Comparison of screw loosening and fracture resistance in different hybrid abutment crown restorations after thermomechanical aging.

Abstract

The purpose of the study is to investigate the screw loosening and fracture resistance of different hybrid abutment crown restorations after thermomechanical aging. Restorations were produced from zirconia, lithium disilicate, and hybrid ceramics with CAD-CAM system (n=10). Restorations and titanium bases (Tibases) were cemented and the abutment screws were torqued to 30 Ncm. They were retorqued after 10 minutes, and removal torque values (RTV) were measured. When 250000 and 500000 cycles of thermomechanical aging were completed, the RTVs were measured again and removal torque loss (RTL) ratios were calculated. The specimens were subjected to fracture resistance test after 750000 cycles of thermomechanical aging. The load at failure was recorded as the fracture resistance (N). The results were statistically analyzed (α=0.05). Aging was effective on the RTVs and RTL ratios (P<0.001). The highest RTV was observed before aging; however, the RTVs significantly decreased and RTL ratios significantly increased after aging (P<0.05). The fracture resistance values significantly differed among the ceramic materials. All lithium disilicate and hybrid ceramic specimens showed restoration fracture, while most of the zirconia specimens were displayed Tibase related failure. Hybrid abutment crowns made of zirconia, lithium disilicate, and hybrid ceramics can withstand the average occlusal forces in the molar region; however ceramic type may affect the failure type. Retightening of the abutment screws after 10 minutes after first tightening and annually may be beneficial to prevent the adverse effects of screw loosening on the integrity of implant-abutment connection.