CLINICS 2011;66(12):2063-2070 Figure 2 - Minor chipping of the anterior zirconia prosthesis on tooth 11. teeth. After a two year follow-up period, Vult von Steyern et al.11 reported minor chip fractures in 15% of their three- to five-unit posterior prostheses. After a three year follow-up period, Sailer et al.7 reported that four out of 33 three- to five-unit posterior prostheses needed to be replaced due to technical problems (poor cementation, fracture, or porcelain chipping). From a four-year follow-up study, Wolfart et al.12 reported a 12 to 13% complication rate due to technical problems. In the current study, 12 units lost cementation, but each could be re-cemented, as seen in previous studies.12 In comparison to previously published reports, the current study observed a lower incidence of veneer chipping and fractures. Several technical and clinical aspects of the study may have contributed to this success rate. Sailer et al.7 emphasized the need for refined zirconia veneers, as surfaces made with crystalline ceramics have a higher fracture rate than do those made with feldspathic ceramics. For the patients in this study, feldspathic porcelain was fused to solid zirconia prostheses, which Figure 3 - Poor gingival response at the 1-year follow-up. 2069 Clinical performance of zirconia crowns Tartaglia GM et al. reduced the likelihood of this occurrence. Additionally, each of the crowns in this study had a sufficient ceramic thickness.15 From a clinical point of view, it was critical that occlusal reduction during preparation be meticulously verified, as suggested by previous investigations.15 In addition, no occlusal adjustments were deemed necessary,15 which may have been related to the proper functioning of the prostheses, as the surface EMG analysis found that all patients had good neuromuscular equilibrium and that the new occlusal conditions had been properly incorporated into the stomatognathic system.21 A strict relationship between neuromuscular equilibrium and variations in occlusal characteristics has been previously described.24,25 For 81% of the crowns analyzed by this study, the anatomical form was scored as sufficient or satisfactory, although no cases were scored as excellent. To avoid fractures in the zirconia prostheses, the occlusion adjust- ments were made after cementation and during maximum voluntary clench. Evidently, minimum and direct occlusal reshaping influenced this sub-category score. In contrast, Ortorp et al.16 reported excellent anatomical form for each of the 25 single crowns in their study (11% of the original group) after three years. Regarding the scores related to the biological properties of the crowns, 57 to 58% of the crowns had a sufficient or satisfactory periodontal response and a similar response regarding the adjacent mucosa (as compared to adjacent, unrestored teeth), which is in accord with the results of prior reports.5,12,16 In previous investigations, the fit of the prosthetic framework was only rated as sufficient, and further improvements in marginal accuracy were deemed necessary.7,15 Sailer et al.7 stated that the lack of marginal accuracy explained the secondary caries that they observed. In our patients, no secondary caries were found; however, marginal accuracy also influences the mucosal response and thus may explain the current findings. This result partially contrasts with data reported by Vult von Steyern et al.11 and C¸ ehreli et al.8 that demonstrated excellent marginal integrity in 80% of the crowns after a one- or two-year follow-up period. The different scores that were obtained for marginal adaptation and periodontal response may be explained by the technical procedure itself, as several patients may have experienced invasion of the biological dental/mucosal width. After analyzing these results, the procedure was modified, and we expect that patients treated with the new, modified protocol will experience improved periodontal responses. Furthermore, customized oral hygiene mainte- nance protocols could be developed with the dental hygienist. The lack of secondary caries in our patients was in contrast with previous studies.5,7 Indeed, Hickel et al.22 suggested that some clinical studies may have overstated the incidence of secondary caries. In fact, the diagnosis of recurrent caries can be made from the clinical point of view only, and the marginal discoloration that is often considered a sign of caries may instead represent a stained restoration margin. Unfortunately, only histological assessments can confirm this clinical diagnosis. Overall, the cumulative three-year prosthetic survival rate observed in this study is in agreement with the litera- ture5,7,12 and is even greater than that reported from investigations where only tooth-supported single crowns were examined.15,16 No differences were observed for either