Implications of spine fixation on the adjacent lumbar levels for surgical treatment of thoracolumbar burst fractures: a finite element analysis

Abstract

Surgical correction with corpectomy and subsequent fusion of neighboring levels is a commonly practiced treatment for thoracolumbar burst fractures. Different fixation constructs are currently implemented to fuse the injured level. However, little is known about the implications of such constructs on the biomechanics of the adjacent spine segments. The objective of this study is to compare the biomechanical alterations at the adjacent segments due to implanted construct. A 3D nonlinear finite element model of the thoracolumbar spine (from T12 to S1) was used for assessing changes in the biomechanics of the adjacent spine segments. First, the vertebral body and the adjacent discs of the operated level (L1) were completely excised leaving only the posterior elements. Next, four constructs were implanted at the thoracolumbar junction (T12-L2): (1) 2RPC which includes two posterior rods, six pedicle screws, an expandable cage, and a transverse plate; (2) 1RPC which includes the same instrumentation of 2RPC with the exclusion of the posterior rod contralateral to the transverse plate; (3) 2RC which includes posterior rods, pedicle screws, and an expandable cage; (4) PC which includes the expandable cage and the transverse plate. Models were validated by comparing their predicted range of motion (ROM) to in vitro biomechanical tests. Subsequently, the stresses generated at all the adjacent levels (L2-L3, L3-L4, L4-L5, and L5-S1) were measured and compared. The results showed that the constructs including pedicle screws (2RPC, 1RPC, 2RC) yielded similar biomechanical performance being the stiffest. Moreover, the same constructs demonstrated comparable stress levels at the adjacent segments, generating 50% higher than that produced from the PC construct. Elevated mechanical stress is believed to be a cause of disc degeneration. Accordingly, the PC construct, although less reliable in stabilizing the injured segment, represented the most conservative approach for preventing potential development of adjacent disc degeneration. Introduction