Clinical study on the success of posterior monolithic zirconia crowns and fixed dental prostheses : preliminary report

OBJECTIVE: The purpose of this report was to present preliminary clinical results regarding the success rates and technical outcomes of posterior monolithic zirconia single tooth crowns (STs) and fixed dental prostheses (FDPs). MATERIALS AND METHOD: Thirty-four patients received 43 posterior monolithic zirconia restorations as single tooth crowns (STs) and/or fixed dental prostheses (FDPs), which were fabricated using a CAD-CAM (Computer Aided Design Computer Aided Manufacturing) system. At baseline and every 6 months, the restorations were examined for survival and technical outcomes. Success of the restorations was defined as the restoration remaining in situ, with no need for removal or replacement at followup visits. Technical outcomes were evaluated with a modified version of the United States Public Health Services criteria. Survival of restorations was estimated by using the Kaplan-Meier survival analysis. For each restoration, duration of follow-up was calculated from the time of placement to the date of its first failure. RESULTS: After a mean observation period of 18.6 ± 3.9 months (between 8-24 months), cumulative survival rates were 86.7% and 92.3% for STs and FDPs, respectively. Technical evaluation revealed good marginal adaptation and crown contours; however, modifications were needed for shade and occlusion of restorations. CONCLUSION: These preliminary results revealed high survival rate and generally successful technical outcomes for posterior monolithic zirconia STs and FDPs.


INTRODUCTION
Zirconia-based ceramics which have superior mechanical properties are used as substructure material, veneered with glass ceramics because of its opacity.In clinical applications, chipping of the veneer is the most frequent complication, thereby reducing the success rate of zirconia fixed dental prostheses (FDPs). 1,2In order to overcome this problem, translucent or monolithic zirconia is needed, which enables the fabrication of monolithic restorations without veneering porcelain.Advantages of monolithic zirconia restorations include increased material quality with presintered homogeneous blocks used in the CAD-CAM (Computer Aided Design -Computer Aided Manufacturing) technique and a reduced production time/cost.In addition, permitting for material thickness of 0.5 mm, due to high mechanical strength, contributes to the preservation of tooth substance and the use of all-ceramic restorations in case the interocclusal space is limited. 3However, information is lacking on clinical results of monolithic zirconia restorations. 4,5Few clinical studies focused on enamel wear caused by monolithic zirconia restorations.However, they reported comparable or lower antagonist enamel wear than that caused by other ceramic materials. 4,5The purpose of this report was to represent preliminary clinical results, including success rates, technical and esthetic evaluations of posterior monolithic zirconia crowns, and FDPs.

MATERIALS AND METHOD
This study consisted of patients who received monolithic zirconia single tooth crowns (STs) and/or fixed dental prostheses (FDPs) supported by teeth at the Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey.The Ethics Committee of Ankara University approved the study (No. 36290600/21), with all participants providing informed consent for clinical procedures.Thirty-four patients were treated with 43 posterior monolithic restorations.The inclusion criteria consisted of having vital or adequately endodontically-treated abutment teeth and good general health, without severe medical or psychological conditions.Exclusion criteria were signs of bruxism, severe periodontal disease, and being less than 18 years old.All teeth were prepared according to a standardized protocol for zirconia-based restorations (Figure 1). 1 In addition, all preparations were standardized: occlusal thickness of the restorations was at least 1.5 mm at the lowest point of the central fossa, and axial reduction was approximately 1-1.5 mm.Also, 1 mm rounded shoulder margins and internal line angles were provided.
Digital impressions were taken by using an intraoral scanner (CEREC Omnicam, Sirona, Bensheim, Germany) by an experienced dentist.Restorations were designed using CAD software (InLab SW 4.2, Sirona).The connectors for FDPs were designed with a mini-mum of a 12 mm 2 area.Restorations were milled from a partially-sintered translucent zirconia block, and colored by being dipped in a staining liquid (InCoris TZI, Sirona), then fully-sintered under the required conditions of the manufacturer.All restorations were checked for marginal fit, inter-proximal contact, and occlusion before cementation; they were steam cleaned, dried, glaze pasted (Ivoclar Vivadent, Schaan, Liechtenstein) and applied according to the manufacturer's recommendations.The restorations were cemented with adhesive resin cement (Panavia 21, Kuraray, Osaka, Japan).No pretreatment of the zirconia surface was done (Figure 2).At baseline (within 2 weeks after cementation), and then every 6 months, the restorations were examined clinically and radiologically by two calibrated clinicians, who were not involved in the treatment.Success of the restorations was assessed and defined as the restorations remaining in situ, with no need for removal or replacement at follow-up visits.Technical outcomes were evaluated by a modified version of the United States-Public Health Services (USPHS) criteria. 6e longevity of restorations was estimated with the Kaplan-Meier survival analysis, using a software package (SPSS 18.0, SPSS Inc, Chicago, IL, USA).For each restoration, the duration of follow-up was calculated from the time of placement to the date of its first failure.

RESULTS
Patient and restoration data are summarized in Table 1.After a mean observation period of 18.6 ± 3.9 months (between 8-24 months), all 43 restorations in 34 patients were examined.According to the Kaplan-Meier analysis, the success rates were 86.7% (26 surviving, 4 failures) and 92.3% (12 surviving, 1 failure) for STs and FDPs, respectively.The cumulative proportion of those surviving at 24 months (survival rates) was 0.612 (standard error: 0.197) for STs, and 0.923 (standard error: 0.74) for FDPs (Figure 3).Failures observed in the study are shown in Table 2. Restoration ratings according to the USPHS criteria are summarized in Figure 4.

DISCUSSION
In this preliminary report of an ongoing clinical study, high success rates (overall 88.4%) were found for posterior monolithic zirconia restorations with up to 24 months of follow-up.In the literature, to the best of the authors' knowledge, no clinical study has reported on success rates of similar restorations.Analyzing failures revealed only two restorations fractured during function, while other failures resulted from biological or esthetic complications.8][9] Considering the previous research, which reported an average posterior biting force of 700 N, 10 fracture of monolithic zirconia restorations should not be expected.However, in vitro studies did not completely mimic the clinical situation.

107
Technical evaluation revealed that marginal adaptation rated as "excellent" or "acceptable" for STs and FDPs.Improved adaptation may be attributed to advanced CAD-CAM technology, including intraoral scanning and precise milling of industrially-sintered blocks. 3his is in agreement with the clinical findings of Batson et al. 6 who reported good marginal adaptation of monolithic zirconia crowns.
Accurately reporting the patient's occlusion to the laboratory is a prerequisite for conventional prosthetic procedures, but the correct simulation of these contacts is also important for the CAD-CAM technique.Thus, restorations can be integrated into the stomatognathic system, and be biologically compatible, esthetically pleasant, and require time for the insertion (without clinical adjustments).In this study, milled and sintered monolithic zirconia restorations, especially FDPs, required considerable occlusal adjustment when placed.This may be attributed to lack of accuracy of CAD software in simulating jaw occlusion, which includes posterior multiple teeth preparations. 11To minimize discrepancies, taking a full-arch digital impression and designing the restoration on full-arch virtual models, as well as gathering bite image impressions before preparation, has been strongly advocated by Arslan et al. 11 The esthetic success of a dental restoration depends on a number of factors, such as crown contour, surface texture, translucency, and color.In the present study, crown contours were generally found to be "excellent".This might be attributed to individualizing the CAD software, which generated the virtual restoration design of an experienced clinician.However, problems were recorded about shade matching of the monolithic zirconia restorations with the remaining dentition.These translucent zirconia materials have been developed to improve light transmittance, and are therefore more translucent than conventional zirconia.On the other hand, they have significantly less translucency compared with conventional glass ceramics. 12Furthermore, translucency decreases with increasing monolithic zirconia thickness. 13Nevertheless, shade matching with the natural dentition may be challenging during clinical application.Further research is needed to enhance the translucency of zirconia.
Limitations of the present study include the absence of a control group for comparison with posterior monolithic zirconia restorations, short follow-up time, and small number of patients.The clinical performance of prosthetic reconstructions should be assessed by welldesigned, longitudinal (minimum of 5 years), randomized controlled clinical trials. 1,3Further clinical studies with longer follow-up and larger cohort size are needed for optimal monolithic zirconia restorations.

CONCLUSION
The preliminary results showed that monolithic zirconia single tooth crowns and fixed dental prostheses had high survival rates and generally successful technical outcomes.

Figure 4 .
Figure 4. Restoration ratings according to the United States Public Health Services criteria (ST: single tooth crown; FDP: fixed dental prosthesis)