Comparison of biomechanical behavior between flat head screw and tapered head screw in internal-hex implant-abutment connection

This investigation aimed to evaluate the screw head configuration on its biomechanical performance. Forty-eight screws specimens with four different screw head conical surface opening angle (β) were divided in four groups (n = 12): flat screw head (FHS) with β = 0°, and tapered screw head (THS) with β = 20°, 30°, and 40°. A preload torque of 35 N·cm was applied to fasten the implant-abutment assembly. The initial removal torque values (RTVs) of half the samples in each group were measured. The remaining specimens were then subjected to fatigue tests and the postload RTVs were measured. Furthermore, four 3D prosthesis models were established for FE analysis. The results suggested that the initial and postload RTVs of FHS groups were significantly lower than THS groups (P < .05). After mechanical cycling, the original regular raised texture features on screw head surface in FHS group were almost removed and transferred, while linear grooves appeared near the bottom of the screw head cone in THS group. The FEA results indicated that the highest stress magnitude and maximum torsion angle of the screws occurred in the FHS group. In contrast, the flat-head screw exhibited larger torque loss, greater stress concentration and more severe friction and wear.