CLINICS 2011;66(12):2063-2070 The wax contour was then impressed on the plaster model. The plaster model, the silicone mask, and then both components together were scanned with a laser scanner (Everest Scan pro, Kavo, Biberach, Germany). The zirconia core was designed with respect to the ceramic support. A computer-aided manufacturing (CAM) device milled the zirconia core in the pre-sintered state (Zirite, Keramo, Tavernerio, Como, Italia) and then sintered it according to the manufacturer’s instructions (TRF, Udine, Italia). Feldspathic porcelain (CZR Noritake Kizai Co. Ldt, Nagoya, Japan) was fused to the core with zirconium oxide margins (Figure 1) by a master ceramist (SST Dental Lab, Segrate, Italy). Proximal contact points and occlusal contacts were adjusted as necessary for maximum intercuspation and to avoid interference from the lateral excursions. The abutment teeth or implants were cleaned prior to cementa- tion. For final cementation, a glass-ionomer cement was used (Ketac, 3M ESPE AG, Seefeld, Germany). A final control step after cementation was performed using a surface electromyographic (EMG) analysis under static conditions to verify the equilibrium of the neuromus- cular system.21 As described by Ferrario et al.,21 the right and left side masseters and the temporalis anterior muscles of each patient were analyzed during maximum voluntary jaw clenching. From the EMG standardized potentials, the percentage overlapping coefficient (POC), an index of neuromuscular symmetry, was computed. For all of the patients, POC indices greater than 85% were found, which corresponded to the values previously reported for patients with good neuromuscular equilibrium.21 Data collection and analysis Following the EMG control analysis, the patients were scheduled for oral-hygiene appointments at six-month inter- vals and were asked to contact the clinic if they experienced any problems with their prostheses. Additionally, the patients were examined by a dentist for follow-up at 12, 24, and 36 months after the procedure and before their scheduled oral hygiene appointments. These recall appointments were standard for patients at this private practice. For each patient, data were collected regarding the patient’s sex, age at the time of crown placement, number of crowns cemented, and the tooth or implant position. Also, clinical findings, including fracture or loss of retention Figure 1 - Three-unit, all-ceramic prostheses for 14-16 rehabilita- tion crowns for implants. The zirconia core was surrounded by feldspathic ceramic. 2065 Clinical performance of zirconia crowns Tartaglia GM et al. of the prosthesis, secondary caries, marginal integrity, wear or surface roughness, and aesthetic characteristics, were recorded. The overall failure rate was computed for fractured and lost prostheses. In accordance with a study by Hickel et al.,22,23 the aesthetic, functional and biological properties of the analyzed crowns were rated. The aesthetic properties examined included surface luster and staining, color stability and translucency, and anatomic form. The func- tional properties that were scored included the existence of fractures, retention, marginal adaptation, contact points and their impact on chewing food, and radiographic aspects. The biological properties scored postoperatively included sensitivity of the endodontically treated teeth, loss of osseointegration for the implants, the recurrence of caries, erosion, abfraction, periodontal and mucosal aspects of the restored teeth, and oral and general health. The overall clinical evaluation was stratified according to the following five levels: clinically excellent/very good (A); clinically good (B); clinically sufficient/satisfactory (minor shortcomings without unacceptable defects but not adjustable without damaging the tooth) (C); clinically unsatisfactory but reparable (D); and clinically poor (necessary replacement of the prosthesis) (E). The overall aesthetic, functional, and biological scores were also compared across the acceptable (excellent, good, and sufficient scores) and the unacceptable (unsatisfactory and poor scores) categories. Patient satisfaction regarding both the aesthetics and the function of the zirconia prostheses was assessed using a questionnaire with five possible rating categories: (A) excellent/very good, the patient was entirely satisfied, and the prosthesis could not be detected with the tongue; (B) good, the patient was entirely satisfied, but the prosthesis could be detected with the tongue; (C) sufficient/satisfac- tory, aesthetic shortcomings and/or discomfort during chewing were reported, but a replacement was not necessary; (D) poor, the patient requested an improvement to be made to the prosthesis; and (E) unsatisfactory, the patient was completely dissatisfied and required a new prosthesis but declined replacements of the same type or made of the same material.22,23 Different groups of dentists comprised the evaluator and operator groups. A training session for the evaluators occurred prior to scoring. Inter-examiner agreement for the examined characteristics was 92% at the beginning of the study, 90% at 12 months, and 88% at 36 months. The clinical (aesthetic, functional, and biological proper- ties) scores and the patient satisfaction scores obtained at the one- and three-year recall visits were compared using the Wilcoxon signed-rank test, and these were examined separately for the single- and multiple-unit crowns and for the implant- and tooth-supported crowns. The significance level was set to 5% (p,0.05). RESULTS A total of 238 single crowns were installed in 108 patients (56 women and 52 men). Of these, 202 tooth-supported single crowns were installed in 96 patients (51 women and 45 men), and 36 implant-supported single crowns were installed in 19 patients (9 women and 10 men) (Table 1). A total of 225 multiple-unit crowns (up to six elements) were installed in 34 patients (24 women and 10 men). Of