EVALUATION OF POSSIBLE FAILURE MECHANISMS OF SHORT SEGMENT FIXATION IN UNSTABLE BURST FRACTURES OF THORACOLUMBAR SPINE IN ANIMAL MODEL

Background - Burst fractures are common cause of disability among the affected young population and instrumented surgical stabilisation remains the treatment of choice for unstable burst fractures of the vertebra. Circumferential fixation and short segment posterior fixation with intermediate screws inserted into the fractured level are the commonly performed surgical stabilisation procedures. But, decreasing the instrumented level in order to preserve the number of motion levels may adversely affect the stability of the construct and can ultimately result in implant failure. Aim - To evaluate the possible mechanisms of failure of the short segment fixation methods in experimentally induced unstable burst fractures of the thoracolumbar spine in calf spine model. Materials and Methods - Eight fresh frozen calf spines specimens were prepared and grouped into two Group A and Group B. After creating an unstable burst fracture by drop weight method at the first lumbar vertebra level, Group A specimens were instrumented using short segment posterior fixation with screws inserted to the fractured level while the specimens in Group B were instrumented using circumferential fixation. The specimens were then subjected to biomechanical testing in a universal testing device (Tinius, Oslon) for failure in axial load and axial rotation. Results - All four specimens in the circumferential fixation group (Group B) failed in axial torque by screw pull out. No failure was observed during axial loading. None of the specimens in the Group A failed in either axial torque or loading. Conclusions - Our findings show that circumferential stabilization constructs can fail in axial torque, and therefore may require additional protection in the form of bracing in the immediate postoperative period. The use of the intermediate screw technique may provide superior stability in axial torque that the circumferential technique.