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The metal ceramic crown preparation
 

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    The metal ceramic crown preparation The metal ceramic crown preparation Document Transcript

    • In many dental practices the metal-ceramic crown is one of the most widely used fixed restorations. This has resulted in part from technologic improvements in the fabrication of restoration by dental laborato- ries and in part from the growing amount of cos- metic demands that challenge dentists today. The restoration consists of a complete-coverage cast metal crown (or substructure) that is veneered with a layer of fused porcelain to mimic the appear- ance of a natural tooth. The extent of the veneer can vary. To be successful, a metal-ceramic crown prepara- tion requires considerable tooth reduction wherever the metal substructure is to be veneered with dental porcelain. Only with sufficient thickness can the darker color of the metal substructure be masked and the veneer duplicate the appearance of a nat- ural tooth. The porcelain veneer must have a certain minimum thickness for esthetics. Consequently, much tooth reduction is necessary, and the metal- ceramic preparation is one of the least conservative of tooth structures (Fig. 9-1). Historically, attempts to veneer metal restora- tions with porcelain had several problems. A major challenge was the development of an alloy and a ce- ramic material with compatible physical properties Fig. 9-1. Recommended minimum dimensions for a that would provide adequate bond strength. In ad- metal-ceramic restoration on an anterior tooth (A) and a dition, it was initially difficult to obtain a natural posterior tooth (B). Note the significant reduction needed appearance. compared to that for a complete cast or partial veneer The technical aspects of the fabrication of this crown. restoration are discussed more in Chapter 24. For now, only a brief description is provided. The metal substructure is waxed and then cast in a special I NDICATIONS metal-ceramic alloy having a higher fusing range The metal-ceramic crown is indicated on teeth that and a lower thermal expansion than conventional require complete coverage, where significant es- gold alloys. After preparatory finishing procedures, thetic demands are placed on the dentist (e.g., the this substructure, or framework, is veneered with anterior teeth). It should be recognized, however, dental porcelain. The porcelain is fused onto the that, if esthetic considerations are paramount, an framework in much the same manner as household all-ceramic crown (see Chapters 11 and 25) has dis- articles are enameled. Modern dental porcelains tinct cosmetic advantages over the metal-ceramic fuse at a temperature of about 960° C (1760° F). Be- restoration; nevertheless, the metal-ceramic crown cause conventional gold alloys would melt at this is more durable than the all-ceramic crown and gen- temperature, the special alloys are necessary. erally has superior marginal fit. Furthermore, it can 216
    • Chapter 9 The Metal-Ceramic Crow n Preparation serve as a retainer for a fixed partial denture be- ities are excellent because all axial walls are in- cause its metal substructure can accommodate cast cluded in the preparation, and it is usually quite or soldered connectors. Whereas the all-ceramic easy to ensure adequate resistance form during restoration cannot accommodate a rest for a remov- tooth preparation. The complete-coverage aspect of able prosthesis, the metal-ceramic crown may be the restoration permits easy correction of axial form. successfully modified to incorporate occlusal and In addition, the required preparation often is much cingulum rests as well as milled proximal and reci- less demanding than for partial-coverage retainers. procal guide planes in its metal substructure (see Generally, the degree of difficulty of a metal- Chapter 21). ceramic preparation is comparable to that of prepar- Typical indications are similar to those for ing a posterior tooth for a complete cast crown. all-metal complete crowns: extensive tooth destruc- tion as a result of caries, trauma, or existing previ- ous restorations that precludes the use of a more DISADVANTAGES conservative restoration; the need for superior re- The preparation for a metal-ceramic crown requires tention and strength; an endodontically treated significant tooth reduction to provide sufficient tooth in conjunction with a suitable supporting space for the restorative materials. To achieve better structure (a post-and-core); and the need to recon- esthetics, the facial margin of an anterior restoration tour axial surfaces or correct minor malinclinations. is often placed subgingivally, which increases the Within certain limits this restoration can also be potential for periodontal disease. However, a used to correct the occlusal plane. supragingival margin can be used if significant cosmetic concerns do not prohibit it or if the restora- tion incorporates a porcelain labial margin (see CONTRAINDICATIONS Chapter 24). Contraindications for the metal-ceramic crown, as Compared to an all-ceramic restoration, the for all fixed restorations, include patients with ac- metal-ceramic crown may have slightly inferior es- tive caries or untreated periodontal disease. In thetics, but it can be used in higher-stress situations young patients with large pulp chambers, the or on teeth that would not provide adequate sup- metal-ceramic crown is also contraindicated be- port for an all-ceramic restoration. cause of the high risk of pulp exposure (see Fig. Because of the glasslike nature of the veneering 7-4). If at all possible, a more conservative restora- material, a metal-ceramic crown is subject to brittle tive option such as a composite resin or porcelain fracture (although such failure can usually be attrib- laminate veneer (see Chapter 25) is preferred. uted to poor design or fabrication of the restoration). A metal-ceramic restoration should not be con- A frequent problem is the difficulty of accurate shade sidered whenever a more conservative retainer is selection and of communicating it to the dental ce- feasible, unless maximum retention is needed-as ramist. This is often underestimated by the novice. for a long-span FPD. If the facial wall is intact, the Since many procedural steps are required for both practitioner should decide whether it is truly neces- metal casting and porcelain application, laboratory sary to involve all axial surfaces of the tooth in the costs generally place the metal-ceramic restoration proposed restoration. Although perhaps technically among the more expensive of dental procedures. more demanding and time consuming, a more con- servative solution usually can be found to satisfy the patient's needs that may provide superior PREPARATION long-term service. The recommended sequence of preparation is illus- trated for a maxillary right central incisor (Fig. 9-2); however, the same step-by-step approach can be ap- ADVANTAGES plied to other teeth (Fig. 9-3). As with all tooth The metal-ceramic restoration combines, to a large preparations, a systematic and organized approach degree, the strength of cast metal with the esthetics to tooth reduction will save time. of an all-ceramic crown. The underlying principle is to reinforce a brittle, more cosmetically pleasing Armamentarium (Fig. 9-4). The instruments material through support derived from the stronger needed to prepare teeth for a metal-ceramic crown metal substructure. Natural appearance can be include: closely matched by good technique and if desired • Round-tipped rotary diamonds (regular grit through characterization of the restoration with in- for bulk reduction, fine grit for finishing) or ternally or externally applied stains. Retentive qual- carbides
    • Section 2 Clinical Procedures-Part I Fig. 9-2. Preparation of a maxillary incisor for a metal-ceramic crown. A, Heavily restored maxillary central incisor. B and C, Rotary instrument aligned with the cervical one third and incisal two thirds to gauge correct planes of reduction. D and E, Guiding grooves placed in the two planes. The cervical groove is made parallel to the path of withdrawal, which usually coincides with the long axis of the tooth. The incisal depth groove is prepared parallel to the facial contour of the tooth. F and G, Incisal guiding grooves are placed. H, Incisal edge reduction. I to K, Facial reduction accomplished in two planes. L, Breaking proximal contact, maintaining a lip of enamel to protect the adjacent tooth from in- advertent damage. M and N, Proximal reduction. O, Placing a 0.5-mm lingual chamfer.
    • Chapter 9 The Metal-Ceramic Crown Preparation R Fig. 9-2, cont'd. P, A football-shaped diamond is rec- ommended for lingual reduction of anterior teeth. Alternatively, a wheel-shaped diamond may be used. Q to S, Finishing the prepa- ration with a fine-grit diamond. T, The completed preparation. Fig. 9-3. Preparation of a maxillary premolar for a metal-ceramic crown. A, Depth holes. B, Occlusal depth cuts. C, Half of the occlusal reduction is completed. D, Occlusal reduction is complete. Guiding grooves are placed for axial reduction. E and F, Lingual chamfer and facial shoulder are prepared on half the tooth. G, Completed preparation. (A to E, Lingual view; F and G, buccal view.)
    • Section 2 Clinical Procedures-Part I Fig. 9-4. Armamentarium for the metal-ceramic crown preparation. Football- or wheel-shaped diamond (for lin- gual reduction of anterior teeth) Fig. 9-5. Depth grooves in the facial wall are placed in Flat-ended, tapered diamond (for shoulder two directions: incisally, parallel to the tooth contour; cervi- preparation) cally, parallel to the path of withdrawal. The grooves Finishing stones should be 1.3 mm deep. Explorer and periodontal probe Hatchet and chisel The actual sequence of steps can be varied prove retention on a tooth with little cingu- slightly depending on operator preference. lum height. On small teeth it may be advis- able to keep the cervical grooves somewhat Step-By-Step Procedure. The preparation is shallower near the margin. divided into five major steps: guiding grooves, in- 3. Place three depth grooves (about 1.8 mm cisal or occlusal reduction, labial or buccal reduc- deep) in the incisal edge of an anterior tooth. tion in the area to be veneered with porcelain, axial This will provide the needed reduction of 2 reduction of the proximal and lingual surfaces, and mm and allow finishing (see Fig. 9-2, F and final finishing of all prepared surfaces. G). Verify the depth of these grooves can be verified with a periodontal probe. On poste- Guiding Grooves rior teeth where the occlusion is to be estab- 1. Place three depth grooves (Fig. 9-5), one in lished in porcelain, 2 mm of clearance must the center of the facial surface and one each exist. If the occlusion is to be established in in the approximate locations of the mesiofa metal, the same minimum clearances are cial and distofacial line angles (see Fig. 9-2, A needed as for a complete cast crown. Poste- to E). These will be in two planes: the cervical rior occlusal reduction incorporates a func- portion to parallel the long axis of the tooth, tional cusp bevel on the lingual cusp, similar the incisal (occlusal) portion to follow the to that for a complete cast crown. When ini- normal facial contour (see Fig. 9-2, D and E). tially positioning the diamond for anterior 2. Perform the facial reduction in the cervical teeth, it may be helpful to observe the long and incisal planes. The cervical plane will de- axis of the opposing tooth in the intercuspal termine the path of withdrawal of the com- position and to orient the instrument perpen- pleted restoration. The incisal or occlusal dicular to that (Fig. 9-6). The grooves must plane will provide the space needed for the not be too deep; otherwise, an overreduced porcelain veneer; it should be approximately and undulating surface will result. 1.3 mm deep to allow for additional reduc- tion during finishing. The incisal grooves Incisal (Occlusal) Reduction. The completed usually extend halfway down the facial sur- reduction of the incisal edge on an anterior tooth face, although (depending on the shape of the should allow 2 mm for adequate material thickness tooth) they may extend to include the incisal to permit translucency in the completed restoration. two thirds. Cervical grooves are generally Posterior teeth generally require less (1.5 mm) be- made parallel to the long axis of the tooth. cause esthetics is not as critical. Caution must be However, they can be adjusted slightly to cre- used, however, because excessive occlusal reduc- ate a more desirable path of withdrawal; in tion shortens the axial walls and thus is a common particular, some labial inclination will im- cause of inadequate retention and resistance form in
    • Chapter 9 The Metal-Ceramic Crown Prepar ation Fig. 9-6. A, Depth grooves 1.8 mm deep placed in the incisal edges to ensure adequate and even re- duction. B, Incisal reduction completed on the left central and lateral incisors. Note the angulation of the diamond, perpendicular to the direction of loading by the mandibular anterior teeth. the completed preparation. This can be particularly problematic on anterior teeth (where as a conse- quence of tooth form, most of the retention is de- rived from the proximal walls). 4. Remove the islands of remaining tooth struc- ture. On anterior teeth, access is usually un- A restricted, and the thickest portion of the cut ting instrument can be used to maximize cutting efficiency (see Fig. 9-2, H). On poste- rior teeth, the same pattern is followed as in preparing depth grooves for a complete cast crown (see Chapter 8). This will include the use of a centric cusp bevel, although addi- tional occlusal reduction will be needed where the porcelain is to be applied (see Fig. 9-3, A to C. Labial (Buccal) Reduction. When completed, the reduction of the facial surface should have pro- B C duced sufficient space to accommodate the metal substructure and porcelain veneer. A minimum of 1.2 mm is necessary to permit the ceramist to produce a restoration with satisfactory appearance (1.5 mm is preferable). This requires significant tooth reduction. For comparison, the cervical diameter of a maxillary central incisor averages between 6 and 7 mm. Fig. 9-7. A, The cervical shoulder is established as the In the cervical area of small teeth, obtaining opti- tooth structure between the depth grooves is removed. The mal reduction is not always feasible (see Fig. 7-4.) rotary instrument is moved parallel to the intended path of withdrawal during this procedure. B, The facial reduction Often a compromise is made with lesser reduction should be completed in two phases, initially maintaining in the area where the cervical shoulder margin is one half intact for assessment of the adequacy of reduction. prepared. Note the two distinct planes of reduction on the facial. The 5. Remove the remaining tooth structure be- proximal aspect parallels the cervical reduction on the fa- tween depth grooves (see Fig. 9-2,1 to L), cre- cial wall. C, Facial reduction completed. A 6-degree taper ating a shoulder at the cervical margin (Fig. has been established between the proximal walls. 9-7). If a restoration with a narrow subgingi- val metal collar is to be fabricated and suffi- cient sulcular depth is present, place the preparation, because a significant amount of shoulder approximately 0.5 mm apical to the tooth structure is being removed and copious crest of the free gingiva at this time. Addi- irrigation (along with intermittent strokes) tional finishing will then result in a margin will expedite the preparation process. Such a that is 0.75 to 1 mm subgingival. Use ade- cautious approach will prevent unnecessary quate water spray during the entire phase of trauma to the pulp. The resulting shoulder
    • Section 2 Clinical Procedures-Part I should be approximately 1 mm wide and use of supragingival margins to posterior teeth (Fig. should extend well into the proximal embra- 9-9) and to un-discolored anterior teeth (in which sures when viewed from the incisal (occlusal) case a porcelain labial margin is preferred; see Chap- side (Fig. 9-8). Where access permits, estab- ter 24). The optimum location of the margin should lishing this shoulder from the proximal gin- be carefully determined with the full cooperation of gival crest toward the middle of the facial the patient. Where a subgingival margin is to be wall is preferred. This will minimize place- placed, careful tissue manipulation is essential; oth- ment of the initial shoulder preparation too erwise, there will be damage that leads to permanent close to the epithelial attachment. If the mar- gingival recession and subsequent exposure of gin is established from facial to proximal, a the metal collar. This is most effectively avoided tendency exists to "bury" the instrument and through meticulous gingival displacement with a encroach on the epithelial attachment. A con- cord before finishing (Fig. 9-10). The configuration of scious effort to maintain proper margin posi- the margin is also finalized at this time (Fig. 9-11). tion relative to the crest of the free gingiva is critical (see Fig. 7-49). The location and spe- Axial Reduction of the Proximal and Lingual cific configuration of the facial margin de- Surfaces. (see Fig. 9-2, M to P). Sufficient tooth pend on several factors: the type of metal- structure must be removed to provide a distinct, ceramic restoration selected, the cosmetic smooth chamfer of about 0.5 mm width. expectations of the patient, and operator 6. Reduce the proximoaxial and linguoaxial sur- preference. faces with the diamond held parallel to the in- From a periodontal point of view, a supragingival tended path of withdrawal of the restoration. margin is always preferred. Its application is re- These walls should converge slightly from stricted, however, because patients often object to a cervical to incisal or occlusal. A taper of ap- visible metal collar or discolored root surface. Such proximately 6 degrees is recommended. On objections are common, even when the gingival anterior teeth, a lingual concavity is prepared margin is not visible during normal function, as in for adequate clearance for the restorative ma- patients with a low lip line. This generally limits the terial(s). Typically, 1 mm is required if the centric contacts in the completed restoration are to be located on metal. When contact is on porcelain, additional reduction will be neces- sary. For anterior teeth, usually only one groove is placed, in the center of the lingual surface. For molars, three grooves can be placed in a manner similar to that described for the all-metal complete cast crown. Fig. 9-8. A, The facial shoulder preparation should wrap around into the interproximal embrasure and extend at least 1 mm lingual to the proximal contact. B, The shoulder Fig. 9-9. Supragingival margins on the maxillary premo- preparation extends adequately to the lingual side of the lars. They were possible because of a favorable lip line hid- proximal contact. Note that on the mesial (visible) side, the ing the cervical aspect of these posterior teeth. The subgin- preparation extends slightly farther than on the distal (cos- gival margins on the mandibular premolars were prepared metically less critical) side. only because of previously existing restorations.
    • Chapter 9 The Metal-Ceramic Crown Preparation 7. Make a lingual alignment groove by posi- 8. As the lingual chamfer is developed, extend tioning the diamond parallel to the cervical it buccally into the proximal to blend with the plane of the facial reduction. When the interproximal shoulder placed earlier (Fig. round-tipped diamond of appropriate size 9-12). Alternatively, a facial approach may be and shape is aligned properly, it will be al- used. Although this is slightly more difficult most halfway submerged into tooth struc- initially, after some practice it should be easy ture. Verify the alignment of the groove, and to eliminate the lingual guiding groove and carry the axial reduction from the groove to perform the proximal and lingual axial re- along the lingual surface into the proximal; duction in one step; however, this requires maintain the originally selected alignment of that the diamond be held freehand parallel to the diamond at all times. the path of withdrawal. The proximal flange Fig. 9-10. A, A gingival displacement cord (under tension) is placed in the interproximal sulcus. B, A second instrument can be used to prevent it from rebounding from the sulcus after it has been packed. A B C D Fig. 9-11. A, After tissue displacement, the facial margin is extended apically. Caution is needed, be- cause if the diamond inadvertently grabs the cord, it may be ripped out of the sulcus and traumatize the epithelial attachment. B, Note the additional apical extension of the shoulder on the distal aspect. C, The entire facial shoulder is placed at a level that will be subgingival after the tissue rebounds. D, The facial margin has been prepared to the level of the previously placed cord.
    • Section 2 Clinical Procedures-Part I Fig. 9-12. A lingual chamfer is prepared to allow ade- quate space for metal. A smooth transition from interproxi- mal shoulder to chamfer is essential. that resulted from the shoulder preparation can be used as a reference for judging align- Fig. 9-13. A, Proximal reduction of the flange with a fa- ment of the rotary instrument (Fig. 9-13). The cial approach. B, Once sufficient tooth structure has been interproximal margin should not be inadver- removed, the cervical chamfer is prepared simultaneously tently placed too far gingivally and thereby with the lingual axial surface. After the distolingual prepa- infringe on the attachment apparatus. It must ration has been completed, the mesial chamfer is blended follow the soft tissue contour (see p. 150). On into a smooth transition with the shoulder. posterior teeth, the lingual wall reduction blends into the functional cusp bevel placed during the occlusal reduction. Anterior teeth require an additional step: After preparation of the cingulum wall, one or more depth grooves are placed in the lingual surface. These are approximately 1 mm deep. 9. Use a football-shaped diamond to reduce the lingual surface of anterior teeth (see Fig. 9-2, P). It is helpful to stop when half this re duction has been completed to evaluate clearance in the intercuspal position and all excursions. The remaining intact tooth struc- ture can serve as a reference. Fig. 9-14. Controlled tissue displacement can be helpful when finishing the margin with a fine-grit diamond or an- Finishing. The margin must provide distinct other rotary instrument. resistance to vertical displacement of an explorer tip, and it must be smooth and continuous circum- ferentially. (A properly finished margin should feel finishing steps for the facial margin depend like smooth glass slab.) All other line angles should on the design of margin chosen (see Table be rounded, and the completed preparation should 7-2 and Fig. 9-15). A porcelain labial margin have a satin finish free from obvious diamond requires proper support for the porcelain. A scratch marks. Tissue displacement is particularly shoulder with a 90-degree cavosurface angle helpful when finishing subgingival margins (Fig. is recommended. This type of shoulder can 9-14). Sometimes this step is postponed until just also be used for a crown with a conventional before impression making after tissue displacement. metal collar and offers the advantage of al- 10. Finish the margins with diamonds, hand in- lowing the collar to be kept narrow. How- struments, or carbides (see Fig. 9-2, Q and ever, there is then the risk of leaving unsup- R). All internal line angles should be ra- ported enamel. For this reason, the margin diused to facilitate the impression-making is often beveled or sloped to create a more and die-pouring steps (see Fig. 9-2, S). The obtuse cavosurface angle (Fig. 9-16). A
    • Chapter 9 The Metal-Ceramic Crown Preparation Fig. 9-17. The shoulder bevel. Fig. 9-15. A, Completed preparation. Note that the tran- sition from incisal to axial walls is rounded, and a distinct 90-degree or slightly sloping shoulder has been established. B, Even chamfer width and a smooth transition between lingual and axial surfaces. The chamfer is distinct and blends smoothly into the facial shoulder. Fig. 9-16. A, 90-degree shoulder. B, 120-degree shoul- der. C, Shoulder bevel. flat-ended diamond in a low-speed hand- piece creates the 90-degree shoulder. Any Fig. 9-18. A, Facial and B, lingual views of metal- unsupported enamel must be removed sub- ceramic preparations. sequently by careful planing with a sharp chisel. Care must also be taken to orient the rotary instrument as it moves around the bur or hand instrument, depending on the tooth if inadvertent undercuts are to be length of bevel required (Fig. 9-17). Gener- avoided. When a metal-collar design of ce- ally a short bevel with a cavosurface angle ramic restoration is planned, the need for a of 135 degrees is advocated, although longer 90-degree shoulder is less critical. A sloping bevels have been recommended for im- shoulder has been advocated to ensure the proved marginal fit. Special care must be ex- elimination of unsupported enamel and to erted where the bevel meets the interproxi- minimize marginal gap width (see Chapter mal chamfer. The chamfer and bevel should 7). Such a shoulder (cavosurface angle of be continuous with each other. Care must be about 120 degrees) can be accomplished taken not to damage the epithelial attach- with a flat-ended diamond by changing its ment during beveling; tissue displacement alignment, paying particular attention to the before preparation of subgingival bevels is configuration of the tooth structure cervical recommended. to the margin. Alternatively, a hatchet can be 11. After a satisfactory facial margin has been used to plane the margin to the correct an- obtained, round all sharp line angles within gulation. Again, be careful to avoid under- the preparation (see Fig. 9-2, S). This will fa cutting the axial wall of the preparation cilitate surface wetting and expedite subse- where it meets the shoulder during finish- quent procedures (impression making, ing. A shoulder-bevel margin is most effec- pouring of casts, waxing, and investing). A tively achieved with a flame-shaped carbide fine-grit diamond operating at low speed is
    • Section 2 Clinical Procedures-Part I particularly useful. However, where access move any sharp transitions (see Figs. 9-2, T; allows, a slightly larger tapered diamond 9-18; and 9-19). may be preferred because the greater diam- eter of its tip prevents "ditching" of the Evaluation. Areas often missed during finish- chamfer. Blend all surfaces together, and re- ing are the incisal edges of anterior preparations and the transition from occlusal to axial wall of pos- terior preparations. The completed chamfer should provide 0.5 mm of space for the restoration at the margin. The chamfer must be smooth and continu- ous, and when evaluated, a distinct resistance to vertical displacement of the tip of an explorer or pe- riodontal probe should be felt. The chamfer should be continuous with the interproximal shoulder or beveled shoulder. The cavosurface angle of the chamfer should be slightly obtuse or 90 degrees. Under no circumstances should any unsupported tooth structure remain, especially at the facial mar- gin. Care is also needed to avoid creating an under- cut between the facial and lingual walls. This aspect of the preparation should be thoroughly evaluated. Fig. 9-19. The "wingless" variation does not exhibit the Excessive convergence should also be avoided, be- defined transition from chamfer to shoulder seen in Fig. 9-15. Rather, the shoulder gradually narrows toward the cause this may lead to pulpal exposure. All residual lingual side. Interproximally, the same criteria for mini- debris is removed with thorough irrigation. (Vari- mum extension of the shoulder apply as for the wing-type ous examples of metal-ceramic preparations are or flange preparation. shown in Figs. 9-20 and 9-21.) Fig. 9-20. Metal-ceramic crowns used to restore maxillary incisor teeth.
    • Chapter 9 The Metal-Ceramic Crown Preparation A B C D A, Metal-ceramic preparations on the maxillary premolars in conjunction with more con- servative preparations on the molars. B, Buccal view of the preparations. Note that, by comparison, con- siderable tooth reduction was needed on the premolars to accommodate metal-ceramic restorations. C, Except for the molars, all remaining teeth in this patient have been prepared for metal-ceramic restorations. Note the subtle variations and modifications of the same underlying theme: wing-type preparations on the anterior teeth, wingless on the premolars. D, Mandibular arch of the same patient. Many of the smaller mandibular teeth were prepared with wingless restorations. Because of previously existing restorations, excessively heavy shoulderlike chamfers resulted on some of the posterior teeth.
    • Section 2 Clinical Procedures-Part I
    • Chapter 9 The Metal-Ceramic Crown Preparation