Biomechanical effects of USS fixation with different screw insertion depths on the vertebrae stiffness and screw stress for the treatment of the L1 fracture.

Abstract

To evaluate the biomechanical effects of internal fixation with different screw insertion depths on vertebrae stiffness and screw stress for L1 fracture. The established L1 fracture was fixed with 10 different depths of screw insertion: 10-100% screw-path length (SPL). Loading on the T12 endplate was simulated. Screws inserted to 60-100% depths has a higher axial displacement of screw against injured vertebrae and maximum stress of screws compared to those of screws inserted to 30-50% depths and 10-20% (P< 0.05). No significant difference was noted among 60-100% SPL groups. Under single loading condition, the incidence rate of maximum stress of each screw ranged from 16.7-50.0%. Chi-square test showed superior screw has a higher incidence rate of maximum stress than inferior screw (P< 0.05). Screws inserted to 60% depth or more can achieve effective strength to withstand the postoperative height correction loss of the L1 vertebrae fracture. However, continuous prolonged depth of screw insertion did not significantly increase the effective strength of the screw against injured vertebrae and maximum equivalent stress of screws. The incidence rate of the maximum stress of each screw in correlated with position of screw insertion but not associated with the screw insertion depth.