Clinical performance of zirconia crowns Tartaglia GM et al. Table 1 - Distribution of the single- and multiple-unit zirconia crowns on teeth and on implants by region. Anterior Posterior Total Single crowns Multiple-unit crowns Teeth 33 159 202 Implants 5 31 36 Teeth 27 54 81 Implants 26 118 144 these, 81 tooth-supported zirconia crowns were placed in 14 patients (8 women and 6 men), and 144 implant-supported crowns were placed in 29 patients (13 women and 16 men). There were two men and four women who received both single- and multiple-unit tooth-supported restorations, four men who received both single- and multiple-unit implant- supported restorations, and nine men and 11 women who received both tooth- and implant-supported zirconia restorations in various combinations. Overall, a total of 463 single or multiple-unit crowns (up to six elements) were installed in 142 patients (67 women and 75 men). Most of the crowns were placed in the premolar to molar area (n = 362; 78%), and 59% of the crowns were tooth- supported (n = 213). Fifty-one percent of the crowns were positioned in 68 women, and there was an average of 3.5 crowns per patient (with a maximum of 21 in a 51-year-old patient). For male patients, there were an average three crowns cemented per patient (with a maximum of 20 in a 66-year-old patient). All single crowns and bridges were in use at the recall appointments one- and two-years post-implantation. Of the 142 patients who had received zirconia crowns three years prior to the recall appointment, four patients had been dropped from the study for various reasons (deceased, no contact, or moved), and these patients represented a total of 18 crowns (4% of the total crowns). The crowns that were deemed lost to follow-up and the cumulative survival rate (CSR) are presented in Table 2. During the follow-up period, no core material in the single-, double-, or triple-unit crowns fractured. In three of the zirconia prostheses that had more than three units, we observed fractures at the interdental connectors or at the zirconia core near the connectors. There were 12 tooth- supported units that lost retention, and these included one 5-unit prosthesis, one 4-unit prosthesis, and three single- tooth prostheses. Each of these was able to be re-cemented. No secondary caries of the abutment teeth were reported. In all patients, the zirconia crowns were occluded against teeth or fixed dental prostheses (Tables 3, 4). At each of the recall visits (one, two, and three years post- procedure), the aesthetic, functional, and biological proper- ties received high ratings for almost all of the crowns examined, and the ratings were equivalent for both the tooth- and implant-supported crowns. Also, no patients Table 2 - Crowns lost to follow-up and the cumulative survival rate (CSR). Examined crowns Lost to follow-up Failed CSR (%) Time Crown cementation 1 year 3 years 463 463 445 0 0 18 0 0 11 100 100 98.2 2066 CLINICS 2011;66(12):2063-2070 reported symptoms related to periapical periodontitis during the follow-up period. Patient satisfaction with the zirconia crowns was also generally high. From a clinical point of view, the lowest scores were obtained in the aesthetic category and were related particularly to the anatomical form of the crowns and the existence of some minor chipping (Figure 2). Relatively low scores were also obtained in regards to the biological properties (for the sub-categories of periodon- tal response and adjacent mucosa) as more than half of the crowns received only sufficient/satisfactory scores (Figure 3). The scores were generally higher for crowns on anterior teeth and for single crowns. The overall aesthetic and biological clinical scores for the analyzed zirconia prostheses were universally acceptable (445 crowns, 100%). In general, the aesthetic and biological ratings did not change during the period of analysis. In contrast, the overall functional score was not acceptable for 23 crowns (5.2%) due to fractures or a loss of retention (most were multiple-unit prostheses on posterior teeth). These scores were significantly higher at the 12-month follow-up visits than at the 36-month follow-up visits for both implant- and tooth-supported crowns (Wilcoxon signed-rank test, implant-supported, p,0.001; tooth-supported, p = 0.002). DISCUSSION In this clinical follow-up study, a three-year follow-up analysis was performed to assess more than 400 zirconia core crowns supported either by teeth or implants. The number of prostheses included in this study was larger than most previously published studies; for example, this current study was approximately two to four times larger than those reported by Ortorp et al.16 and Encke et al.15 The number of tooth-supported prostheses (283) was compar- able to that reviewed by Schley et al.5 To the best of our knowledge, this is the first investigation to analyze the long-term outcomes of implant-supported zirconia prostheses. Furthermore, in previous studies, only zirconia prostheses with up to five elements were long- itudinally followed, whereas we included several six- element units that were both implant- and tooth-supported. For example, one prior laboratory investigation that tested zirconia prostheses with only as many as five units found that these can withstand main chewing and clenching stresses.3 Furthermoe, only the study from Papaspyridakos and Lal10 described a 12-unit mandibular prosthesis, although this study did not examine longitudinal data regarding their rehabilitation performance. In this private-practice study, the loss of patients at the time of follow-up was minimal, as only 4% of the prostheses were unavailable for analysis at the three-year recall visit. Previous studies have documented much larger drop-out frequencies, including 10%,5 13%,15 17%,16 and 24%.7 The higher participation rate of our patients is in line with their high satisfaction towards the zirconia core prostheses.7,16 Perhaps the inclusion of patients from a private practice where the patient-dentist relationship is influenced by monetary considerations contributes to general satisfaction. According to the results of our study, only 11 crowns fractured during the time period analyzed, and each of these fracture events occurred between the two- and three- year follow-up visits. As expected, the prostheses with a