Effects of Different Screw Head Designs on Screw Head Slippage

• Main Authors: Chen-huei Lin, 林承輝
• Other Authors: Ching-Kong Chao
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/80191874644968684945

碩士 === 國立臺灣科技大學 === 機械工程系 === 102 === Locking plates have widely been used to treat different kinds of fractures in the body during surgical operation, for it can strengthen the bone-plate structures to make the fixation more rigid. However, the screw head slippage happened frequently during insertion and removal of the screws. Slippage occurs when a screw cannot be driven since the user’s power or torque is not transmitted properly and is commonly seen in small size hexagonal screws made of titanium. There have been no studies with comparisons of different kinds of screw head designs on the slippage of screw head. Therefore, the purpose of this study is to investigate the effects of a variety screw head designs on screw slippage. In this study, four types of screw head designs were investigated, including Hexagonal, Torx, Cross, Iron Cross using Finite Element Method (FEM) and Biomechanical experimental testing to validate the results in order to evaluate which type of screw head produces better torque to avoid screw slippage under the condition of cutting the same area and volume. Moreover, the effect on using different angle of twisting screw driver during the insertion and removal of screw was also observed. In this study, Titanium and SUS420 stainless steel were utilized in the manufacturing of screw head and screw driver, and torques of each designs were measured and calculated after torsion tests were applied to all specimens for up to 50 degrees rotation. The result of FEM reveals that Iron Cross screw head has the best performance at the screw driver rotated 0.1 degrees to the screw head, then Cross, Torx , and Hexagonal screw head . The results show that Iron cross provides for up to 5 times higher moment than Hexagonal. However, the mechanical testing shows that Hexagonal has the best performance, then Torx, and Iron Cross. The moment Hexagonal has provided is 2 times higher than that of Cross. The differences between numerical analysis and biomechanical experimental tests was caused by not considering the torsion of screw driver in numerical computation, and the dimensions of Iron Cross cannot be manufactured exactly the same as defined in numerical model. During the experiment , we also see that in different angle and the driver is not all engage to the screw head can lead Hexagonal slippage. Although Hexagonal screw head can produce highest torque among all the other, its shape approaches to circle as the screw size gets smaller. Having screw driver being used for a couple of period, wear and tear could cause inefficient force transmission, which eventually bring about slippage easier during different angle of insertion and removal of screws. There are difficulties simulating the slippage of screw head with FEM, and can only be determined by experiment. In the future, this study could be extended to reduction and avoiding slippage of screw head.