Crowns

The Role of All-Ceramic Crowns in Contemporary Restorative Dentistry

Terry E. Donovan, DDS, and George C. Cho, DDS

Authors

Terry E. Donovan, DDS, is a professor and co-director of Advanced Education in Prosthodontics at the University of the Southern California School of Dentistry.

George C. Cho, DDS, is an associate professor and clinical director of Advanced Education in Prosthodontics and director of Pre-doctoral Implant Dentistry at USC School of Dentistry.

Disclosure: The authors have no monetary interest whatsoever in any of the products mentioned in this article.

Copyright 2003 Journal of the California Dental Association.

A large number of all-ceramic alternatives to metal-ceramic restorations are being marketed aggressively to the dental profession. Most of these all-ceramic alternatives have little or no scientific evidence to support their use. This article present guidelines for clinicians to analyze new ceramic systems and specific indications and contraindications for use. It is suggested that before practitioners consider using an extensively using an all-ceramic system, they make sure that published clinical trials document a survival rate of at least 95 percent over five years. In addition, each system should be evaluated to determine whether it delivers the esthetic results anticipated.

The past decade has seen an unprecedented introduction of myriad all-ceramic crown systems. The introduction of many of these systems has been accompanied by an equally unprecedented blizzard of marketing activity proclaiming the benefits of these restorations. At most major dental meetings, superb clinicians dazzle their audiences with slides of beautiful all-ceramic restorations that rival nature in appearance and restore the smiles of their grateful patients.

The result of these converging activities is that the practicing dentist is often confused by the countless claims and counterclaims of various laboratories, manufacturers, and clinicians. This article has been written in an attempt to sort out fact from fiction in the area of all-ceramic restorations and to provide a philosophical matrix to assist clinicians in their choices for esthetic crown restorations.

It is important to understand that metal-ceramic crowns continue to be the gold standard for complete-coverage restorations. With proper tooth preparation, margin geometry and soft-tissue management, extremely esthetic and functional restorations can be fabricated; and these restorations will provide the best longevity of the esthetic alternatives.1-7 With the use of full-contour wax patterns with controlled cut-back techniques and proper alloy selection, the fracture rate can be close to zero. The use of porcelain labial margins using any one of a variety of techniques will ensure excellent esthetics in the cervical area.8-14 The authors prefer to terminate the metal on the labial or buccal surface about 1 mm from the shoulder margin to permit improved light transmission without compromising strength.15 Although some authorities recommend the use of 360 degree porcelain margins, in the authors’ opinion this creates unnecessary complexity in the laboratory phase while providing minimal improvements in esthetics.

The primary rationale for use of an all-ceramic crown is improved esthetic potential. Given that the potential longevity of all-ceramic crowns is generally less than that of metal-ceramic crowns, the former should be utilized only in those situations where the esthetic result is paramount. The need for exceptional esthetics on molars is rare for the majority of patients; and, because the failure rate is significantly higher on posterior teeth, it would seem prudent to limit use of all-ceramic crowns to anterior teeth (Figures 1 and 2). It is appalling to see complete arch all-ceramic restorations in trade journals done in the pecuniary pursuit of “metal-free” dentistry.16 Many of the posterior teeth in some of these articles either do not require restoration or could optimally be restored with a partial veneer restoration without any effect on the esthetic result.

Thus, the primary indication for all-ceramic crowns is single-unit restorations on anterior teeth and first premolars. Fixed partial dentures should not be fabricated with current all-ceramic systems.17 It is technically possible to fabricate all-ceramic fixed partial dentures with many systems, but it appears that the failure rate is relatively high with all of them. If a patient insists on a metal-free fixed partial denture, the clinician should provide that service only if the patient assumes total responsibility should the restoration fail soon after placement. One large dental laboratory reported a 75 percent failure rate of Empress II three-unit fixed partial dentures (Ivoclar North America, Amherst, N.Y.) at three years (Personal communication, David Avery, Drake Dental Laboratory, Charlotte, N.C., June 2002). This material was marketed extensively on its introduction as a material suitable for fixed partial dentures. Recently introduced zirconia-based materials have impressive physical properties, but the reader is cautioned that until controlled clinical trials are conducted, it is impossible to know if those improved physical properties will translate into improved clinical performance.18-20

All-ceramic crowns have the important advantage of potential to provide improved esthetics, but they have several disadvantages compared to metal-ceramic crowns. These include reduced marginal integrity, more-aggressive tooth preparation, potential wear of the opposing dentition, increased technique-sensitivity, and difficulty in dealing with a tooth preparation that varies significantly from the ideal.7

Acceptable marginal integrity can be achieved with most all-ceramic systems and with ceramic margins on metal-ceramic crowns. In spite of manufacturer’s claims of superior marginal integrity with specific all-ceramic systems, several studies have concluded that better fit is obtained with metal margins.21- 24 In spite of their initial promise, most machined margins using various CAD/CAM technologies have not yet provided the superb marginal integrity anticipated. Although much has been written regarding marginal integrity with various systems, it should be noted that it has never been demonstrated that the differences seen in marginal integrity are clinically significant.

Although it is not commonly understood, all-ceramic tooth preparations are more aggressive than their metal-ceramic counterparts. While there is not universal consensus on this issue, in the opinion of the authors, all-ceramic preparations should remove 1.5 mm of tooth structure circumferentially around the tooth, and 2 mm off the occlusal surface of posterior teeth to achieve maximum strength and optimal esthetics (Figures 3 and 4). With metal-ceramic crowns, slightly less reduction is required on the labial or buccal surfaces and significantly less reduction can be accomplished interproximally and on the lingual surfaces. Some manufacturers of all-ceramic systems claim that 1 mm of reduction is adequate, and clearly crowns can be fabricated with such minimal reduction. However, these crowns never meet the optimal esthetic potential that can be achieved with the more aggressive reduction; and it is likely that the strength of the restoration is compromised as well.

One significant disadvantage of all-ceramic crowns is the inability to provide adequate support with non-ideal preparations. With metal-ceramic crowns, a full-contour wax pattern followed by a controlled cut-back technique will provide optimal support for the ceramic veneer, independent of the underlying preparation (Figure 5). Use of this technique also results in predictable esthetics; and, because a uniform layer of porcelain is created, minimal stress is generated at the porcelain/metal interface during cooling of the restoration after firing. This results in improved metal-ceramic bonding. With all-ceramic crowns, the cores are generally milled to create a uniform thickness of about 0.4 mm that conforms to the basic shape of the preparation. Thus, if the preparation is less than ideal (which in the clinical setting is often the rule rather than the exception), optimal support of the veneering porcelain is not provided. Clinically, this often results in chipping of the ceramic veneer off of the internal core. Anecdotally, a rather high incidence of failures of this type has been reported with several of the milled alumina- and zirconia-based systems.

Clinicians have noted many cases in which ceramic crowns have caused excessive wear of opposing dentition (Figures 6 and 7). While wear is a complex phenomenon, and is obviously multifactorial, it is clear that ceramic materials have a greater potential to cause wear than metal. Many manufacturers have claimed that their specific brand of porcelain causes less wear than their competitor’s products, but this has never been demonstrated by clinical data. Laboratory studies on wear are notoriously inadequate in predicting clinical performance and clinicians are cautioned to interpret such data with a healthy level of skepticism. Until good clinical data is available to the contrary, the prudent clinician should assume that when any ceramic material is in repetitive gliding contact with the opposing dentition, it has significant potential to cause pathologic wear. In this regard, metal-ceramic crowns have the obvious advantage that metal contact with the opposing dentition can be developed both in maximum intercuspation and throughout most of the lateral and protrusive excursions (Figure 8). This, of course, must be accomplished with considered use of the appropriate cut-back design.

Once it has been determined that all-ceramic crowns are indicated for a specific patient, a choice must be made among the myriad products available. The clinician should utilize a clear set of criteria to apply to assist in this decision.

Given that the primary indication for use of all-ceramic crowns is improved esthetics, the clinician should analyze available systems in terms of their ability to deliver on that promise. Many all-ceramic systems achieve their improved strength characteristics by virtue of an internal opaque core. These systems will not provide any better esthetic result than that of metal-ceramic restorations and thus should be avoided. Additionally, to achieve maximum esthetic potential, color must be able to be developed internally to mimic tooth structure. With some systems, colorants are painted on the surface and little or no light transmission occurs. Again, the esthetic potential of these systems is limited; and these should also be avoided. Systems should be selected with core materials that permit light transmission and for which the basic color of the restoration is determined internally.

The second important criteria that should be applied when selecting an all-ceramic crown system is that it should be supported by appropriate scientific evidence. Laboratory studies conducted to determine the physical properties or strength of all-ceramic crowns have virtually no value in predicting clinical performance.25-28 All-ceramic crowns fail by propagation of microscopic defects called Griffith’s flaws, or defects created during fabrication or adjustment.29-30 Such defects undergo static fatigue and stress-corrosion in a moist environment, and crack propagation can occur in the absence of excess occlusal stress.31 These facts mandate that manufacturers provide evidence from properly conducted clinical trials before materials are used routinely. It has been suggested that a minimum length of such clinical trials be three years and optimally five years and that the failure rate be no higher than 5 percent.32-33

Based on the preceding discussion, it would seem that the system that best meets these criteria and possesses the strongest evidence base at the time of this writing is the IPS Empress system (Ivoclar North America, Amherst, NY).34 The translucent internal pressed core with this system combines adequate strength characteristics with improved light transmission. Several clinical trials have demonstrated good clinical performance in the short term.35-36 Other systems worth considering are the ProCera AllCeram crown (NobelBiocare USA, Yorba Linda, Calif.) and Inceram Alumina (Vident, Monrovia, Calif.).37-39

Although a limited number of clinical trials have been published in recent years, it is a basic fact that such data is not available to the clinician until many years after a system is introduced to the profession. Clinicians wishing to utilize new systems in the absence of clinical data should proceed with caution. Systems should be analyzed in terms of their ability to provide improved esthetics and their potential for longevity. Experts in the field should be consulted, and a thorough knowledge of the system requirements (preparation design, requirements for bulk reduction, margin geometry, etc.) should be obtained from the manufacturer. It would seem prudent to then use the system (with the appropriate informed consent) in a few patients and then to observe the results before placing large numbers of such restorations. Placing large numbers of essentially experimental restorations is unfair to patients and potentially very expensive for the clinician.

Conclusion

All-ceramic crowns have one advantage and numerous disadvantages. With some systems, crowns can be fabricated that demonstrate superior esthetics to that achieved with metal-ceramic crowns. However, in general, the life span of all-ceramic crowns is shorter, the fit is inferior, tooth preparation is more invasive, and cementation is more difficult. All-ceramic crowns should not be used with less than ideal preparations and may cause excessive wear of opposing tooth structure in some patients.

Thus, the use of all-ceramic crowns should be limited to those situations in which esthetics is of primary importance. They are contraindicated on molars and for fixed partial dentures. All-ceramic systems should be selected on their ability to provide superior esthetics and on the results of controlled clinical trials. It is suggested that a survival rate of 95 percent or better after five years would be necessary to demonstrate success. Using these criteria, IPS Empress (Ivoclar North America, Amherst, N.Y.) would be the current system of choice for use when all-ceramic crowns are indicated.

References

1. Donovan TE, Daftary F, Alternatives to metal-ceramics. J Cal Dent Assoc 16(2):10-7, 1988.

2. Chee WWL, Daftary F, Alternatives to metal-ceramic restorations. J Cal Dent Assoc 15(11):28-32, 1987.

3. Donovan TE, Contemporary ceramic restorations: A comparative analysis. Alpha Omegan 81:57-64, 1988.

4. Cho GC, Donovan TE, The versatile and esthetic metal-ceramic crown. J Cal Dent Assoc 24:39-43, 1996.

5. Donovan TE, Cho GC, Soft tissue management with metal-ceramic and all-ceramic crowns. J Cal Dent Assoc 26(2):107-12, 1998.

6. Donovan TE, Cho GC, Predictable esthetics with metal-ceramic and all-ceramic crowns: the critical importance of soft-tissue management. Periodontology 2000 27:121-30, 2001.

7. Cho GC, Donovan TE, Chee WWL, Rational use of all-ceramic crown systems. J Cal Dent Assoc 26(2):113-20, 1998.

8. Toogood GD, Archibald JF, Technique for establishing porcelain margins. J Prosthet Dent 40:464-6, 1978.

9. Prince J, Donovan TE, Presswood RG, The all-porcelain margin for metal-ceramic crowns: A new concept. J Prosthet Dent 50:793-6, 1983.

10. Prince J, Donovan TE, The esthetic metal-ceramic margin: A comparison of techniques. J Prosthet Dent 51:273-5, 1984.

11. Vryonis P, A simplified approach to the complete porcelain margin. J Prosthet Dent 42:592-3, 1979.

12. Cooney JP, Richter WA, MacIntee MI, Evaluation of ceramic margins for metal-ceramic restorations. J Prosthet Dent 54:1-5, 1985.

13. Kessler JC, Brooks TD, Keenan MP, The direct lift-off technique for constructing porcelain margins. Quintessence Dent Technolog 10:145-50, 1986.

14. Donovan TE, Prince J, An analysis of margin configurations for metal-ceramic crowns. J Prosthet Dent 53:153-7, 1985.

15. O’Boyle KH, Norling BK, et al, An investigation of new metal framework design for metal ceramic restorations. J Prosthet Dent 78:295-301, 1997.

16. Donovan TE, Metal-free dentistry: Consumer generated or marketing hype? J Esthet Rest Dent 14(2):71-3, 2002.

17. McLean JW, High strength ceramics. Quintessence Int 18:97-106, 1987.

18. Luthardt RG, Sandkuhl O, Reitz B, Zirconia-TZP and alumina: Advanced technologies for the manufacturing of single crowns. Eur J Prosthodont Rest Dent 7: 113-9, 1999.

19. McLaren E, White SN, Glass-infiltrated zirconia/alumina-based ceramic for crowns and fixed-partial dentures. Pract Periodont Aesthet Dent 11:985-94, 1999.

20. McLaren E, All-ceramic alternatives to conventional metal-ceramic restorations. Compendium 19:307-26, 1998.

21. Hung SH, et al, Marginal fit of porcelain-fused-to-metal and two types of ceramic crown. J Prosthet Dent 63:26-31, 1990.

22. Morris HF, Department of Veterens Affairs Cooperative Studies Project No. 242. Quantitative and qualitative evaluation of marginal fit of cast ceramic, porcelain shoulder, and cast metal full crown margins. J Prosthet Dent 67:198-204, 1992.

23. Beschnidt SM, Strub JR, Evaluation of the marginal accuracy of different all-ceramic crown systems after simulation in the artificial mouth. J Oral Rehabil 26:582-93, 1999.

24. Boenig KW, Wolf BH, Clinical fit of Porcera AllCeram crowns. J Prosthet Dent 84:419-24, 2000.

25. Kelly JR, Campbell SD, Bowen HK, Fracture surface analysis of dental ceramics. J Prosthet Dent 62:536-41, 1989.

26. Kelly JR, Giordino R, et al, Fracture surface analysis of dental ceramics. Clinically failed restorations. Int J Prosthodont 3:430-40, 1990.

27. Hondrum SO, A review of the strength properties of dental ceramics. J Prosthet Dent 67:859-65, 1992.

28. Kelly JR, Clinically relevant approach to failure testing of all-ceramic restorations. J Prosthet Dent 81:651-61, 1999.

29. McLean JW, Sced LR, The bonded alumina crown. 1. The bonding of platinum to aluminous dental porcelain using tin oxide coatings. Aust Dent J 21:119-27, 1976.

30. Anusavice KJ, Recent developments in restorative dental ceramics. J Amer Dent Assoc 124:72-84, 1993.

31. White SN, Zhao XY, et al, Cyclic mechanical fatigue of a feldspathic dental porcelain. Int J Prosthodont 8:413-20, 1995.

32. Scharer P, All-ceramic crown systems: Clinical research versus observation in supporting claims. Signature Spring Issue:1-2, 1996.

33. Scherrer SS, De Rijk WG, et al, Incidence of fractures and lifetime predictions of all-ceramic crown systems using censored data. Am J Dent 14:72-80, 2001.

34. Wohlwend A, Scharer P, The Empress technique: A new technique for the fabrication of full ceramic crowns, inlays and veneers. Quintessence Int 16:966-78, 1990.

35. Sorensen JA, Choi C, et al, IPS Empress crown system: Three-year clinical trial results. J Cal Dent Assoc 26:130-6, 1998.

36. Lehner C, Studer S, et al, Short-term results of IPS Empress full-porcelain crowns. J Prosthodont 6:20-30, 1997.

37. Oden A, Andersson M, et al, Five-year clinical evaluation of Procera AllCeram crowns. J Prosthet Dent 80:450-6, 1998.

38. McLaren EA, White SN, Survival of In-Ceram crowns in a private practice. A prospective clinical trial. J Prosthet Dent 83:216-22, 2000.

39. Segal BS, Retrospective assessment of 546 all-ceramic anterior and posterior crowns in a general practice. J Prosthet Dent 85:544-50, 2001.

To request a printed copy of this article, please contact/Terry E. Donovan, DDS, 1715 Kaweah Drive, Pasadena, CA 91105-2177.

Legends

Figures 1 and 2. The fracture rate of all-ceramic crowns increases when they are placed on molar teeth.

Figures 3a and b. All-ceramic preparations optimally remove 1.5 mm of tooth structure circumferentially around the tooth.

Figure 4. This diagram illustrates the primary differences between preparations for veneers, and metal-ceramic and all-ceramic crowns. All-ceramic crowns require greater reduction on the lingual and proximal surfaces. A = veneer reduction. B = metal-ceramic reduction. C = all-ceramic reduction.

Figure 5. Use of a full-contour wax-up with a controlled cut-back technique ensures optimal metal support for the ceramic veneer with metal-ceramic restorations.

Figures 6 and 7. These teeth demonstrate excessive wear resulting from gliding contact with porcelain restorations on the opposing teeth.

Figure 8. With metal-ceramic restorations, centric and excursive contacts can often be placed on metal, thus minimizing the risk for excess wear.