Clinical performance of metal-free polymer crowns after 3 years in service.

Objective: To investigate the influence of location and preparation design on the survival and complication rate of metal-free polymer crowns within a prospective clinical study.
Methods: After randomization, a total of 114 (68 posterior and 46 anterior) single crowns were prepared, either with a chamfer finishing line or with a shoulder combined with occlusal reduction of at least 1 mm. Build-ups were made with a composite material, using the corresponding dentin adhesive. After making impressions with polyether material, polymer crowns were manufactured on stone dies and adhesively luted with resin cement. Follow-ups were scheduled after 1 month, 1 year, 2 years and 3 years. Documentation included failures and other complications, as well as ratings of esthetic and functional performance. After 3 years, data from 100 single crowns were statistically evaluated.
Results: Within a minimal observation period of 3 years, 10 complications occurred, including 3 total fractures, 3 partial fractures and 3 decementations. Only 4 crowns (3 total fractures +1 partial fracture) had to be replaced, whereas 2 partial fractures could be repaired and all loosened crowns could be recemented. Kaplan-Meier survival analysis did not show any influence of location or preparation design on the complication rate. Esthetic and functional evaluation by the patients revealed a high acceptance of single polymer crowns, indicated by medians of 9-10 on a 10-point-scale.
Conclusion: Within the 3 year observation period, it can be concluded that 0.5 mm chamfer and shoulder preparation ensure that the stability of metal-free polymer crowns for anterior and posterior teeth is acceptable. Long-term stability and wear behavior has still to be evaluated.