2022, Vol. 26 ›› Issue (18): 2789-2794

Biomechanical properties of traditional trajectory screw technique and modified cortical bone trajectory technique: a finite element analysis

Zhou Zhihao1, Alafate·Kahaer1, Wang Yixi2, Liu Dongshan1, Xieraili·Maimaiti1, Shi Wenjie2, Paerhati·Rexiti1   

Abstract: BACKGROUND: Based on the shortcomings of the existing cortical bone trajectory, we proposed a new screw trajectory of modified cortical bone trajectory, but it is not clear how the actual mechanical performance of the modified cortical bone trajectory is.   
OBJECTIVE: To compare the biomechanical difference of screws between modified cortical bone trajectory and traditional trajectory in osteoporotic lumbar spine by finite element method.
METHODS:   CT scan data of four human osteoporosis specimens were obtained. The finite element model of L4 vertebral body was established, and the screw model was created. The screws with specifications (diameter=4.5, 5.0, 5.5, 6.0, length=40 mm) were selected for modified cortical bone trajectory, and (diameter=6.0, 6.5, length=45 mm) for traditional trajectory. Firstly, the safety of screws was tested in turn. Those who caused damage to the cortical bone of pedicle during screw placement were excluded. After determining the safe diameter, the pull-out strength, screw stability, and vertebral body stability were analyzed.   
RESULTS AND CONCLUSION: (1) Screw safety test: When traditional trajectory (diameter=6.5 mm) and modified cortical bone trajectory (diameter=6.0 mm) were implanted, there was a risk of bone destruction, and there was no risk for other diameter screws. Therefore, the above two diameter screws were excluded from subsequent tests. The safety diameter of traditional trajectory was 6.0 mm, and that of modified cortical bone trajectory was 4.5, 5.0, and 5.5 mm. (2) Pullout force: The force of traditional trajectory (diameter=6.0 mm) was lower than that of modified cortical bone trajectory (diameter=5.5, 5.0, 4.5 mm) (P < 0.05). (3) Screw stability: The load-displacement ratio of modified cortical bone trajectory (diameter=5.5, 5.0 mm) in the upper, down, left and right directions was greater than that of traditional trajectory (diameter=6.0 mm) (P < 0.05). (4) Vertebral stability: The load-displacement ratio of modified cortical bone trajectory (diameter=5.5 mm) was greater than that of traditional trajectory (diameter=6.0 mm), and the load-displacement ratio of modified cortical bone trajectory (diameter=5.0 mm) under flexion and extension conditions was greater than that of traditional trajectory (diameter=6.0 mm) (P < 0.05). (5) Results have verified that in L4 vertebral body, considering the safety and mechanical properties of the screw, modified cortical bone trajectory (diameter=5.0 mm) is an ideal common diameter choice.
Key words: osteoporosis, lumbar spine, finite element analysis, modified cortical bone trajectory technique, biomechanics, traditional trajectory screw technique