Biomechanical analysis of the effect of ‘intermediate screws’ in short segment posterior fixation of unstable burst fractures of thoracolumbar spine in calf spine model

Abstract

Background: Efforts in preserving motion levels in unstable thoracolumbar burst fractures steered to short segment fixation. However, short segment spanning fixation in clinical scenario reported high failure rates. Augmentation of spanning fixation by inserting intermediate screws into the fracture level is proposed to enhance stability. An experimental comparative study was performed to assess the biomechanical role of the ‘intermediate screws.’ Methods: Five calf spine specimens were freshly prepared to record the biomechanical characteristics, range of motion (ROM), and stiffness. CT scan confirmed an unstable burst fracture in each specimen. Each specimen was instrumented with short-segment posterior fixation with an intermediate screw. The same test protocols were repeated with and without intermediate screws. Results: Intermediate screws contribute to 20.2%, 16.5%, 14.5% and 23% decrease in ROM and 15.4%, 25.6%, 48.3%, and 160.2% increase in construct stiffness. Conclusions: Intermediate screws significantly increase the construct stiffness and decrease the ROM.