All-ceramic crowns and fixed prostheses have had many false starts across the past several decades. Most dentists have frustrating memories of placing beautiful all-ceramic restorations, only to have them fracture after a few months or years of service. An obvious exception to this phenomenon is the success of pressed ceramic restorations for the anterior portion of the mouth. Millions of beautiful crowns and fixed prostheses using pressed ceramic have served successfully for many years, primarily in the anterior portion of the mouth. However, dentists long have sought stronger all-ceramic restorations for crowns and fixed prostheses in both the anterior and the posterior portions of the mouth.
In the past several years, numerous brands of zirconia-based all-ceramic crowns and fixed prostheses have been introduced to dentistry. Some popular brands in the United States are Cercon (Dentsply, Milford, Conn.), Everest (KaVo, Lake Zurich, Ill.), IPS e.max (Ivoclar Vivadent, Amherst, N.Y.) and Lava (3M ESPE, St. Paul, Minn.). These materials are having a significant effect on the fixed prosthodontic laboratory industry, as well as on practitioners and their patients. Many clinical reports and research articles have been published on use of zirconium oxide milled by computer-aided design/computer-aided manufacture procedures, sintered and used to create substructures for ceramic that subsequently is layered or pressed on the external surfaces.1–6
Porcelain-fused-to-metal (PFM) restorations have served the profession well for nearly 50 years. Why are dentists and laboratory technicians moving to zirconia-based all-ceramic crowns and fixed prostheses instead of staying with PFM restorations?
This article expresses my observations on the advantages and disadvantages of zirconium oxide substructure crowns and fixed prostheses in relation to PFM restorations.
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INITIAL ESTHETIC APPEARANCE
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When restoring a single maxillary central incisor and attempting to achieve an optimum esthetic result, most dentists prefer to make the restoration from all-ceramic materials. Most dentists would agree that all-ceramic crowns and fixed prostheses usually have a better appearance than do PFM restorations. This advantage is related to the difficulty in blocking out the color and opacity of the metal substructure. However, with superior laboratory fabrication, which is available but not commonly seen, PFM restorations can be made to simulate natural tooth structure. In addition, the popular ceramic collar often placed on the facial margins of PFM restorations has made them even more acceptable from a long-term esthetic standpoint. As the gingival tissues shrink apically, the margins associated with the ceramic collar on the PFM restoration do not have the same objectionable appearance as would margins fired over metal.
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LONG-TERM COLOR STABILITY OF PFM VERSUS ALL-CERAMIC RESTORATIONS
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Experienced dentists know that the color of most PFM restorations becomes lighter during service because most dental laboratories characterize the color of PFM restorations by placing superficial ceramic stains on them. Initially, the stained crowns are esthetically acceptable, but after a few years of service, they become lighter in color and create an obvious mismatch with adjacent teeth. The superficial stains are dissolved by patients’ consumption of acidic beverages and foods, and the color of the PFM restorations becomes the same as it was before the stains were placed. Unfortunately, lightening of restorations during service also may be observed on all-ceramic restorations for the same reasons.
In our ongoing research at the CRA Foundation on zirconia-based all-ceramic restorations and from our discussions with laboratory technicians, another factor appears to be a challenge. Accuracy of the fusing temperature of the superficial ceramic layered and fired on zirconia all-ceramic restorations is critical for the success of the restorations, and long-term research is not available on the longevity of superficial stains placed on the fired ceramic on such restorations. In addition, pressed ceramic is being placed over zirconia substructures, and it also requires superficial staining for an optimum esthetic appearance.
In summary, I cannot predict whether the long-term color stability of zirconia-based all-ceramic crowns will be better than that observed for PFM restorations. My guess is that this characteristic will be similar to that attained with PFM restorations.
I cannot predict whether the long-term color stability of zirconia-based all-ceramic crowns will be better than that observed for porcelain-fused-to-metal restorations.
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GINGIVAL RECESSION AND EXPOSURE OF RESTORATION MARGINS
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The margins of PFM restorations in observable areas of the mouth usually are placed sub-gingivally. After a few years, gingival tissues always recede—and the margins of the PFM restoration are exposed to view. Even when all-ceramic margins have been placed on PFM restorations, the exposed margins of these restorations often still are unattractive and are immediately identifiable when the patient smiles.
Will zirconia-based all-ceramic restorations be better esthetically than PFM restorations when the gingiva recedes? Some of the current zirconia products use nearly white zirconia for restoration substructures, whereas others use pigmented zirconia. Depending on the color of the underlying zirconia substructure, the overall expertise of the technician and whether the laboratory technician cuts back the zirconia from the facial margins and fires or presses ceramic to the margins, the margins exposed by gingival recession may or may not be more acceptable than the exposed margins of PFM restorations. In my experience, the exposed margins of the zirconia-based all-ceramic restorations are less objectionable esthetically than are exposed PFM margins.
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LONG-SPAN FIXED PROSTHESES
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Important research has been conducted regarding the acceptability of zirconia-based all-ceramic fixed prostheses, as cited previously. From this clinical research, some of which spans seven years, it is apparent that substructures of three-unit fixed prostheses in the studies served well through the study period. As might be expected, the superficial layered ceramic materials exhibited some fracture and pitting across time. Fracture and pitting also are common with PFM restorations, but our recent research results show somewhat more superficial ceramic challenges with the all-ceramic restorations.7
It appears that at this relatively early date, zirconia-based all-ceramic restorations are serving adequately in three-unit fixed prostheses and, in some studies, in prostheses of longer spans. It has been my observation among practitioners that they are pleased with the esthetic result and the clinical strength of these restorations for the few years they have been available in the United States. However, more long-term research is needed.
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PRECISION ATTACHMENTS
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Metallic precision attachments and stress breakers occasionally are incorporated into PFM restorations. These are placed relatively easily by laboratory technicians, and they are known to serve well for long periods.
Currently, precision attachments and stress breakers are not available for zirconia-based all-ceramic restorations. This limitation will not hinder their use in most day-to-day clinical situations but will not allow their use in some complicated restorative situations that require sophisticated laboratory support.
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WEAR OF OCCLUDING TEETH
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Older, large–particle-size ceramic materials fired over metal were well-known to be destructive to opposing natural teeth and some types of restorations. However, the smaller particle size of current ceramics and their overall modified and improved ingredients have greatly reduced the objectionable wear of opposing teeth experienced with older ceramics. Currently, similar ceramics are fired or pressed on zirconia substructures and metal. It is unlikely that the characteristics of long-term wear on opposing teeth will be much different with the zirconia-based all-ceramic restorations than they are with the current generation of ceramics for PFM restorations.
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POTENTIAL METAL SENSITIVITY
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Because the zirconia-based crowns and fixed prostheses do not have conventional metal castings as substructures, it is assumed that the well-known objectionable gingival reactions often observed with PFM restorations probably will be reduced or eliminated. After having placed hundreds of restorations, I have found these reactions to be reduced with the use of all-ceramic materials.
Significantly more clinical research is needed regarding zirconia-based all-ceramic crowns and fixed prostheses.
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COST OF THE RESTORATIONS
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The cost of zirconia-based all-ceramic restorations is higher than that of PFM restorations. The higher cost generally is related to a surcharge by milling centers for milling the zirconia frameworks.8 The average clinical fee, including laboratory cost, as reported by the American Dental Association for a PFM crown in 2005 was $808, and the average cost of a ceramic crown in 2005 was $834.9 The fees undoubtedly are slightly higher now. Will this cost be a deterrent to the growth in use of these restorations? I doubt it. The cost will come down eventually as milling is perfected and milling devices of lower cost are introduced and used.
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SUMMARY
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Use of zirconia-based all-ceramic crowns and fixed pros-theses is rapidly growing. Zirconia-based crowns and fixed prostheses have undergone only a few years of basic science research and laboratory and clinical observation. To date, the research results are promising. However, significantly more clinical research is needed regarding this concept.
These are my observations regarding zirconia-based all-ceramic restorations when compared with PFM restorations:
- – They are better esthetically than typical PFM restorations.
- – The long-term color stability probably will be the same as that observed with PFM restorations.
- – The margins of the restorations have a more acceptable appearance than those of PFM restorations when gingiva recedes.
- – Research shows that the strength and service record of PFM restorations and zirconia-based restorations in three-unit prostheses is similar, but longer clinical observation is necessary for final judgment.
- – Prostheses requiring precision attachments or stress breakers are best made with PFM restorations.
- – The long-term wear characteristics on opposing teeth for either material probably will be similar, because the external ceramic materials are similar.
- – Gingival sensitivity to metal will be reduced or eliminated with use of zirconia-based restorations.
- – The cost of zirconia-based restorations is higher than that of PFM restorations, but it probably will decrease as further developments take place.
Overall, the potential for zirconia-based all-ceramic restorations appears to be very good.
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INITIAL ESTHETIC APPEARANCE
LONG-TERM COLOR STABILITY OF...
