Esthetic Dentistry

Complete-crown and partial-coverage tooth preparation designsfor bonded cast ceramic restorationsStephen P. Broderson"^

A new concept for tooth preparation design for complete- and partial-coverage all-eeramierestorations is presented. Because ofthe efficacy of third-generation dentinal bonding agents,the preparations for complete-coverage and partial-coverage restorations can be made withless emphasis on retentive form. For partial-coverage restorations, cavosurface angles shotddbe large so that the resultant configuration ofthe enamei rods is conducive to optimal bond-ing. The new preparations are simple, with extreinely tapered axial walls, to allow maximumthiekness ofthe ceiamic material. These types of preparations result in fmished restorationsthat are stronger and have better margins and less chance of microleakage. Long-term successof these types of restorations will be determined by the success or failure ofthe dentinal bond-ing agent and resin cement system used. (Quintessence Int 1994;25:535-539.)

Introduction

The concept of dentin-enamel bonding has revolution-ized restorative dentistry. Many third-generation den-tinal bonding agents are available to bond ceramic res-torations. These bonding agents, when combined withresin cements, have bond strengths to dentin that ap-proach those to enamel.

Etched porcelain and Dicor {Dentspiy Internation-al) are available to provide conservative esthetic resto-rations.'"* Several articles have provided recommenda-tions for tooth preparation design for all-ceramic resto-rations. The consensus seems to be that all-porcelainrestorations require absolutely precise work, more sothan any other type of prosthetic restoration. '

In this article the preparation designs currently advo-cated for complete- and partial-coverage all-ceramicrestorations will be reviewed. Changes in the basic de-sign concept will be recommended to enable the dentistand laboratory to construct all-ceramic restorationsthat will fit more precisely and be fabricated more effi-

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ciently. Although special emphasis will be placed onDicor restorations, the preparation design for any all-ceramic restoration is the most important factor in thelong-term success of the restoration, regardless of theceramic material utilized. It has been my experiencethat the success rate of restorations bonded to prepara-tions described in this paper is much higher than that ofrestorations bonded to preparations utilizing conven-tional retention form. Long-term success also dependson the strength of the dentinal bonding agent.

Com pie te-coverage all-ceramic restorations

Current concepts of tooth preparation for all-ceramicrestorations have evolved from the need for retentionform—because of the limitations of the types of ce-ments available (zinc phosphate, polycarboxylate, andglass-ionomer)—and from the technical mechanicalneeds for the fabrication of the original porcelain jack-ets.

Dodge et aP reported that the ideal convergenceangle for cast metal complete crowns is 16 degrees.Their castings were cemented with zinc phosphate ce-ment. Dodge et aP found that, as parallelism decreases,castings seat moTe completely and retention decreases.However, Dodge et aF also stated that in the mastica-tory process and in oral habits there are compressive

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Esthetic Dentistry

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Fig 1 Anterior and posterior com-plete-crown preparations. Com-piete-crown preparations shouldhave a cavosurface angle ot 120 to140 degrees and a convergenceangle of greater than 20 degrees.The axiogingivai internai line angleshould be rounded and as smoothas possible.

forces with oblique and apical components that tend totip or unseat the casting. The occurrence of pure tensileforces is infrequent. In real clinical situations, severalauthors have found the average convergence angles tobe 20 to 25 degrees.

Hung et al" compared Cerestore (Coors Biomédi-cal), Dicor, and porcelain-fuscd-to-metal (PFM)crowns. They concluded that marginal openings in-crease in size after cementation and after thermocy-cling, and that the PFM crowns have a significantly bet-ter marginal fit than do the Dicor or Cerestore crowns.The PFM crowns were prepared with a 45-degree cir-cumferentiai bevel on a shoulder, and the Dicor andCerestore crown preparations had 90-degree shoul-ders. Convergence angles for the preparations were notdiscussed. The finished crowns were cemented withzinc phosphate cement. Hung et aP reported that thetwo major factors resulting in acceptable margins arecement film thickness and margin design. When thereis less hydrostatic cement pressure" the castings seatmore completely.

Leempoel et al'' showed that preparation design, ce-ment utilized, and laboratory quality are the most im-portant factors influencing the retention of a castingand that occlusal convergence angles of 15,5 to 30.2 de-grees provide sufficient retention for a complete castmetal crown.

The restorative process for all-ceramic restorationsis technique sensitive and the utmost care must be tak-en to develop a cavity preparation that makes use ofthe strengths and minimizes the weaknesses of the ce-

ramic material.-' Ease of laboratory fabrtcation shouldalso be a primary consideration for the tooth prepara-tion.

Hobo and Iwata'', in introducing Cerapearl (Kyo-cera), described the ideal preparation as having 2.0-mm occlusal reduction, 1.5-mm axial reduction, and a1.2-mm shoulder with smooth internal hne angles andno bevels. Malamed,'" preparing for a complete Dicorcrown, described a marginal design with a 135-degreechamfer with a cavosurface junction of 90 degrees. Thedepth ofthe shoulder should be 1.2 to 1,5 mm. Henselet al" described the ideal margin for a complete Dicorcrown as having a 1,2-mm shoulder perpendicular tothe axial surface and occlusal reductions of 1.5 to 2,0mm. None of these authors discussed convergenceangles.

New preparation design

It is this author's opinion that the availability of third-generation dentinal bonding agents has had a major ef-fect on the preparation design; that is, retention formcan be less emphasized. Complete-crown preparationswith greater convergence angles, which allow greaterseating of the restorations, are recommended; thiscloser fit results in a higher compressive strength,'̂Sharp internal or external line angles and preparationmargins with sharp corners should be avoided. Becausethe casting must be seated with passive pressure, thepreparation design must have a greater convergencethan is necessary for gold restorations. It is imperative

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Fig 2 The six maxillary anterior teeth have been restoredwith Dicor Plus compiete-crown restorations. Note the con-servative tooth reduction at tbe marginal area, the roundedexternal and internal iine angles, and tbe convergenceangles.

Fig 3 Design for an onlay type of preparation as recom-mended in tbe Dicor Ciinical Procedures Manual, This typeof preparation creates too many stress points in the ceramicrestoration.

that the preparation be as smooth as possible, so thatoptimal accuracy in impressioning, waxing {in the caseof Dicor), and die duplication can be obtained.

This author's experience with Dicor over the past 8years has led to modifications in the complete-crownpreparation for cast ceramic restorations (Fig 1):

1. The occlusal convergence angle should be no lessthan 20 degrees. This allows for better seating of therestoration.

2. Al! line angles should have exaggerated rounding.Sharp shoulders should be eliminated, because theycan result in stress points in the restoration. It is alsovery important to smooth and round all axio-occlu-sal line angles. For the fabrication of castings onsharp shoulder preparations with near-parallel axialwalls, it is usually necessary to relieve the inside ofthe shoulder area of the casting to allow completeseating of the casting on the die. This leads to weak-ness in the cemented casting,

3. The marginal configuration should be a chamfer ofapproximately 120 to 140 degrees with a roundedinternal line angle to the axial wall.

4. The occlusal reduction should be at least 2.0 tnm.The finished occlusal surface of the preparation onpremolars and molars should have a V shape mesio-distally, similar to the finished intercuspal angle de-sired in the final restoration, to enable the laborat-ory to develop optimum occlusal anatomy

The ultimate strength of the cast ceramic material isdependent on its thickness. Tlie material responds to

stress in accordance with the engineering Law ofBeams (Dicor Laboratory Techniques Course, York,PA. Dentsply International, 1984), which states thatdoubling the thickness of the material increases itsstrength fourfold. Creating a tooth preparation withlarge convergence angles allows greater thickness inthe axial occlusal area of the restoration. The finishedcast ceramic restoration fabricated in accordance withthe above-mentioned guidelines will be stronger andhave better marginal fit (Fig 2).

Partía I-coverage all-ceramic restorations

The partial-coverage bonded cast ceramic restorationhas many advantages over conventional types of rest-orations (cast gold and porcelain-fused-to-metalcrowns). Maxillary and mandibular posterior teeth cannow be restored with conservative tooth reduction,providing the pahents with the esthetics they demand.No longer does the dentist have to make complete-coverage ceramometal restorations to satisfy esthetics.Preparation design is the secret to success in utilizingpartial-coverage all-ce ramie restorations.

Jackson and Ferguson^ described the ideal inlay-on-lay preparation as having walls flared 11 to 15 degrees,rounded internal line angles, rounded proximal boxeswith a chamfered margin and no bevels, and ocelnsalreduction of at least 1,5 mm.

The Dicor Chnical Procedures Manual" shows an on-lay-type preparation to be very similar to that for a castgold restoration, with rounded line angles and no bev-els (Fig 3), This type of preparation uses classic reten-

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Buccal and linijua! cavostirface angles onmanditiular molars and premolars 130°

Gingival eavosurfaceangle !2ü"-140°

Bueeal eavosurfaee angles onmaxillary molars and premolars SO'-âO"

Fig 4 Complete-crown preparations for maxillary and mandiublar premolars and molars. All line angles are rounded. The ca-vosurface angle is 130 degrees on the buccal and lingual of surfaces mandibular of premclars or molars, 150 degrees on theinterproximal aspect of all posterior teeth, and 50 to 60 degrees on the buccal surface of maxillary molars and premolars. Thisleaves an optimum amount of enamel with properly oriented enamel rods for the best bonding.

tion form with complex line angles and boxes, that, inmy opinion, are unnecessary. Tliese features also pro-vide stress points in the casi ceramic, which may lead tobreakage. It ¡s the author's experience that Dicor resto-rations fabricated for this type of preparation design,even when acid etched and dentin bonded, have a highincidence of breakage.

New preparation design (Fig 4)

1. The preparation should have as much enamel tobond to as possible.

2. The occlusal reduction should be at least 2 mm to al-low optimal occlusal anatomy and strength.

3. The eavosurface angle should be nearly 130 degreeson the bucea! and lingual surfaces and nearly 150 de-grees interproximally. All internal line anglesshould be rounded, and remnants of the box fromprevious preparation should be eliminated as muchas possible. The preparation will have a horizontalflat appearance with very little retention form.

4. There should be no occlusal function at the margins.5. The preparation should have smooth, rounded cor-

ners and line angles, and the margin should be a con-tinuous, smooth flowing line.

6. For maxillary molars and premolars, the buccaleavosurface angle should be 50 to 60 degrees.

Rationale for preparation design changes

The continued development of third-generation denti-

nal bonding agents should enable preparations withless retention form. Cast ceramic material has higherstrength when all line angles are grossly rounded. Thistype of preparation leads to a casting that is more easilyseated with a thinner cement interface; therefore a res-toration that is superior in both fit and strength isachieved. In a partial-coverage preparation, exposingas mueh enamel as possible for bonding, especiallyinterproximally, results in a finished bonded restora-tion with minimal chance for mieroleakage. Figure 5,views of onlays placed on maxillary right premnlarsand first molar, reveals how much enamei is availablefor bonding, even interproximally. Note the smooth-ness of the preparations, the lack of retention form, andthe flowing margins.

It was shown by Retief et al" that severe microieak-age occurs in sealant-lined butt-joint restorations inetched cavities. They also found that bonding to sur-faces of longitudinally cut etched enamel prisms ob-tained in butt-joint preparations is not as strong asbonding to cross-cut etched enamel surfaces obtainedin beveled preparations. Cheung,''* in a scanning elec-tron microscopic investigation of acid-etched cervicalmargins of Class II cavities, found that beveling resultsin an oblique orientation of the enamel rods, resultingin a better pattern for bonding after acid conditioning.Cheung''' concluded that beveled eavosurface marginsresult in reduced mieroleakage.

The horizontally oriented long bevel described forthe cast ceramic preparation results in the same orien-tation of enamel rods. Tliis author's clinical experience

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Figs 5a and 5b Dicor onlay restorations on the maxillary right first and second premoiars and first molar. The etched prepara-tions show how much enamel is available for bonding. The buccai margins are exceiient.

Fig 5c Oniay-complete-crown preparation. Note thetapered chamfer on the mesiolingual aspect, the roundedmesio-axio-occlusal line angle, and the smooth-fiowingcavosurface margin.

has been that the laboratory fabrication becomes much

easier and that the casting has a better clinical fit than

that obtained with a shoulder type of preparation.

Summary

Concepts for preparation of complete- and partial-

coverage cast ceramic restorations were reviewed.

Recotnmendations for changes from more conven-

tional types of retention form were presented. It has

been my clinical experience that these changes will re-

sult in faster preparation, more accurate duplication of

the preparation, easier laboratory fabrication, and a

stronger cemented finished restoration. These recom-

mendations can be utilized with any type of all-ceramic

restoration. Ultimately, the long-term success of these

restorations will be determined by the success or fail-

ure oí the dentinal bonding agent and resin cement

system utilized. To the present time, however, this

system of preparation design has been successful clin-

ically.

References

1. Shaffer H. Zobler C. Complete restoration witti resin bondedporcelain inlays. Quintessence Int 1991;22:87-!)3.

2. HungSH.etal. Marginal fit of porcelain fused to melaJ and twotypes of ceramic crowns. J Prosthel Dent 1990;63:26-31.

."i. Cavel W,et al. A pilot study of (lie clinical evaluation of castableceramic inlays and a dual cure cement. Quintessence Int1988:19:257-262.

4. Hobo S. Iwata I. Castable apatite ceramic as a new biocompat-ible resiorative materials. Quintessence Int 1985;16:207-216.

5. Jackson KD, Ferguson RW. An esltietic, bonded inlay-onlaytechnique for posterior te et ti. Quinlessence Int 1990;21:7-12.

6. Dicor Ctinicat Procedures Manual. York, PA' Dantsply Inler-national, 1984.

7. Dodge WW, et al. Tbe effect of convergence angle on retentionand resistance form. Quinlessence Int 1985;16:191-194.

8. GaviMs JR, et al. The effect of various finish line preparationson the marginal seal and occlusal seat of full crown prepara-tions. J Prosthet Denl 19S1;45:13K-145

9. Leempoel PJB. et al. The convergence angle of tooth prepara-tions for complete crowns. J Prosthet Dent 1987;58:414-416.

10. Malamed KA. Considerations in posterior glass-ceramic res-toraiions. Int J Periodont Rest Dent 1988:8(4) ;33^9.

11. Hensel W, ct al. A clinical case report utilizing a eastable ce-ramic crown system. Quintessence Int 1987;18:323-334.

12. BrukI CE, Philip GK. The fil of molded all ceramic, twin foil,and conventional ceramic crowns. J Prosthet Dent 1987;58:408-413.

13. Relief DH, et al. Surface topography ot the enamel margins ofbutt and beveled class V preparations. J Prosthet Dent1982;48:]66-17O.

14. Cheung G. A scanning electron microscopic investigation oí theacid-etched cervical margin of Class II cavities. Quintessence

9-M2. •

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