In-Ceram Fixed Partial
Dentures: Three-Year
Clinical Trial Results

By John A. Sorensen, DMD, PhD;
Seung-Koo Kang, DDS;
Tony J. Torres, CDT;
and Helmut Knode, ZTM, DMD

In-Ceram is a sintered, high alumina content, glass infiltrated ceramic core material reported to have sufficient strength for all-ceramic fixed partial dentures. While Vita/Vident recommends that In-Ceram should be used only for anterior FPDs, the purpose of this study was to push the sintered alumina material to its limits by testing posterior FPDs with premolar and molar pontics. This prospective clinical trial tested the longevity of 61 three-unit In-Ceram alumina FPDs. The failed specimens were analyzed to determine factors contributing to failure. The abutment teeth were prepared for full crown retainers with shoulder margins and 1.3 mm of axial reduction. All FPDs were cemented with an encapsulated glass ionomer. None of the patients reported postcementation sensitivity. During the three-year period, seven FPDs fractured through the connector area. By location of the pontic, failure rates were 0 percent for anteriors, 11 percent for premolars and 24 percent for molars. Based on the results of this clinical study at the three-year point, In-Ceram alumina can be reliably utilized for anterior FPDs as indicated by a 100 percent success rate. The findings do not support the use of In-Ceram alumina for posterior FPDs as was advised by the porcelain manufacturer. Glass ionomer cement can be predictably used to cement In-Ceram FPDs with few clinical side effects.

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

Figure 1. Master die duplicated in a special plaster with a mold for the pontic. Figure 4. FPD substructure is carved to desired dimensions. Figure 5. Substructure has been sintered. Note how moisture has been driven off and the die has shrunk away from core.

Dr. Michael Sadoun in 1988 reported the development of a new approach to the fabrication of all-ceramic fixed prostheses.1 The master dies are duplicated in a special plaster with a mold to shape the undersurface of the pontic (Figure 1). This technique was taken from the ceramics manufacturing industry using a process called "slip casting" whereby a fine particle size high-alumina content powder is mixed with a special liquid and applied as a slip material to the plaster dies.

Figure 2. Moisture absorbed into die stone agglomerating or packing alumina particles. Figure 3. During sintering the alumina particles fuse together at points of contact producing a highly stable, organized crystalline structure. Figure 6. At elevated temperatures, infiltration glass moves inside from external surface to fill air spaces between particles by capillary action. Figure 7. Glass infiltrated FPD substructure with translucency increased and shade conferred.

As the slip is applied to the die, moisture is absorbed into the stone agglomerating or tightly packing the particles onto the stone (Figure 2). The coping is carved to the desired dimension and sintered at approximately 1,140 degrees Celsius in a special oven over an 11-hour firing cycle. The particles fuse at the points of contact producing an organized, highly stable crystalline structure (Figure 3). From the sintering procedure, the moisture is driven from the die material causing the die to shrink away from the coping, eliminating the need for aluminum oxide abrasion devestment (Figures 4 and 5). At this stage, the coping is white and opaque and has low strength. A process developed by Dr. Sadoun called "infiltration glass firing" is then performed. At elevated temperatures, this glass material -- applied as a slurry on the external surface of the coping -- flows into the interstitial spaces by capillary action (Figure 6). An altered refractive index results in a translucent substructure with the selected shade inherent to the high strength core material (Figure 7). The patent rights2 to this material were then transferred to Vita Zahnfabrik (Bad Sackingen, Germany) and after several years of research and development was launched on the market as In-Ceram. The exact fabrication procedures are presented in a technical journal.3

In vitro tests have demonstrated excellent material properties for fixed prosthodontics. A three-point bend test recorded flexural strengths of 446 MPa, nearly three times stronger than any of the other ceramic tested.4 Cross-sectional marginal fidelity studies where crowns were indirectly fabricated and cemented on their respective dies yielded mean vertical marginal discrepancies of only 24 m for In-Ceram crowns while metal-ceramic crowns with a metal collar had marginal discrepancies of 27 m.5,6 The best marginal fit was achieved with a shoulder margin, but even a feather-edge or shoulder-bevel margin (not recommended for all-ceramic crowns) produced excellent marginal fit of 67 m.5 With similar experimental methodologies, three-unit posterior In-Ceram FPDs yielded a mean vertical marginal discrepancy of only 58 m.7 This is excellent marginal adaptation considering the complexity in seating an FPD on two abutments during cementation. Unlike metal-ceramic FPDs, which have metal substructure distortion resulting from the porcelain firing procedure yielding a degradation of the post ceramic firing fit,8-10 the In-Ceram material is highly stable; and its fit does not deteriorate with veneer porcelain firings.

Figure 8. Transillumination reveals increased translucency resulting from glass infiltration procedure. Figure 9. In-Ceram FPD Nos. 8 through 10 demonstrates translucency even with a Vita shade A5. Empress crown No. 22 for comparison.

Many problems inherent to metal-ceramic restorations can be overcome with metal-free ceramics. The advantages of an all-ceramic FPD include:

Greatly improved esthetics from light transmission through the core material and enhanced shade match between core materials and veneering ceramic (Figures 8 and 9);

Greatly reduced thermal conductivity resulting in reduced temperature sensitivity and adverse pulpal responses;

A radiolucent material, which allows for greater effectiveness of diagnostic radiographs;

Reduced iatrogenic periodontal disease due to diminished plaque accumulation from the inherently smoother properties of ceramic compared to the metal-opaque-porcelain junction;

Reduced iatrogenic periodontal disease due to the fact that better contours can be achieved compared with typical overcontouring of metal-ceramic crown margins; and

Reduced health hazard because all-ceramic bridge materials are essentially inert, while more than 80 percent of dentists in the United States use base metal alloys containing nickel, beryllium and chromium.

Figure 10. Dynamic seating of In-Ceram crown for cementation.

In-Ceram restorations gain their high strength from the core material and do not rely on the adhesive cementation technique for strengthening of the restoration like other all-ceramic crown systems. IPS Empress (Ivoclar North America, Amherst, N.Y.)11 and OPC (Jeneric/Pentron, Wallingford, Conn.)12 must be adhesively cemented to achieve maximum strength for clinical longevity. In-Ceram is the only all-ceramic system that can be handled clinically and cemented like a metal-ceramic restoration. Figure 10 illustrates how the dynamic seating method can be used for cementation of an In-Ceram crown similar to a metal-ceramic crown.

Dr. Sadoun worked with a number of practitioners in France to clinically test his new ceramic. Their work resulted in success rates of 100 percent for single anterior and posterior crowns. For FPDs, they observed a 100 percent success rate for anteriors and a 91 percent success rate for posteriors.1 After analyzing the clinically failed specimens, he determined that most of the posterior FPD failures occurred at the occlusal-proximal line angles of the abutments adjacent to the pontic. They then modified the posterior FPD preparation design to include preparation of small boxes on the interproximals adjacent to the pontic. After 1.5 years of clinical testing, they had a success rate of 100 percent.13 An in vitro study recorded a 30 percent higher breaking strength of In-Ceram posterior FPDs with the proximal box design.14

Since metal-ceramic FPDs are the standard of care in practice, their clinical survival rate should be used as the criterion for new all-ceramic systems. Considering the extent of worldwide usage of metal-ceramic restorations, relatively little information is available on the survival rate and average service times of esthetic fixed prostheses in clinical practice. The fact that many dental insurance companies will pay for the fabrication of a new FPD every five years is testament to the low state of the art. The following clinical studies yielded positive results with relatively low failure rates. Leempoel and colleagues15 studied the survival rate of crowns in 40 Dutch general practices. For 1,323 anterior metal-ceramic crowns, survival rates at five years were 95 percent and at 10 years were 82 percent. For 2,011 premolar metal-ceramic crowns, survival rates at five years were 98 percent and at 10 years were 97 percent. A failure rate of 2.8 percent for metal-ceramic crowns was observed over a seven-year period at the University of Zurich.16

Coornaert and colleagues17 followed 2,181 metal-ceramic units over seven years and observed a 2.4 percent failure rate. The follow-up rate was 85 percent during the seven-year study with most failures occurring within one year after cementation. Inadequate gold framework thickness resulting in porcelain fracture near the gingival margin and framework fracture through the FPD connectors were most often the source of failure. These failures were indicative of efforts to improve the esthetics by reducing the dimensions of the metal framework in order to increase the porcelain thickness. Leempoel and colleagues18 evaluated 86 FPDs with 213 restored abutments in general practice over seven years and found a 4.4 percent failure rate.

To summarize the data on the few clinical studies that have been performed -- most of them well more than a decade ago -- the metal-ceramic prostheses failure rate at approximately seven years was about 2.5 percent to 6 percent. An equally low failure rate of In-Ceram FPDs is expected if this system is to be accepted for general clinical use.

The Vita/Vident company (Brea, Calif.) recommends that In-Ceram be used for single anterior and posterior crowns as well as anterior three-unit FPDs. It does not recommend its application for posterior FPDs. The purpose of this prospective clinical trial was to push this material to its limits by testing both anterior and posterior FPDs. A secondary purpose was to determine the factors that contributed to any failures. This paper is primarily concerned with longevity factors of the In-Ceram FPDs.

Materials and Methods

Patients were recruited from the greater Los Angeles area and treated at the School of Dentistry Clinical Research Center of the University of California at Los Angeles. Qualification requirements for acceptance into the study were that subjects had a minimum of 20 teeth, did not wear a complete denture, had at least moderately good oral hygiene, had no active periodontal disease, had one missing tooth needing replacement, and had teeth or fixed prostheses opposing the pontic space. Multiple three-unit FPDs could be placed in the same subject. Potential subjects were not rejected if they had a history of bruxism. Since bruxers are a normal part of the dental population, it was believed they should be included in the study. Location and extent of wear facets as well as type of contact in lateral excursions were recorded. Patient subjects were given informed consent and all Human Subject Protection Committee guidelines were followed.

Figure 11. Standard shoulder tooth preparation design for anterior abutments. Figure 12. Cemented In-Ceram FPD.

Tooth Preparation

All teeth were prepared with a flat-ended diamond aiming for 1.3 mm of axial reduction and 1.5 to 2 mm for incisal or occlusal reduction. This is the same amount of reduction or less than needed for metal-ceramic FPDs. The margin design was a shoulder configuration with a rounded axial-gingival line angle. The shoulder margin design provides the greatest strength and best marginal fit.5 A flat-ended diamond can penetrate the tooth to varying degrees and still produce a shoulder configuration free of lipping. The bulk reduction was performed with a flat-ended diamond, followed by refinement of the margin with tissue-protecting end cutting burs. The margins were finalized with hand instruments to achieve as smooth, clean and linear a finish line as possible.

Figure 11 illustrates the standard abutment tooth preparation for an anterior FPD with full circumferential shoulder margins. The anterior FPD was then cemented (Figure 12).

Figure 13. Posterior In-Ceram FPD with increased bulk of core material created by the proximal box preparations. Figure 14. Cemented In-Ceram FPD Nos. 3 through 5.

A modified abutment tooth preparation design was used for posterior FPDs. Full circumferential shoulder margins were prepared with the addition of small boxes on the interproximals adjacent to the pontic. Often these can be placed where previous fillings had been. These abutment tooth box design features serve to increase the bulk of core material at the proximal-occlusal line angle (Figure 13). Figure 14 shows the cemented posterior FPD.

Margins on posterior FPDs were placed either equi- or supragingival when possible and extended subgingivally when needed for establishment of margins on sound tooth structure or replacement of existing fixed prostheses. Anterior FPD margins were placed just below the crest of the gingiva when esthetic margins were needed or otherwise on sound tooth structure equigingivally.

Gingival displacement was achieved by placement of a No. 2 Gingibraid cord containing 0.65 mg/inch of aluminum sulfate in the sulcus of the abutment teeth for 10 minutes. The cord was then moistened with Hemodent (ESPE, Seefeld, Germany) and removed; the sulcus was dried and a vinylpolysiloxane (Reprosil, Caulk, Dentsply, Milford, Del.) impression material syringed around the abutment teeth and followed by seating of the impression tray with a heavy body vinylpolysiloxane impression material. After seven minutes of setting time, the impression tray was removed. Opposing alginate impressions were made and interocclusal records made when needed. A provisional restoration was fabricated from acrylic resin with a vacuum matrix and cemented with calcium hydroxide (Dycal, Caulk).

FPD Fabrication Techniques

The impressions were poured in an improved die stone (Die-Keen, Modern Dental Materials Inc., St. Louis) and mixed according to manufacturer's specifications. The dies were pinned with the PinDex System (Whaledent, New York), sectioned and trimmed. Three layers of paint-on die spacer (Belle de St. Claire, Chatsworth, Calif.) were applied to the axial-gingival line angle.

The In-Ceram FPDs were fabricated according to the Vita instructions. This included duplication of the master die in a special plaster, slip cast application of the alumina to the die, carving the FPD to the desired shape and sintering at 1,120 degrees Celsius for the 11-hour procedure. The sintered framework was returned to the master die for verification of fit, adjusted when necessary and then infiltration glass fired with the appropriate shade of glass. The excess infiltration glass was removed and the framework dimensions were measured with a digital micrometer at 28 points and recorded. The veneering porcelain (Vitadur-N, Vita Zahnfabrik) was applied to the desired occlusion, contours and shape.

Delivery and Adjustment Protocol

The provisional crowns were removed, temporary cement debrided and teeth cleaned with a pumice slurry and rubber cup. Proximal contacts were evaluated with Shim Stock marking tape (Micro-O-Reg Shim Stock, Jackson Heights, N.Y.) and adjusted if necessary. The internal adaptation and marginal integrity were evaluated using Fitchecker (GC America, Chicago) and any areas of binding adjusted until the fit was determined to be excellent. Finally, the occlusion and contours were adjusted. If any adjustments were made, the crowns were autoglazed. The dimensions of the FPDs were measured at 28 points. The internal surface of the FPD was aluminum oxide abraded at 40 pounds/inches2 to debride the surface for cementation. The In-Ceram alumina is of sufficient strength to be unaffected by aluminum oxide abrasion.

Cementation Protocol

The abutment teeth were again cleaned with a pumice slurry and rubber cup. The teeth were then rinsed with water and dried with care being taken not to desiccate them. The FPDs were cemented with a glass ionomer cement (Ketac-Cem Applicap, ESPE). The encapsulated form of glass ionomer prevented measuring and mixing inaccuracies. The capsules were activated and mixed in an amalgam mixing machine, and a thin layer of cement was applied to the internal walls of the retainers. The FPDs were seated with an orangewood stick using a dynamic seating method by having the patient bite down on the stick and wiggling the stick to gain maximum seating of the FPD. A full setting time of 10 minutes was observed leaving the cement undisturbed. The excess set cement was removed with scaling instruments. The occlusion and proximal contacts were reverified.

Recall Evaluations

Measurements were made at baseline, and recall visits were scheduled annually. Evaluation appointments consisted of intraoral photographs, polyvinylsiloxane impressions of the FPD and antagonist teeth and direct clinical measurements. Parameters measured included:

General oral hygiene;
Plaque index of tooth and FPD;
Gingival index of tooth and FPD;
Pocket depth measurement with pressure-sensitive periodontal probe (Florida Probe);
Measurement of attached tissue around fixed restorations compared to contralateral teeth;

Occlusal analysis with determination of:

Location of centric contacts,
Notation of contacts on veneer porcelain or In-Ceram core material,
Occlusal scheme in lateral excursions, and
Notation of excursive contacts in veneer or core material;
Marginal integrity rating (A, B, C);
Condition of FPD;
Inquiry as to patient comfort with all teeth and crown; and
Polyvinylsiloxane impression of antagonist teeth and FPD for measurement of contour changes with MTS Tooth Profiling System.

The primary objective of this article is to explore the longevity factors of all-ceramic FPDs. The details of the other clinical parameters will be presented in other papers.

Results

Table 1 Distribution In-Ceram FPDs as defined by location of pontic Anterior 21 Premolar 19 Molar 21 Maxillary 31 Mandibular 30

A total of 61 three-unit In-Ceram FPDs were cemented in 47 subjects. Subjects ranged in age from 19 to 66. Table 1 shows the distribution In-Ceram FPDs as defined by the location of the pontic. Roughly one-third were anterior, premolar and molar pontic FPDs.

None of subjects experienced postcementation sensitivity with the glass ionomer cementation protocol. None of the abutments needed endodontic therapy during the three-year follow-up.

At the three-year recall point with 61 FPDs placed, seven FPD fractures occurred and one patient with one FPD died. In Table 2, the failure rate by location of the pontic is calculated.

Table 3 presents information on where the FPDs fractured, the period to fracture, and the occlusal-gingival connector height at the fracture location. All FPDs fractured through the connector, usually in the more posterior connector. The FPDs failed over a variety of time periods, but after 15 months the FPDs seemed to have stabilized and no more fractures occurred. The mean occlusal-gingival connector height of all the posterior FPDs was 4 mm. Figure 15 shows an FPD that had been fractured for three weeks when the patient presented. Figure 16 shows that respecting the soft tissues with the gingival embrasure contours and the opposing cusp contacting on the connector along with the sharp occlusal anatomy limited the occlusal-gingival connector height to 2.7 mm.

Figure 15. Clinical presentation of fractured FPD through posterior connector. Figure 16. Small occlusal-gingival dimension of connector due to gingival tissues and occlusal contact on connector area.

One patient, JQ, was a bruxer. This was determined by the presence of wear facets and the results of a history taken from the patient at the initial examination. The occlusal-gingival connector height was 4.4 mm. It was the only failed FPD with a connector height greater than the mean of 4 mm.

Fractographic evaluation of the failed specimens showed that in approximately 70 percent of the cases, the origin of failure was usually at flaws located at the interface of the veneer-core materials on the tissue surface of the connector area.19

Discussion

The Vita Company recommends that the In-Ceram System be used only for anterior FPDs. The present clinical study attempted to push the material to its limits by testing the In-Ceram ceramic for posterior FPDs as well. When broken down by location, the failure rate was 0 percent for anterior pontics, 11 percent for premolar pontics and 24 percent for molar pontics. In reviewing the seven failed FPDs, fracture always occurred through the connector. No fractures of the retainers occurred. There was no clear typical period until failure. Several FPDs fractured in the first four weeks of service, some fractured at five to six months, and some at 12 months. The longest period before failure was 15 months. Fractographic evaluation of the failed specimens showed that in 70 percent of the cases the origin of failure was located at flaws of the veneer-core material interface on the tissue surface of the connector area.19

The results of this study were similar to the Coornaert and colleagues16 study of metal-ceramic restorations in that the majority of failures occurred within the first year after cementation. The early failures were most likely due to technical problems in fabrication of the In-Ceram FPDs, such as inclusion of large flaws.

Hüls'20 clinical data on In-Ceram FPDs had similar results to the authors' study. At three to six years, he recorded no failures of anterior FPDs and a 19 percent failure rate of posterior FPDs.

The authors' results showed that the In-Ceram alumina cannot be reliably used for posterior FPDs. However, if one wanted to use In-Ceram in posterior applications, the results indicate that the veneer porcelain application on the tissue surface of the connector is not necessary and may eliminate the possibility of voids being formed at the veneer core interface. Further, this would allow for increased occlusal-gingival connector height made of the core material. The study did not show evidence of breakdown of the exposed alumina core material.

Attempts to develop other more conventional all-ceramic FPD systems have been frustrating. Claims were made by the Jeneric/Pentron Company that the Optec system had sufficient strength for fabrication of FPDs even when only veneer retainers rather than full crown retainers were used. Christensen and Christensen21 tested 40 FPDs with a variety of retainer designs and at two years found an 80 percent failure rate for posterior FPDs and a 22 percent failure rate even with full crown retainers on anterior FPDs. In the present study, since none of the In-Ceram FPD failures occurred in the retainers, it can be assumed that single crowns made of In-Ceram should have failure rates equal to or better than metal-ceramic crowns.

It is not yet known how many years of evaluation are needed to reliably predict great clinical longevity and sufficient resistance to the adversarial oral fatigue phenomena for an all-ceramic system. However, the 100 percent success rate of anterior In-Ceram FPDs in the present study and in Hüls'20 research is encouraging.

The main purpose of this paper was to present longevity factors for the In-Ceram FPDs.
Future papers will present the results of other clinical parameters.

Conclusions

At the three-year point in this clinical study of In-Ceram alumina three-unit FPDs, the following conclusions can be made:

- Seven out of 61 FPDs failed by fracture through the connector area.
- By location of the pontic, failure rates were 0 percent for anteriors, 11 percent for premolars and 24 percent for molars.
- The results are supportive of the manufacturer's suggested utilization of the In-Ceram system for anterior FPDs.
- Glass ionomer cement can be predictably used to cement In-Ceram FPDs with few clinical side effects.

Acknowledgments

This study was funded by Vita Zahnfabrik (Bad Sackingen, Germany) and Vident Corporation (Brea, CA). The authors would like to thank Sushma Nachnani, MS, for her aid in coordinating the clinical study at UCLA in the authors' absence and Dr. Creighton Choi for treating patients in the authors' absence.

Authors

John A. Sorensen, DMD, PhD, is the ODA Centennial Professor of Restorative Dentistry and director of the Clinical Research Center at Oregon Health Sciences University, Portland. He is also formerly an associate professor and director of advanced prosthodontics at the University of California at Los Angeles.

Seung-Koo Kang, DDS, maintains a practice limited to prosthodontics in Seoul, Korea. He is formerly a research assistant at UCLA.

Tony J. Torres, CDT, is a technician at Valley Dental Arts Laboratory, Stillwater, Minn. He is formerly the manager of education at Vident, Baldwin Park, Calif., and formerly a research assistant at UCLA.

Helmut Knode, ZTM, DMD, maintains a private practice in Triie, Germany. He is formerly a visiting assistant professor at UCLA.

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To request printed copies of this article, please contact/John A. Sorensen, DMD, PhD, Oregon Health Sciences University, School of Dentistry, 611 S.W. Campus Drive, Portland, OR 97201-3097.