The Effect of Fatigue Loading on the Biomechanical Characteristics of Thoracic Cortical Screw

• Main Authors: Ru-Yi Chang, 張如意
• Other Authors: 王兆麟
• Format: Others
• Language: zh-TW
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/8e8239

碩士 === 國立臺灣大學 === 醫學工程學研究所 === 106 === Objective: To compare the fixation strengths of pedicle screws inserted via the cortical bone trajectory and the traditional trajectory, and to investigate the effect of fatigue loading on fixation strength. Introduction: Cortical bone trajectory is a relatively new technique for pedicle screw insertion. The cortical bone trajectory aims to improve fixation strength by increasing the contact between the screw and the denser cortical bone. Many papers studied the biomechanical characteristics of insertion using the cortical bone trajectory. However, the effect of fatigue loading on fixation strength of the cortical bone trajectory has received comparatively less attention. This study investigated the fixation strengths of thoracic cortical bone trajectory and traditional trajectory with the use of cortical pedicle screw and traditional pedicle screw by human cadaveric biomechanical tests both before and after fatigue loading. Materials and methods: Thirty-nine human cadaveric thoracic spine vertebrae (T7-T12) were used. The mean bone mineral density (BMD) of the specimens was 0.961 g/cm2 (range: 0.634- 1.273 g/cm2). The experiment consisted of two groups. In both groups, the cortical bone trajectory and traditional trajectory were used in the left and right pedicles of each vertebra, respectively. Group 1 used traditional poly-axial pedicle screws (4.35 mm x 35 mm) in both trajectories. Group 2 used traditional poly-axial pedicle screws (5.0mm x 40 mm) in the traditional trajectory and cortical screws (5.0mm x 40 mm) in the cortical bone trajectory. All specimens were randomly distributed into the control group and fatigue group. The peak-to-peak force and loading frequency of fatigue loading were 10-100 N and 1 Hz. The loading was vertically applied at the screw end to simulate the axial body force. The displacement history along the loading line during fatigue loading were recorded. The increase of the displacement at the end of fatigue loading was defined as the "fatigue damage". The "fatigue damage" was further divided into "stiffness damage" and "creep damage". From the force and displacement curve of the pullout test, the "pullout strength" and "pullout stiffness" were calculated. Results: The results of fatigue loading showed; both group 1 and group 2 exhibited less fatigue damage in the cortical bone trajectory than in the traditional trajectory (group 1: 0.29(0.17) mm vs. 0.49(0.34) mm, p=0.061; group 2: 0.27(0.11) mm vs. 0.37(0.11) mm, p=0.008). Before the fatigue loading, the pullout strength of the cortical bone trajectory was lower than that of the traditional trajectory in group 1 (438(188) N vs. 536(226) N, p=0.061), and the pullout strength of the cortical bone trajectory was greater than that of the traditional trajectory in group 2 (799(369) N vs. 683(210) N, p=0.092). After 10,000 cycles fatigue loading, no difference in pullout strength was observed between the cortical bone trajectory and the traditional trajectory within either group 1 or group 2 (group 1: 337(122) N vs. 394(103) N, p=0.193；group 2: 765(465) N vs. 735(397) N, p=0.446). Conclusion: The results of this study showed that the use of traditional screw with traditional trajectory reached better strength than the traditional screw with cortical bone trajectory before fatigue loading; hence it is not suggested to use the traditional screw in the cortical bone trajectory. The pull out strength of cortical screw with cortical bone trajectory was better than the one of traditional screw with traditional trajectory before fatigue loading. However, we found both screw types at both trajectories reached similar fixation strength after fatigue loading. This may imply that the surgeon may choose the screw type and trajectory at their convenience if the fixation strength at the acute stage may not be a critical issue.