Rational Use of Contemporary
All-Ceramic Crown Systems

By George C. Cho, DDS;
Terry E. Donovan, DDS;
and Winston W.L. Chee, BDS

There are a host of contemporary all-ceramic systems available today, as well as improved techniques for metal-ceramic restorations. Perhaps the most important factor in achieving predictable success with these restorations is excellent soft tissue management. Metal-ceramic restorations provide the most predictable service in terms of clinical longevity, versatility and prevention of wear to the opposing dentition.

Article copyright 1998 Journal of the California Dental Association.
Photographs copyright of the authors.

Figure 1. Recession is perhaps the most common esthetic failure with anterior crowns. Figure 2. Inflammation frequently occurs with anterior crowns as a result of placing the margins too deep in the gingival sulcus.

During the past 20 years, several all-ceramic crown systems have been developed and introduced to the profession. Some of these systems have represented significant technological innovations, and some have been evolutionary generations of earlier ceramic systems. All have been developed in an attempt to compensate for some of the inherent deficiencies in metal-ceramic restorations. ^1-4

However, what has not been primarily understood is that most esthetic failures with anterior restorations are a result of inappropriate soft tissue management rather than deficiencies that can directly be attributed to the restorations themselves. ⁵ Such deficiencies in tissue management will result in esthetic failure regardless of the type of restoration utilized (Figures 1 and 2).

Figure 3. By placing color internally on a textured opaque surface, the refraction of light can create a natural appearance. Figure 4. Translucent incisal and body porcelains allow the internal color to predominate.

The metal-ceramic crown has been the predominant restoration of the past two decades. Any discussion on indications and contraindications for all-ceramic crowns must first consider the advantages and disadvantages of metal-ceramic restorations.

There have been two significant developments with metal-ceramic restorations in the past several years that have dramatically improved their inherent esthetic potential. The first is the development of techniques to build color internally within the ceramic veneer and the concomitant development of lateral segmental build-up techniques. The ability to place the color internally and veneer it with more translucent incisal and body porcelains through these layering techniques has added the dimension of depth to metal-ceramic crowns and significantly improved the appearance of these crowns (Figures 3 and 4). Any restoration today that cannot provide such depth of appearance will run the risk of failure due to specular reflection and a high incidence of metameric mismatches.

As seen in Figures 5 and 6, Porcelain margins on metal ceramic crowns can provide excellent fit and esthetics.

The second improvement in metal-ceramic technology has been the development of numerous techniques for simplifying fabrication of all porcelain labial margins (Figures 5 and 6). ^6-12 Some authorities even recommend 360 degree all-ceramic margins to improve light transmission in the cervical third. While a comparison of techniques for fabricating porcelain margins is beyond the scope of this article, it is safe to state that several effective techniques are available to the ceramist today, and all can result in esthetic margins with acceptable marginal integrity. Routine use of such margins for anterior restorations can result in esthetic restorations and eliminate the necessity of placing margins deep into the sulcus to hide them.

Given that metal-ceramic restorations can be fabricated to provide both excellent esthetics and good longevity, this article will compare results with these restorations with a number of contemporary all-ceramic systems.

Such an analysis may provide the reader with important guidelines when making a choice between metal-supported and all-ceramic systems, and also differentiate the all-ceramic systems. In this process, several important principles regarding metal-ceramic and all-ceramic crowns will be delineated and highlighted as a guide to the practicing clinician.

The parameters that will be compared include strength, fit, conservation of tooth structure, wear potential, ease of fabrication, use in multiple unit situations, economic factors, biocompatibility, and esthetic potential.

Strength and Clinical Life Span

While the need for extremely esthetic and natural restorations has become predominant in recent years, in most situations the requirement that such restorations also provide a reasonably predictable long-term clinical Life Span is also paramount. One of the deficiencies of early all-ceramic crowns such as the feldspathic porcelain jacket crown was its lack of such predictability. Many of the newer systems have been marketed heavily on the basis that these crowns are stronger or are reinforced with a core material that will prevent clinical fracture.

Figure 7. The incidence of fracture with all-ceramic crowns has been quite high.

The underlying assumption that strengthened porcelains will result in improved clinical performance simply lacks validity. Ceramic restorations do not fail because they lack strength. Flaw-free glass has been shown to be stronger than stainless steel.13 The real problem with dental ceramics is that with our current techniques it is impossible to produce restorations that are free of microscopic defects, or what are known as Griffith's flaws. ¹⁴ Such flaws will propagate under even minor occlusal loads and will undergo static fatigue. ^15,16 The result is often sudden, catastrophic failure of the restoration under low-stress situations (Figure 7).

Clinicians should be guided in their choice of an all-ceramic system on the basis of controlled clinical trials. Unfortunately, these are only infrequently conducted, and often the data is not available on a system until long after the system has been introduced, been a commercial failure, and has disappeared from the scene. It is apparent that increases in strength, documented by laboratory studies using static loads, are useless in predicting clinical performance.

In general, the clinical trials that have been conducted indicate that most available systems have high failure rates when used on posterior teeth. As a simplistic guide, most studies report a failure rate of 33 percent or higher within three to five years on posterior teeth. Given that the primary indication for an all-ceramic restoration is high esthetic demand, and that this is rarely a problem with posterior restorations, it would seem prudent to avoid placing all-ceramic crowns on posterior teeth.

Many clinical trials were conducted with all-ceramic crowns luted with conventional cements. It is generally accepted today that most all-ceramic crowns should be internally etched and bonded into place with a resin cement. Bonding these restorations results in somewhat greater strength, which should translate into improved clinical service; however, this has not been well-documented. Clinicians should avoid using resin-modified glass ionomer cements with all-ceramic crowns. There have been numerous anecdotal reports of post-cement expansion of such cements, which has reportedly resulted in fracture of the restoration. More research in this area is indicated.

With regards to strength of all-ceramic restorations, three important principles emerge:

• Principle No. 1. Gains in strength of all-ceramic restorations, as documented by laboratory studies, are of no value in determining clinical performance.
  
  
• Principle No. 2. Controlled clinical trials are essential to indicate criteria for clinical success with all-ceramic systems.
  
  
• Principle No. 3. All-ceramic crowns should not be used indiscriminately to restore posterior teeth.
  
  

Figure 8. Many systems demonstrate excellent marginal integrity when used on a cylindrical test die.

• Principle No. 4. No all-ceramic system provides inherently superior marginal integrity. Overall, all-ceramic crowns have slightly poorer marginal integrity than metal-ceramic crowns.

• Principle No. 5. Any all-ceramic crown system that utilizes refractory dies will be technique-sensitive unless each batch of refractory material is carefully calibrated in the laboratory.

A basic tenet of almost all restorative procedures is that it is important to conserve tooth structure. Tooth preparations for metal-ceramic crowns are inherently radical, and teeth are weakened considerably with such preparations. It is important to understand that preparations for current generations of all-ceramic crowns are even more radical.18 With the advent of porcelain laminate veneers that can be successfully bonded to etched enamel, such radical preparations would seem to be headed in the wrong direction.

Figure 9. All-ceramic preparations (B) generally require more tooth reduction on the lingual and interproximal surfaces than preparations for metal-ceramic crowns. (C)

To provide sufficient thickness of ceramic material for strength and esthetics, a uniform circumferential 1.5 mm of reduction is recommended with most all-ceramic systems (Figure 9). It is important that an all-ceramic crown is fabricated with a relatively uniform cross-sectional bulk of porcelain so that minimal stress is built up internally in the restoration as it is heated and cooled during the firing procedures. This means more tooth structure is reduced axially from the interproximal and lingual surfaces with all-ceramic preparations than with metal-ceramic preparations.

• Principle No. 6. All-ceramic crowns, as they are presently conceived, require more tooth reduction and are therefore less conservative than metal-ceramic crowns.
  

Figure 10. All-ceramic crowns are not indicated with non-ideal tooth preparation because of lack of support for the restorative material.

Most tooth preparations in a clinical setting are done because of the presence of significant caries or fractured cusps and because the tooth cannot be restored by more conservative means. This means that many tooth preparations for both metal-ceramic and all-ceramic crowns are considerably less than ideal (Figure 10).

With metal-ceramic crowns, the loss of tooth structure and less than ideal tooth preparations can be compensated for with metal copings (Figure 11).

Figure 11. Metal copings can provide optimum support for the ceramic veneer.

Thus, a uniform amount of ceramic veneer can be applied to the coping and minimal stress will accumulate at the bond. This also will ensure that the porcelain is adequately supported by the metal sub-structure.

With all-ceramic crowns, there is no adequate way to compensate for lack of an ideal tooth preparation. This often means there will be considerable variability in thickness that corresponds to unsupported porcelain in the final restoration that may well result in early clinical fracture of the crown.

• Principle No. 7. With all-ceramic crowns it is difficult, if not impossible, to compensate for lack of an ideal tooth preparation.
  

Figure 12. Porcelain in gliding contact with natural dentition will cause attrition.

When porcelain is in gliding contact with opposing dentition, it will cause accelerated attrition of the opposing teeth (Figure 12). ¹⁹

Figure 13. Design of the cutback and resultant metal framework on anterior crowns can provide optimum compression bonding and reduce wear. Figure 14. Metal occlusal surfaces on posterior restorations require less tooth reduction, permit more precise occlusal contacts, and will prevent wear of the opposing dentition.

This is a major problem when restoring anterior teeth since the occlusal scheme usually advocated is anterior group function with disclusion of the posterior teeth in protrusive and lateral excursions. There is no clinical documentation for the claim that certain porcelain systems do not wear the opposing dentition. There is also no adequate laboratory test that can predict the clinical effect of any system in this regard.

With anterior metal-ceramic crowns, this problem of wear can be addressed with proper design of the metal framework. Generally, centric occlusion contact can be established in metal as well as most of the lateral and protrusive excursions (Figure 13). Metal occlusals are preferred for posterior crowns because less tooth reduction is required, a more precise occlusal pattern can be obtained, and wear of the opposing dentition is prevented (Figure 14).

• Principle No. 8. All types of porcelains will cause attrition of the opposing dentition when in gliding contact with natural teeth.
  

All-ceramic restorations as currently conceived should only be used as single units. There are techniques for fabricating fixed partial dentures with many of the available systems, but clinical results have been very poor and usually limited to three units.

Metal-ceramic restorations can be used as single units, fixed partial dentures, multi-unit fixed splints, or as bonded (Maryland) prostheses. They can be cast in one piece, or either pre- or postsoldered (Figures 15 and 16).

• Principle No. 9. All-ceramic restorations are primarily single-unit crowns.
  

As seen in Figures 15 and 16, Metal-ceramic restorations can be used in simple and complex restorative situations.

One of the most important factors in success with any restorative modality is lack of technique sensitivity. The success of silver amalgam as a restorative material is a prime example. Most all-ceramic systems are technique-sensitive. They often require use of alternative die materials or refractories. ²⁰ Special equipment such as casting machines, investing units, computer-controlled ovens, CAD-CAM, etc. are required; and the laboratory technician is required to learn new procedures and techniques, each of which may have its own learning curve.

In addition, the clinician must provide a very specific tooth preparation for all-ceramic crowns that requires rounded internals without any sharp angles. This may often be subtly different from what the clinician normally prepares for metal-ceramic preparations. Initially, many all-ceramic crown systems experienced unduly high failure rates because the tooth preparations received from clinicians were often inadequate. This commonly occurred because the clinicians had not been properly educated regarding the critical nature of the all-ceramic preparations.

In a global sense, all dentists will continue to place many metal-ceramic restorations, since fixed partial dentures and splinted restorations will continue to be a necessity in patient treatment plans. The clinical and laboratory procedures required for fabrication of esthetic and durable metal-ceramic restorations have been well-researched and utilized for more than 30 years. It would seem a reasonable philosophy to attempt to educate dentists on optimal tooth preparations for metal-ceramic restorations and at the same time ensure that technicians understand how to achieve optimal esthetics with metal-ceramic restorations. In the long run, such an approach may be preferable and may yield improved results, rather than requiring both dentists and technicians to learn and re-learn the subtleties involved with each new ceramic system.

• Principle 10. Optimum results are obtained with standardized procedures and techniques. Any system that is classified as technique-sensitive will have a lack of predictability in the clinical setting.
  

When evaluating laboratory costs for the practitioner, it is difficult to generalize on the differences between metal-ceramic and all-ceramic crowns. However, it is relatively safe to state that the fees for all-ceramic crowns are usually not less than those charged by the average laboratory for metal-ceramic units.

Often dental laboratories incur significant expenses when installing a new all-ceramic system. In an effort to recoup these expenses, a laboratory will usually undertake an aggressive marketing campaign to gain new clients and to maximize the utilization of the new system. This frequently results in both the dentist and laboratory providing less than an optimum service, as both are in the early stages of the learning curve specific for that system. The costs associated with remakes and loss of patient confidence should be considered when analyzing the overall impact of utilizing a new all-ceramic system in the practice.

• Principle No. 11. There are no economic advantages associated with all-ceramic crowns for either the patient or dentist.
  

It has often been stated that certain all-ceramic crowns are more biocompatible than their metal-ceramic counterparts. While this statement has scant documentation, it is true that ceramic materials tend to be less biologically active than metal alloys. Whether this is of any real benefit is questionable, except in the case of documented allergy to a component of a metal alloy.

There are some patients who have real or imagined sensitivity to certain materials. These patients often request certain all-ceramic restorative materials to prevent systemic reactions. The reader is warned that these patients should be treated with great caution. Often, emotional and systemic problems will be attributed to reactions to dental materials that are totally out of the control of the operator.

Patients with sensitivity to metal alloys can be treated with all-ceramic restorations, but they should be informed prior to treatment that fracture incidence is significantly higher with all-ceramic crowns and they will be financially responsible for any necessary remakes.

• Principle No. 12. All-ceramic crowns systems are less biologically active than those using metal alloys. These ceramic crowns may be of use in treating select patients with documented metal allergy. These patients should be informed of the likelihood of an increased fracture rate.
  

The primary indication for all-ceramic crowns is the need for optimum esthetics. However, it is important to understand that not every all-ceramic system automatically provides an esthetic result that is superior to metal-ceramic crowns.

Figure 17. If an all-ceramic crown has an internal opaque core, the esthetic potential will be no better than that of a metal-ceramic crown.

Given the assumption that porcelain labial margins with metal-ceramic crowns eliminate the problem of metal show through or excess opacity in the cervical third, the major advantage possessed by all-ceramic crowns is the potential for improved light transmission in the cervical third. The strengthening mechanism for many all-ceramic systems is the use of a high-strength internal core. Most of these internal cores are quite opaque, and the transmission of light with these systems is no better than that achieved with a metal core (Figure 17). Given that the primary indication for all-ceramic crowns is improved esthetics, it would seem prudent to avoid those systems that do not provide this.

• Principle No. 13. If an all-ceramic crown system uses an internal opaque core, the esthetic results achieved with that system will be no better than what could achieved with a metal-ceramic crown.

Figure 18. Dicor restorations are shaded with extrinsic coloration. This can result in high surface reflectivity.

Many other all-ceramic systems consist of cast or pressed ceramics with no potential for internal characterization or layering effect. With these systems, all the color is on the external surface; and with these restorations it is difficult to control the desired texture of the labial surface, and the surface tends to be highly reflective resulting in metameric mismatches. These restorations do not look natural because of the lack of depth in color and specular reflectance (Figure 18). Restorative systems that do not allow internal color characterization should be avoided.

• Principle No. 14. If internal characterization is impossible with an all-ceramic crown system, the restorations will likely lack color depth and exhibit an unnatural appearance.

Figure 19. Willi's Glass restorations are highly esthetic but have a poor clinical track record.

Attempts have been made with some all-ceramic systems to use the cast component as a coping that is then veneered with conventional porcelain (Figure 19). ²¹ While these systems exhibit exceptional esthetics, the clinical track record has been extremely poor. The high early clinical fracture rate may be due to loss of strength from the thinning of the coping material, or due to stress created because of not properly matching the coefficients of thermal expansion and contraction between the core and the veneering material. Crown systems that provide excellent esthetics but limited clinical service should be used with great caution, and the patient should be informed of the assumed risks.

One system that appears to have great promise is IPS Empress (Ivoclar North America, Inc. Amherst, N.Y.) when used as a veneered crown. The core material with this system has excellent translucency, and the esthetic results obtained with the system are excellent and predictable. One study of 75 IPS Empress restorations placed on first molars, bicuspids and anterior teeth reported only 1 fracture with the length of service ranging from 15 to 43 months. ²²

While many practitioners believe that all-ceramic restorations are inherently more esthetic than their metal-ceramic counterparts, this may be primarily because typically more tooth structure is removed with all-ceramic preparations. Most laboratory technicians could produce metal-ceramic restorations with equivalent esthetic results if this increased tooth reduction is provided.

• Principle No. 15. Excellent esthetic results can be achieved by talented technicians with both all-ceramic and metal-ceramic systems if adequate tooth reduction is accomplished by the dentist.

Summary and Conclusions

There are a host of contemporary all-ceramic systems available today, as well as improved techniques for metal-ceramic restorations. Perhaps the most important factor in achieving predictable success with these restorations is excellent soft tissue management. Metal-ceramic restorations provide the most predictable service in terms of clinical longevity, versatility and prevention of wear to the opposing dentition.

Many all-ceramic systems are strengthened with an internal opaque core and seem to have few advantages over metal-ceramics and have numerous disadvantages. The same statement is true for all-ceramic restorations that have no provision for intrinsic coloration.

A few all-ceramic systems have the potential for intrinsic coloration and do not utilize an internal opaque core. With uniform tooth reduction of 1.5 mm. these systems can provide excellent esthetics. With less tooth reduction, the esthetic result may be somewhat compromised.

It is suggested that metal-ceramic crowns will continue to be the standard and will be used extensively in the future. Certain all-ceramic systems should be used with discrimination in situations where they are indicated.

Authors

George C. Cho, DDS, is an assistant professor and director of clinical education for advanced education in prosthodontics at the University of Southern California School of Dentistry.

Terry E. Donovan, DDS, is an associate professor and executive associate dean at the USC School of Dentistry. He is also chairman of the Department of Restorative Dentistry and co-director of advanced education in prosthodontics.

Winston W.L. Chee, BDS, is an associate professor and the Ralph W. and Jean L. Bleak Professor of Restorative Dentistry at the USC School of Dentistry. He is also director of implant dentistry and co-director of advanced education in prosthodontics at USC.