Influence of abutment-to-fixture design on reliability and failure mode of all-ceramic crown systems.

Objective: Evaluate the effect of implant connection designs on reliability and failure modes of screw-retained all-ceramic crowns.

Methods: Central incisor ceramic crowns in zirconia abutments were screwed and torqued down to external hexagon (EH), internal hexagon (IH) and Morse taper (MT) implant systems. Single-load-to-fracture (SLF) test (n=4 per group) determined three step-stress fatigue profiles with specimens assigned in the ratio of 3:2:1. Fatigue test was performed under water at 10 Hz. Use level probability Weibull curves and reliability for missions of 50,000 cycles at 400 N and 200 N were calculated (90% confidence bounds-CB). Weibull probability distribution (90% CB) was plotted (Weibull modulus vs characteristic strength) for comparison between the groups. Fractographic analyses were conducted under polarized-light microscopy and SEM.

Results: Use level Weibull probability calculation indicated that failure was not associated with fatigue in groups EH (β=0.63), IH (β=0.97) and MT (β=0.19). Reliability data for a mission of 50,000 cycles at 400 N revealed significant reliability differences between groups EH (97%), IH (46%) and MT (0.5%) but no significant difference at 200 N between EH (100%) and IH (98%), and IH and MT (89%). Weibull strength distribution (figure) revealed β=13.1/η=561.8 for EH, β=5.8/η=513.4 for IH and β=5.3/η=333.2 for MT. Groups EH and IH exhibited veneer cohesive and adhesive failures. Group IH also presented adhesive failure at zirconia/titanium abutment insert while MT showed fracture at abutment neck.

Significance: Although group EH presented higher reliability and characteristic strength followed by IH and MT, all groups withstood reported mean anterior loads.