Factors Essential for Successful All-Ceramic Restorations
Terence E. Donovan, DDS
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ABSTRACT
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Background. The use of all-ceramic crowns is increasing, and this trend will continue. However, all-ceramic systems are not all the same. They differ considerably in their relative esthetic potential, their physical properties and evidence base relative to longevity. The use of an all-ceramic system does not guarantee outstanding esthetics.
Conclusions. Some all-ceramic systems can provide superior esthetic results compared with metal-ceramic restorations. Zirconia-cored crowns are the strongest all-ceramic system and may provide improved esthetic results compared with metal-ceramic crowns. No all-ceramic restoration has been shown to have a life span equivalent to that of metal-ceramic restorations. Further clinical trials are needed.
Practice Implications. Clinicians should choose appropriate all-ceramic restorations on the basis of their patients’ needs. Currently available evidence indicates that clinicians should not use all-ceramic crowns on molars; in addition, posterior fixed partial prostheses fabricated with all-ceramic materials have a high likelihood of failure.
Key Words: Zirconia-cored restorations; transformation toughening; all-ceramic; esthetic potential; translucent
Patients increasingly are demanding dental restorations that are both esthetic and functional. Manufacturers have introduced numerous all-ceramic alternatives to metal-ceramic restorations, and all-ceramic restorations are being marketed to patients and dentists in an aggressive manner. I think it is safe to state that many dentists are somewhat confused about what these restorations offer in terms of improved esthetic potential, as well as their indications, contraindications and potential life span.
In this article, I describe factors that are essential for success when using all-ceramic restorations. Many of these factors also are essential for success with any type of indirect restoration.
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PRIMARY INDICATIONS
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The primary indications for all-ceramic restorations are improved esthetics and lower cost. Although the physical properties and strength of all-ceramic crowns have improved, no study has demonstrated that they can provide the same length of service as properly fabricated metal-ceramic restorations. Thus, clinicians should use them with patients for whom the esthetics of the restoration are more important than absolute longevity.
The esthetic potential of different all-ceramic systems is not equal. When restoring anterior teeth, dentists should use the most esthetic system for which there is a documented evidence base regarding longevity. The criteria suggested by Schärer1 as an adequate evidence base seem appropriate. These criteria include the need for published independent clinical trials of three to five years’ duration, with survival rates of 95 percent or greater.
Such a system should have a relatively translucent core, the capability for the technician to build in color intrinsically, and esthetic layers of body and incisal porcelain. In my opinion, etched and bonded feldspathic porcelain jacket crowns and restorations fabricated with the IPS Empress System (Ivoclar Vivadent, Amherst, N.Y.) meet these specifications. Restorations with zirconia cores do not have equivalent esthetic potential, and because of their improved physical properties, they are indicated primarily for crowns on premolars and molars and for anterior fixed partial prostheses.
With the escalating cost of precious metals, all-ceramic crowns are competitive with metal-ceramic crowns from a cost perspective. An informal survey of several major dental laboratories indicated that at the current cost of gold, platinum and palladium, all-ceramic alternatives are considerably less expensive, especially when the costs of providing all-porcelain margins in metal-ceramic crowns are factored in (T. Donovan, unpublished data, January 2008) (Table
). Usage data from the first part of the year indicate that 50 percent of all crowns fabricated in the United States in 2008 will be all-ceramic restorations, which is a significant increase over previous years (T. Donovan, unpublished data, January 2008).
Esthetic success.
It is important to understand that simply placing an all-ceramic restoration instead of a metal-ceramic restoration will not guarantee outstanding esthetics. The clinician must accomplish a number of details meticulously to ensure success. One of the most important details is proper tooth preparation, including finish-line geometry.2,3 To attain optimum esthetics and adequate strength, the dentist must achieve adequate reduction to give the ceramist room to create a restoration with excellent esthetics, as well as to achieve physiological crown contours. The clinician must remove a minimal cross-sectional thickness of between 1.2 and 1.5 millimeters of enamel and dentin circumferentially to provide sufficient room for the core and veneering porcelains. These are aggressive preparations, and a biological price may need to be paid for the sake of esthetics.
Pulpal status.
In this regard, the clinician must assess carefully the pulpal status of teeth to be restored before preparing them. Many teeth that require crowns have undergone multiple restorative procedures over the years, each of which stresses the pulp and increases the likelihood of eventual pulpal death.4,5 If there is any question regarding the viability of the pulp and an all-ceramic crown is to be used, the clinician should perform endodontic therapy before preparing the tooth. Creating an endodontic access cavity through an all-ceramic crown is difficult and often results in a defect that becomes more extensive, eventually requiring fabrication of a new restoration.
The cervical finish line should be a rounded shoulder or deep chamfer at 90o to the external angle of the root, and it should be as smooth as possible. The location of the finish line is critical to long-term gingival health. If we assume that achieving an optimum esthetic result is one of the goals of placing an all-ceramic crown, the dentist will, in most situations, place the cervical margins in a subgingival location. Margins need to be placed deeply enough into the sulcus so that minor gingival recession will not expose them, but not so deep that they violate biological width and result in a chronic inflammatory response6–8 (Figure 1). Clinicians can use the anatomical landmarks suggested by Block9 and Kois10 as guides for predictable margin placement. The gingival tissues must be brought to a state of optimum health before the dentist determines the final location of the cervical margin and makes the impression. Quality acrylic resin provisional restorations are essential to achieving this,11 and Sorensen and colleagues,12 Flemmig and colleagues13 and Sorensen and Newman14 described useful gingival enhancement procedures.
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Figure 1. This chronic inflammatory response to all-ceramic crowns is a result of biological width violation.
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Patient-related factors.
The decision to use an all-ceramic restoration should be made in the context of a number of patient-related factors. What are the patient’s esthetic expectations? Are there complicating occlusal relationships, and what is the level of parafunctional activity? Is the anticipated restoration on an anterior tooth or a molar, and is it a single-tooth restoration or a fixed partial prosthesis? If there is evidence that the patient experiences nocturnal bruxism, will he or she wear an occlusal nightguard every night?
In ideal circumstances involving anterior single-unit restorations, I recommend that the dentist choose what I believe is the most esthetic option (IPS Empress). In compromised situations or when restoring posterior teeth or placing a fixed partial prosthesis, the dentist should choose a stronger, zirconia-cored system. It is critical for patients to understand that long-term survival rates for all-ceramic restorations are likely to be lower than those for metal-ceramic restorations, and that they give appropriate informed consent.15,16
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ZIRCONIA-CORED RESTORATIONS
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When considering use of a zirconia-cored restoration, clinicians should understand that all zirconia materials are not the same. A system that mills the zirconia core in the softer "green" state and then sinters it is superior to one that mills the core in the sintered state. This is because the latter requires a robust milling machine and high-temperature milling that will result in near-surface damage and defect formation, which will significantly shorten the anticipated life span of the restoration.17 Milling in the green state followed by sintering allows lower-temperature milling, and the sintering "heals" any milling-induced defects.
Zirconia-based technology in dentistry is in its infancy, and much remains to be learned before dentists use it routinely in practice. To my knowledge, there are no published studies with five-year data that meet Schärer’s criteria1 for restorations on posterior teeth or for fixed partial prostheses.15 The physical properties of the zirconia core are excellent, with their high strength and fracture toughness. In addition, the cores possess a property called "transformation toughening" that prevents defect propagation through transformation from the tetragonal to the monoclinic form. Many clinical trials are being conducted, and the fact that most involve posterior teeth or fixed partial prostheses indicates that the manufacturers of these materials are confident of a high level of success.16
Chipping.
One problem that has plagued almost all trials of zirconia-cored restorations has been a relatively high rate of chipping of the veneering ceramic. The fracture rate of the veneering ceramic has ranged from 8 to 50 percent at one to two years in these trials, while the reported rate of veneer fracture with metal-ceramic restorations has been between 4 and 10 percent after 10 years.16 The cause of this chipping is not known, and both core flexure and bond failure have been suggested.
Another possible cause of chipping is the lack of uniform support of the veneering ceramic by the core. A well-established principle for metal-ceramic restorations is that the metal core should support a uniform thickness of veneering ceramic and that there should be a maximum of 2 mm of unsupported porcelain. This is accomplished with an anatomical-contour wax-up and controlled cutback (Figure 2).
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Figure 2. An anatomical-contour wax-up with controlled cutback provides optimum metal support and a uniform thickness of the veneering ceramic for metal-ceramic crowns.
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Veneering ceramic thickness.
Laboratories make zirconia cores by scanning the die and then milling a uniform core of 0.3 mm for anterior teeth and 0.5 mm for posterior teeth. Because of the bell-shaped nature of teeth that must be reduced to remove undercuts, uneven thicknesses of the veneering ceramic will result, some of which may exceed the industry standard of 2 mm. The technician can correct this problem before he or she mills the core, but this rarely is done in commercial laboratories. Clinicians should request that the laboratory send them a digital image of the proposed core for approval before milling the core to ensure a uniform thickness of veneering ceramic with adequate core support.
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CEMENTATION
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The cementation protocol for all-ceramic crowns can be essential for success. Clinicians can effectively etch feldspathic porcelain jacket crowns and IPS Empress restorations with hydrofluoric acid and bond them in place by using a resin cement. Clinical studies have indicated that this protocol significantly increases their expected clinical life span.18,19 Clinicians can accomplish this with one of two approaches.
One approach is to use the immediate dentin sealing concept advocated first by Paul and Schärer20 and more recently by Magne and colleagues21 and Magne.22 With this approach, the dentist seals the dentin with a highly filled dentin bonding agent (such as OptiBond FL, Kerr, Orange, Calif.) immediately after tooth preparation and before making the impression.
Although there is no long-term documentation, another approach is to use a self-adhesive dual-cure resin cement and thus avoid the bonding step entirely. This is the simplest approach, although there are some minor concerns regarding potential long-term hydrolysis due to the hydrophilic nature of the self-adhesive cement.
High-strength all-ceramic materials (zirconia and alumina) cannot be etched and bonded readily. This is not really a disadvantage because it permits cementation with more conventional resin-modified glass ionomer cements (such as RelyX Luting Plus Cement, 3M ESPE, St. Paul, Minn.), which are less technique-sensitive. With traditional preparations, which provide mechanical retention and resistance form, adhesive cementation is not needed.
Some authorities have advocated the use of air abrasion and cementation of zirconia-cored restorations with a cement containing specific adhesive monomers (such as Panavia F2.0, Kuraray America, New York City)23 or air abrasion along with tribochemical bonding in order to adhesively cement zirconia restorations. Clinicians should exercise a great deal of caution when taking this advice. First, adhesive cementation is not necessary or critical to improve success rates. Second, the results of studies of air abrasion of zirconia-cored structures have been variable at best.24,25 Air abrasion of the intaglio surface of zirconia crowns can cause a transformation change from the tetragonal to the monoclinic phase, which severely ages the restoration and reduces its life expectancy. The problem is the particle size and it is pressure-dependent, but it is a potentially significant problem and I believe that the benefits do not equal the risk.
As a routine part of maintenance, dentists should provide a nightguard to any patient suspected of performing parafunctional activities, such as diurnal or nocturnal bruxism. I prefer a hard and soft nightguard (for example, Comfort Zone Bite Splint, Drake Precision Dental Laboratory, Charlotte, N.C.). The hard occlusal surface permits precise adjustment of the occlusion, while the soft intaglio surface provides comfort, which improves patient compliance. The dentist should instruct the patient to avoid immersing such splints in solutions containing alcohol and to bring the splint to all recall appointments so that he or she can monitor compliance.
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CONCLUSIONS
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The use of all-ceramic restorations is increasing, and this trend will continue. Higher-strength ceramic materials have expanded the indications for all-ceramic restorations; however, at this time, their efficacy is not similar to that of metal-ceramic restorations.
Simply using an all-ceramic restoration will not ensure predictable esthetic success. Precise attention to detail with regard to tooth preparation, cervical margin design and location, soft-tissue management and impression-making are crucial to success. Proper selection of materials and the ceramist also are essential, as are correct shade-matching procedures. Correct luting protocols are important to long-term success. Finally, I suggest that patients with all-ceramic restorations routinely use an occlusal nightguard.
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FOOTNOTES
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Dr. Donovan is a professor, Operative Dentistry, School of Dentistry, The University of North Carolina at Chapel Hill, 437 Brauer Hall, CB 7450, Chapel Hill, N.C. 27599-7450, e-mail "terry_donovan{at}dentistry.unc.edu". Address reprint requests to Dr. Donovan.
Disclosure. Dr. Donovan did not report any disclosures.
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REFERENCES
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- Schärer P. All-ceramic crown systems: clinical research versus observation in supporting claims. Signature 1996;(Summer):1.
- Scoble HO, Donovan TE. Tooth preparation for indirect esthetic restorations. J Calif Dent Assoc 1990;18(1):31–37.[Medline]
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- Ingber JS, Rose LF, Coslet JG. The "biologic width": a concept in periodontics and restorative dentistry. Alpha Omegan 1977;70(3): 62–65.[Medline]
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ABSTRACT
PRIMARY INDICATIONS
