Yield-point detection can improve the mechanical performance of cancellous bone screws

Abstract

Appropriate torque applied to screws is an insurance for strong and durable fracture fixation. We compared the mechanical characteristics of four types of bone screws during insertion to facilitate the development of an optimal screw torquing strategy. Bone screws were inserted into synthetic cancellous bone blocks. For each group, five screw–block constructs were subjected to a tightening test. The yield point was determined by calculating and comparing the torque and compression. Two screws were selected based on the torque–rotation curve, and axial pull-out tests were conducted under five different rotation conditions. The pull-out strength and compression were used to identify the screw insertion state and estimate the mechanical characteristics of the screws at different tightening angles. The torque and axial compression increased concomitantly during screw torquing. The torque–rotation slopes of the different screws varied significantly. The yield-to-stripping torque ratio ranged from 61% to 72%, and the yield-to-peak axial compression ratio ranged from 61% to 78%. The maximum pull-out strength occurred at the yield point and was significantly higher than that at a stripping angle of 90% (27.51% and 30.35%; p < 0.05). These outcomes demonstrate the viability and broader significance of yield-point detection.