- resistant to the action of acids, bases and salts; it is not dissolved by royal water (3 parts nitric acid, one-part hydrochloric acid);
- resistant to corrosion.
- in industry: catalytic role, for electrochemical gilding;
- in dentistry: in the form of alloys for prosthetic work;
- in general medicine: gold salts for the treatment of rheumatism.
The very low hardness does not recommend it to be used for the production of prostheses.
It is alloyed with other metals (Ag, Cu, Ni, Pt, Pa, Ir) to improve its physical and mechanical characteristics.
Gold alloys recommended for dental use have varying percentages of pure gold depending on the mechanical qualities required of the alloy for a certain type of prosthetic work.
Gold alloys are used in dentistry because they resist very well to the corrosive action of the oral environment and because they result in dental prostheses with precision unmatched by other alloys.
Metals in the composition of gold alloys
– specific grey color,
– malleable and ductile, easy to work,
– melting point is 1071°C,
– specific weight is 10,5
– in the fluid state it absorbs gases, especially oxygen, which it releases during the solidification phase, resulting in porous castings
– it has an affinity for sulphureous products and blackens,
– combines with gold, changes its color and increases its hardness and resistance to abrasion.
– characteristic red color. Imparts its color to alloys
– malleable and ductile,
– very good thermo-electric conductivity,
– it alloys easily with gold and darkens its color,
– increases the strength and hardness of the alloy.
In gold alloys enters in equal proportion with silver (42 + 42)
Platinum – is an element of the noble metal group
– its color is silver-grey, it is influenced by the color of the alloy, becoming yellow-grey;
– malleable and ductile;
– the melting point is very high, 1754,44° C;
– specific gravity is 21.4 higher than gold;
– increases the resistance to breakage and increases the hardness of the alloy, making it more resistant to abrasion;
– high melting point increases the melting range of the alloy;
– the alloy’s shine is better maintained for longer;
Platinum foil was once the support on which the ceramic mass was deposited and sintered. Today the technology is outdated.
Palladium, an element of the group of mobile metals
physical and chemical characteristics:
– silvery-grey color, much lighter in color than the alloy;
– malleable and ductile;
– melting point is 1548.88°C;
– specific gravity 12;
– increases malleability, ductility, hardness, fracture strength and resistance to elastic and plastic deformation of the alloy;
– increases the melting range of the gold alloy;
– the percentage in the composition of gold alloys is variable, but it has high ennobling power.
Iridium, element of the noble metal group,
physical and chemical characteristics:
– color similar to silver;
– specific gravity is 22,4;
– very high hardness;
– resistant to breakage, bending and deformation;
– brittle to shocks;
– resistant to very active acids and alkalis including royal water;
– high melting point, 2454°C;
– the alloying percentage is variable, up to 25%;
– the crystalline structure is very fine, transmitting it to the alloy.
TYPES OF GOLD ALLOYS.
GOLD ALLOY 916 ‰ = 22 KARATS (SOFT GOLD ALLOYS)
916 g are pure gold, the rest up to 1000 g (84 g) is silver and copper, generally in equal proportions. If the proportion changes in favor of copper, the alloy is harder and has a red color. If the percentage of silver is higher, the alloy has a lower hardness and the color is less intense.
The 916‰ gold alloy is the most malleable of all gold alloys, which is why it is used for the following micro prostheses: inlays, cast crowns. The edges of the inlays may turn brown.
– yellowish color;
– very malleable and ductile;
– corrosion resistant;
– does not decompose.
Gold alloy 833‰ = 20 karats (medium hard gold alloys)
Per 1000 g of gold: 833 g are pure gold, and the remaining 167 g are made up with copper and silver. This alloy has a higher percentage of copper than silver in its structure. Copper gives the alloy a higher hardness, abrasion resistance and breakage resistance. Due to these characteristics, it is used in the manufacture of: RCR, crowns, inlays, dental bridges, bridge body 1-2 teeth.
Due to the high percentage of silver and copper there is a danger that during melting – casting there is a de-homogenization followed by a change in the physical – chemical behavior, which can become sensitive to corrosion.
Gold alloy 833‰ with silver, copper, zinc, palladium and platinum (hard alloys)
In this alloy silver and copper are represented in small percentages, being replaced by platinum. The alloy has very high hardness, is break and shock resistant with a high degree of elasticity. The presence of platinum in the composition improves the physico-chemical qualities of the alloy, it is very resistant to the action of oxidizing substances.
– replacement crowns,
– 3/4 and 4/5 partial crowns,
– dental bridges (total),
– skeletal prostheses.
Palladium 20/50 ‰, iridium up to 25 ‰ are added to the composition in varying percentages.
Gold alloys 750‰ = 16 carats
In the physico-chemical composition silver is represented by 83 parts and copper by 167. In a high percentage, copper gives the alloy superior physico-chemical characteristics compared to the 833 ‰ alloy, it is harder and has much higher elasticity.
Platinum, palladium and iridium have been introduced in this alloy up to 50% to increase hardness and resistance to the action of corrosive substances.
Physico-chemical characteristics of noble alloys
Noble alloys are suitable for all types of fixed and removable dental prosthetics (skeletal prosthesis). In the fluid phase, these alloys are very fluid, a characteristic which favors flowing in very small molds (thickness) of only 0.25-0.35 mm, resulting in very thin prostheses (cast crowns with controlled thickness, etc.).
The shrinkage coefficient at the transition from the fluid to the solid state is very low, 1.1-1.3%, fully compensated by the expansion of the packing mass. The result is prosthetic parts of the highest accuracy (precision).
Chemically they are particularly resistant to the corrosive action of acids and bases. In the oral environment they are very well tolerated.
From a financial point of view, the value of the material is maintained after removal of the work from the prosthetic field, because after refining the alloy it can be recovered for reuse in dentistry or jewelry.
Silver alloys with palladium.
These alloys represent a category of noble alloys different from gold or platinum alloys. The physical-chemical composition is dominated by the presence of silver and palladium, to which other elements are added in micro-percentages.
These alloys have a lower cost price than gold or platinum alloys. Silver alloys with palladium were created to partially or totally replace gold or platinum alloys.
Ni and Cr alloys
The main element present in the composition of these alloys is Ni.
* nickel = 65 – 70%
* chromium = 14 – 16%
* secondary elements (aluminium, molybdenum, tungsten, boron, silicon, carbon, manganese) = 3-11%. The percentage varies depending on the manufacturer.
Secondary elements are introduced into the composition for the following purposes:
– obtaining the microcrystalline structure by adding boron, silicon, carbon and aluminium;
– deoxidation, by adding silicon and boron;
– obtaining the appropriate fluidity in the liquid phase by adding boron and silicon;
– obtaining corrosion resistance by adding manganese, molybdenum and tungsten. The Cr oxide film is the corrosion protection element.
Mechanical resistance (to deformation, breakage, abrasion) is given by Cr.
– the color of these alloys is silver-grey. It can be said that this color is the reference color compared to yellow alloys (gold and bronze alloys).
It should be mentioned that alloys that are white do not exist in reality. Only in some publications it is written, by mistake, “white alloys”. Only non-metallic materials in the resin (acrylic), ceramic and composite groups have a white color similar or identical to that of dental crown enamel. There is a tendency to use the terms “white alloys” and “yellow alloys” but these are not recommended in academic circles but, at most, only in the trade.
This color is a disadvantage compared to bronze alloys.
– the specific weight is 7 – 9 gr/cm³. It is much lower than gold alloys. Dental prosthetics have less weight, less alloy is consumed.
The low specific weight requires a different technology for the packaging of the models, flow channels with diameters of 3 – 4mm and a sufficiently high alloy pushing force (much higher than for noble alloys).
– hardness, in Brinell units, is 180-280 kg/mm². This hardness is an advantage for reducing the size of the prosthetic parts and a disadvantage for the physical processing phase, for the occlusal faces of antagonist teeth (abrasion occurs) and for the time of removal by sectioning of the microprostheses.
– the melting-turning range is 1150 – 1359°C. These temperatures are obtained in the dental laboratory with other apparatus than those used for noble alloys. The heat source must operate over the entire surface of the alloy mass to be cast;
– the cooling shrinkage coefficient of these alloys is 2.2 – 3.5% depending on the thermal value of the melting range;
– corrosion resistance in the oral environment is satisfactory if the technological process in the laboratory is carried out according to the conditions written in the product leaflet;
– biological behavior. In general, these alloys are very well tolerated but it is not excluded that the following pathological manifestations may occur:
- allergic dermatitis, in dental technicians, due to contact with the powders during processing of prosthetic parts;
- eyelid oedema, also of an allergic nature;
- chronic eczema.
These manifestations are produced by the presence of nickel powder and the soil on which it acts.
The wipla stainless alloy
This alloy is a stainless steel which is characterized by a very high percentage of iron and the presence of carbon.
¬ iron = 70-72%
¬ Chromium = 18% ¬ Iron = 70% iron
¬ nickel = 8%
¬ additional elements represented by manganese, molybdenum, silicon, tantalum, titanium, which are used to fix carbon in the alloy.
Alloys containing more than 50% iron and up to 2% carbon are called steels.
If the percentage of carbon exceeds 2%, the alloy is called cast iron. Alloys of iron and carbon in these proportions form solid, homogeneous solutions and are known as austenitic alloys.
Iron has a melting point of 1539°C, Brinell hardness of 55 kg/mm² and four allotropic states with distinctive structures and properties (alpha, beta, gamma, delta). Alloys used in dentistry contain gamma iron. Austenitic steel resulting from the combination of iron and carbon is not stainless.
Chromium 18% and nickel 8% are added to make it stainless steel. To prevent the formation of iron and chromium carbides, microprocents of manganese, molybdenum, silicon, tantalum and titanium are added to the alloy to fix the carbon in the alloy. Chromium makes the alloy resistant to the action of oxidizing agents, increasing its malleability, ductility and mechanical strength. Chromium and nickel stabilize the austenitic structure of the alloy.
The alloy used in dentistry is known as wipla “Wie Platin”, from the German language, having a platinum-like color. Because of the chromium content, wipla is also called 18/8 stainless alloy.
– The color is grey-silvery, similar to platinum;
– Hardness is 160 kg/mm² (Brinell);
– Melting range 1375 – 1420° C;
– The brightness of the machined surfaces is resistant to the oral environment;
– Resistance to corrosive action of the oral environment, due to the presence of chromium oxide formed on the surface;
– In the liquid phase the viscosity is increased, which prevents the obtaining of thin castings (0,30 – 0,35mm);
– Heat treatment is necessary, because the austenitic structure changes during processing. The treatment consists of reheating the prosthetic part to 1000 – 1100°C, followed by quenching;
– The alloy is not suitable for joining two elements. There is an alloy for soldering, but it is unsuitable, contains a lot of silver, has low mechanical and chemical resistance;
– The shrinkage coefficient at the transition from fluid to solid phase is high, 2.5-3%, not compensated by the packing mass. Shrinkage results in smaller castings;
The precision of the castings is not possible, because of the high viscosity and the high shrinkage coefficient.
Forms of commercial presentation:
¬ 0.10 – 0.15 mm board for orthodontic rings and caps;
¬ Wire of very different sizes:
– 0.2 mm, malleable, for perimeters and ligatures
– 0.6 – 0.8 mm, very flexible, used for making acrylic partial denture brackets
– 1-2.5 mm, very hard ferder-hart, used for surgical splints and orthodontic appliance springs. This wire in cross-section can be round or semi-round…
¬ Elements with different shapes to obtain prefabricated crocks by molding;
¬ Prefabricated bars which are shaped using special clamps to make a lingual bar or a palatal bar.
The commercial representative in Romania is the product “Gaudent”.
This category of alloys has the following elements in its composition:
– copper = 80 – 82%
– aluminium = 8,5 – 10%
– nickel = 3 – 4%
– manganese = 1,5 – 2%
– Iron = 1%.
The complex chemical composition has created a special physico-chemical behavior, these alloys are known as “special bronzes”.
- Resistance to agents that promote and produce corrosion. This resistance is due to the protective aluminium oxide film which forms on the surface of the alloy and which, if removed mechanically (by abrasion), recovers immediately;
- hardness is 130 Brinell units for castings;
- specific weight: 8,5 gr/cm³;
- melting range: 1025 – 1050° C;
- thermal conductivity is 2.5 times lower than noble alloys;
- flowability is suitable for obtaining microprostheses and dental bridges. This characteristic ensures flowability in molds with dimensions (thickness) of 0.30 – 0.35 mm;
- tensile strength is 40 kgf/mm². After heat treatment this increases to 65 kgf/mm²;
- the color is yellow, a particularly favorable factor.
Gaudent has long dominated dental work for the morphological restoration of dental crowns and arches, progressively succeeding in replacing wipla stainless steel alloy.
Today, Cr-Ni alloys, produced in various assortments, tend to be used but they have the following unfavorable characteristics:
- grey-silver color, not accepted by many patients;
- Vicker hardness 240, much higher than other dental alloys;
- high melting range: 1280 -1350°C (requires special heat sources and equipment).
Gaudent is appreciated in prosthetic work for:
– resistance to corrosion and oxidation in the oral environment
– hardness close to that of gold alloys
– melting range similar to that of mobile alloys
– packing mass compensates for shrinkage coefficient
– processing in the laboratory is carried out with ordinary abrasion instruments
– resistance to plastic and elastic deformation
– the removal of microproteins from dental bridges is not a very difficult operation.
Bronzes are an alternative to other types of alloys (noble and non-noble) due to their characteristics and low cost price.
Very useful practical recommendations:
– Channels for flowing the fluid alloy should be properly sized (diameters 3 – 4mm).
– The alloy fluid reservoir should be located at 15mm from the top of the mold.
– Expansion of the packing mass (hydroscopic and thermal) to be obtained in a scientific way.
– The calcination of the silicone mold is obtained at 650°.
– The alloy heated in the furnace to 700°C
– The ideal heat source is high-frequency currents, associated with the introduction of the fluid alloy into the mold in a vacuum using compressed air.
– Cooling of the mold should be done in a very short time (30 – 40 sec.) after casting. This rapid cooling reduces the possibility of the formation of Al-rich gamma phase with low corrosion resistance.
Cr-Co alloys (stellite)
Chromium-cobalt alloys are superior to wipla (iron, chromium-nickel) stainless alloys.
They are complex combinations of many metals: chromium, cobalt, nickel, molybdenum, silicon, carbon, magnesium, aluminium, tantalum, tungsten, titanium, tungsten, wolfram, vanadium, niobium. The highest percentage is represented by chromium (15-30%) and cobalt (1-64%). For each product there is a certain recipe, known only to the specialists who elaborate the alloy.
The possible methods of forming these alloys are:
Chromium is a silver-grey metal harder than iron.
The melting point is 1876°C and the specific gravity 7.1. It is sensitive to hydrochloric acid. In a percentage of more than 12% it makes the alloys passive to the action of oxygen. It is contained in the alloy in the proportion of 15-30%.
Cobalt is a metal with a chromatic appearance similar to silver. Its hardness is higher than that of iron and nickel. In combination it gives the alloy high hardness and chemical stability, protecting it against the action of acids and bases.
The melting point is 1481°C and the specific gravity 8.9. The alloy contains 2 to 64% of the alloy.
Nickel is a malleable and ductile silver-grey metal with a melting point of 1441°C and a specific gravity of 8.6. In combination with the other elements, it gives the alloy the following properties: increased hardenability, reduced hardness (thus making cold working possible), reduced oxidation and a more homogeneous structure. In the composition of alloys, the proportion is variable 5-55%.
Molybdenum is a relatively malleable metal with a common silver-grey color appearance. The melting point is 2606°C and the specific gravity is 10, 2. In the alloy composition it is introduced in percentages between 5-18%. The alloy has the following properties: hardness, resistance to breakage by forming a fine crystalline structure.
Tungsten is a very hard metal with a high melting point of 3380,57°C. It is particularly resistant to the action of most acids and bases. Together with tantalum, vanadium and niobium they belong to the same subgroup of the periodic system. Tantalum is very tough, malleable and ductile and can be drawn and drawn. Tantalum is electro-chemically inert, very well supported by tissues. These characteristics recommend it for making intraosseous implants. In alloys it is introduced in a proportion of 4 – 5%.
Magnesium is a high hardness metal. The specific weight is 7.12. The melting point is 1216°C. Ductility and malleability are low. In alloys it is about 5%.
Aluminium is a low hardness metal. Specific gravity is 2.7. The melting point is 646°C. In the composition of bronze-type alloys it is introduced in a percentage of 8 – 10%. Aluminium and manganese are added in order to increase the fluidity of the alloy (in the fluid phase). For these alloys fluidity is a specific characteristic.
– The specific gravity is unique (6.5 – 8), so they are much lighter than noble alloys.
– Resistance to corrosive action of oxygen, acids and bases.
– Melting range is between 1300 – 1500°C.
– Mechanical strength, manifested by resistance to abrasion, to breakage and increased hardness (on the Brinell scale 180 – 340kg/mmp). This characteristic causes difficulties for the machining phase. After finishing and polishing, the glossy appearance of the surfaces is maintained.
– The high fluidity of the liquid phase due to the low viscosity favors the penetration into small molds, thus obtaining thin casts.
– The cooling shrinkage coefficient is 1.7 – 2%, which is compensated by the expansion of the mold.
– The melting range, represented by high temperatures and high shrinkage coefficient, requires the use of special packaging masses.
– Crystallizes on cooling homogeneously; an austere structure is formed.
– Products obtained by drawing (wire drawing) have greater flexibility than components of prostheses made by casting.
– The glossy appearance of mechanically or chemically processed surfaces remains for a long time in the oral cavity.
These alloys have been designed and produced for the production of metal components of skeletal dentures.
– The physico-chemical characteristics they possess are suitable for this type of prosthesis. Other dental prostheses (bridges, crowns or replacement crowns) have also been made out of necessity. The very high hardness is an important weakness for the time of removal after the prosthetic field by cutting. Subperiosteal implants are made of these alloys and are generally tolerated by the tissues.
– The form of commercial presentation differs according to the company’s preferences. They are: hemisphere-shaped, parallelepiped-shaped, hexagonal, square or rectangular base, cubes or cylinders 10-12 mm long and 4-5 mm in diameter.
Commercial names of alloys
– Vitalium, on which are the initials V-260 and V-180 (the figures represent the hardness value in Brinell units). V-100 is the lower hardness product indicated for dental bridges.
– Wisil, Remanim, Rubonite, Niranium.
– Wiplam wire for splints, it acquires flexibility by cold shaping with pliers.
– Special elements for maintaining support and stabilization of skeletal prostheses, abutments, hinges, bars with rider, Ceka system, staples.
The metal crown is a micro protection that completely covers the faces of the crown portion of the tooth, restoring its morpho functional integrity.
Cast crowns are made of noble or semi-noble alloys; the side walls can have different thicknesses; if they have unequal thicknesses the crown is full thickness, and if the walls are equally sized it is directed thickness.
The characteristics of the full-thickness crown are:
¬ the inner faces of the crown are in contact with the dental bridge;
¬ the side walls are unevenly sized;
¬ between the inner faces of the crown and the faces of the dental bridge there is a frictional force which gives it its most effective stability;
¬ temperature variations in the oral cavity are transmitted entirely to the dental bridge;
¬ removal from the dental bridge is made difficult by cutting the occlusal and buccal surfaces;
¬ a lot of material is consumed for its fabrication;
¬ the major indication is lateral teeth with reduced dimensions in the cervical-occlusal direction.
The characteristics of the thickness-directed crown are:
¬ the side walls are 0.3 mm thick;
¬ the side walls are at a distance from the faces of the dental bridge;
¬ the crown is in contact with the dental bridge on the occlusal face and in the cervical area for a distance of 2 mm;
¬ partially transmit temperature variations;
¬ less material is consumed for fabrication;
¬ are indicated on lateral teeth with large cervical-occlusal dimensions.
Regardless of the type of crown cast, from a clinical-technical point of view, the same steps are followed, steps that must be strictly observed to avoid errors that force the technician to remake the crown.
One of the main benefits of the cast crown is its durability. A cast crown can last for many years, sometimes even a lifetime, with proper care and maintenance. This makes it a great option for restoring a damaged tooth, as it can provide long-term protection and function.
Another benefit of the cast crown is its natural appearance. Because the crown is custom-made to fit the tooth, it can blend in seamlessly with the patient’s natural teeth. This can be especially beneficial for restoring a front tooth, as a natural-looking crown can greatly improve the appearance of the smile.
In summary, the cast crown is a type of dental restoration that is used to fully encase a damaged or decayed tooth. It is typically made of gold alloy or a combination of gold and other metals, and is used to restore the function and appearance of a damaged tooth. The process of getting a cast crown involves a consultation with a dentist, preparation of the tooth, taking an impression of the tooth, and custom making the crown. The cast crown is known for its durability and natural appearance. With proper care, a cast crown can last for many years, making it a great option for restoring a damaged tooth.