8 THE COMPLETE CAST CROWN PREPARATION
Although esthetic factors may limit its application, the all-metal complete cast crown should always be offered to patients requiring restoration for badly damaged posterior teeth. The complete cast crown has the best longevity of all fixed restorations. It can be used to rebuild a single tooth or as a retainer for a fixed dental prosthesis. It involves all axial walls, as well as the occlusal surface of the tooth being restored (Fig. 8-1).
Fig. 8-1 Complete cast crowns used to restore the molar teeth. A and B, The canines and premolars, which are more visible because of their more anterior arch position, have been restored with metal-ceramic crowns.
Preparation for a complete cast crown requires that adequate tooth structure be removed to allow restoration of the tooth to its original contours. Tooth structure should be preserved when possible, but reduction should produce a crown of acceptable strength.
Because all axial surfaces of the tooth are included in the preparation, the complete cast crown has greater retention than a more conservative restoration on the same tooth (e.g., a seven-eighths or three-quarter crown [see Fig. 7-34]).
Normally, a complete cast crown preparation also has greater resistance form than does a partial-coverage restoration on the same tooth. For a partial veneer crown to rotate off the tooth, only the tooth structure immediately lingual to the occlusal portion of the proximal groove or box need fail. However, if the axial walls of a complete cast crown have been prepared with the proper degree of taper or convergence, a significant amount of tooth structure must fail before the crown can be torqued off.
The strength of a complete cast crown is superior to that of other restorations. Its cylinder-like configuration encircles the tooth and is reinforced by a corrugated occlusal surface. Just as an O-shaped link in a chain resists deformation better than a C-shaped link, this restoration is less easily deformed than its counterparts, which are more conservative of tooth structure.
A complete cast crown allows the operator to modify axial tooth contour. This can be of special significance with malaligned teeth, although the extent of possible recontouring is limited by periodontal considerations. Similarly, it is possible to allow better access to furcations for improved patient oral hygiene through recontouring of buccal and lingual walls (Fig. 8-2). When special requirements exist for axial contours, such as when retainers are needed for partial removable dental prostheses, a complete crown is often the only restoration that allows the necessary modifications for the creation of properly shaped survey lines, guide planes, and occlusal rests (Fig. 8-3) (see Chapter 21).
Fig. 8-3 Complete cast crowns used as retainers to accommodate a mandibular partial removable dental prosthesis. Metal-ceramic crowns have been placed on the mandibular left canine (A) and the maxillary first molar (B). Note the occlusal rests (A, arrows) and the survey contours (B), which extend to form reciprocating guide planes. (See Chapter 21.)
The restoration permits easy modification of the occlusion, which is often difficult to accomplish if a more conservative restoration is made. This is especially important when supraerupted teeth are present or when the occlusal plane needs to be reestablished.
Because all coronal surfaces are involved in the preparation for a complete cast crown, removal of tooth structure is extensive and can have adverse effects on the pulp and periodontium. Because of the proximity of the margin to the gingiva, it is not uncommon to see inflammation of gingival tissues (although a properly fitting complete cast crown with good axial contour should minimize this).
After cementation, it is no longer feasible to perform electric vitality testing of an abutment tooth. The conductivity of the metal interferes with the test. This can be a disadvantage if future complications occur, although thermal tests occasionally yield the necessary information.
Patients may object to the display of metal associated with complete cast crowns, and in those with a normal smile line, the restoration may be restricted to maxillary molars and mandibular molars and premolars.
The complete cast crown is indicated on teeth that exhibit extensive coronal destruction by caries or trauma. It is the restoration of choice whenever maximum retention and resistance are needed. On short clinical crowns or when high displacement forces are anticipated, such as for the retainer of a long-span fixed dental prosthesis, grooves should be included as additional retentive features.
This restoration is fabricated when correction of axial contours is not feasible with a more conservative technique. The restoration also may be used to support a partial removable dental prosthesis, because obtaining the necessary contours with a partial-coverage restoration is more difficult. Although proximal guide planes can sometimes be prepared through simple enamel modification, arriving at properly oriented reciprocal guide planes and survey contours is often impractical. The minimum dimensions required for occlusal rests of a partial removable dental prosthetic framework necessitate removing significant amounts of enamel and, if the dentin is exposed, restoring the tooth with a cast crown.*
The complete cast crown is indicated on endodontically treated teeth. Its superior strength compensates for the loss of tooth structure that results from previous restorations, carious lesions, and endodontic access.
The complete cast crown is contraindicated if treatment objectives can be met with a more conservative restoration. Wherever an intact buccal or lingual wall exists, use of a partial-coverage restoration should be considered. In particular, if less than maximum retention and resistance are needed (e.g., on a short-span fixed dental prosthesis), a preparation more conservative of tooth structure is called for. Similarly, if an adequate buccal contour exists or can be obtained through enamel modification (enameloplasty), a complete crown is not indicated. If a high esthetic need exists (e.g., for anterior teeth), a complete cast crown is also contraindicated.
The occlusal reduction must allow adequate room for the restorative material from which the cast crown is to be fabricated: type III or IV gold casting alloy or their low–gold content equivalent. Minimum recommended clearance is 1 mm on nonfunctional (noncentric) cusps and 1.5 mm on functional (centric) cusps. The occlusal reduction should follow normal anatomic contours to remain as conservative of tooth structure as possible. Axial reduction should parallel the long axis of the tooth but allow for the recommended 6-degree taper or convergence, which is the angle measured between opposing axial surfaces.
The margin should have a chamfer configuration and is ideally located supragingivally. Sometimes crown lengthening is indicated to obtain a supragingival margin, rather than risk future periodontal disease (see Chapter 6). The chamfer should be smooth and distinct and allow for approximately 0.5 mm of metal thickness at the margin. Typically, it is an exact replica of half the rotary instrument that was used to prepare it. (The recommended dimensions for reduction are shown in Fig. 8-4.)
Fig. 8-4 Recommended dimensions for a complete cast crown. On functional cusps (buccal mandibular and lingual maxillary), the occlusal clearance should be equal to or greater than 1.5 mm. On nonfunctional cusps, a clearance of at least 1 mm is needed. The chamfer should allow for approximately 0.5 mm of metal thickness at the margin.
Proper tooth preparation for a complete cast crown results in the reduction’s being directly beneath the cusps of the crown (see Fig. 7-45). This is important for ensuring optimum restoration contour with maximum durability and conservation of tooth structure. Proper placement of the functional cusp bevel achieves this outcome. Because additional reduction is needed for the functional cusps (to provide 1.5 mm of occlusal clearance), the bevel must be angled flatter than the external surface (Fig. 8-5). On most teeth, the functional cusp bevel is placed at about 45 degrees to the long axis.
All complete crown preparations should be assessed for adequate reduction at the occlusoaxial line angles of the nonfunctional cusps. A minimum of 0.6 mm of clearance is needed here for adequate strength. Maxillary molars in particular often require an additional reduction bevel in this area (Fig. 8-6). Without it, an overcontoured restoration that does not follow normal configuration may result. Such additional reduction is often unnecessary for mandibular molars, however, because they are lingually inclined and their profile is relatively straight.
Increasing the faciolingual width of a complete crown is a common error in practice and is a leading cause of periodontal disease associated with restorations. Adequate chamfer width (minimum 0.5 mm) is important for developing optimum axial contour. On small premolars, however, it may be advantageous to prepare a slightly narrower chamfer to conserve tooth structure and retention form. This requires increasingly careful manipulation of the wax pattern during fabrication of the restoration and careful assessment to ensure that the crown is not excessively contoured.
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