The biomechanical effect of screw size applied to the level of fracture in posterior segmental instrumentation

Introduction and Purpose: In the treatment of thoracolumbar junction fractures using short segment posterior segmental instrumentation, the pedicle screw applied to the fracture level significantly enhances biomechanical and clinical stability. There is no clear information in the literature about the ideal screw size applied to the fracture level. The aim of this study is to evaluate the effect of the pedicle screw size placed on the fractured vertebra on biomechanical stability in short segment posterior instrumentation applied in the surgical treatment of thoracolumbar vertebral fractures. Material and Method: 21 fresh sheep cadaver thoracolumbar spines were used in the study. 21 cadavers were randomly selected and divided into 3 groups. An experimental unstable thoracolumbar fracture model was created with the falling mass model at the L1 level. Each sample was cast proximally and distally to preserve the T12-L2 level. Short segment posterior segmental instrumentation was applied in 3 different ways. Separate groups comprised the study: in the 1st group, without intermediate screws, in the 2nd group, with an intermediate screw of a length not exceeding 50% of the vertebral body length (20 mm), and in the 3rd group, with an intermediate screw of a length not exceeding 70% of the vertebral body length (30 mm). Axial neutral loading, flexion/extension, lateral bending and axial rotation tests were applied to each fixed sample with an electromechanical actuator (10 kN AG-X; Shimadzu, Kyoto, Japan) and the results were recorded. Shapiro-Wilk test to evaluate the distribution of data; Kruskal Wallis test and Mann Whitney U test in independent groups and Wilcoxon Signed Rank test in dependent groups was used. Results: It was observed that screw placement in L1 after unstable burst fractures significantly improved the stability of the system in axial loading, flexion/extension, lateral bending and axial rotation directions compared to the traditional short segment posterior instrumentation technique. No statistically significant difference was observed between the two systems in the axial load, flexion/extension, lateral bending and axial rotation directions of the short or long pedicle screw added to L1 and the short segment posterior instrument (G1