EFFECT OF ROD DIAMETER ON BIOMECHANICS OF POSTERIOR LATERAL MASS FIXATION IN CERVICAL SPINE: AN IN VITRO AND IN SILICO STUDY

Abstract

Spinal fusion is a common surgical method often performed for the treatment of various disorders associated with the spine, including degenerative spondylosis, spondylolisthesis, secondary to trauma, or other clinical causes of spinal instability. In the cervical spine, posterior fixation and fusion using rods, screws, plates, hooks, etc. have been the most common method in re-stabilizing the spine following injury.(1, 2) Several studies have shown the clinical and biomechanical advantages of pedicle screw fixation over other internal fixation approaches in the cervical spine.(3, 4) While using pedicle screws for stabilization of the spine is very common in thoracic and lumbar spine regions, it has been considered risky in the cervical region due to the surrounding neurovascular structures, except at C2 and C7 levels.(5-7) Mal-positioning of the pedicle screws into cervical vertebrae can result in injury to the vertebral artery and nerve root.(8, 9) The risk of screw mal-positioning can be significantly high when the screws are positioned by freehand technique in patients suffering from complicated anatomical conditions due to rheumatoid arthritis or spondylosis.(10) To overcome the limitations of posterior fixation through the pedicle, modified screw insertion techniques have been developed over the years.(11, 12) One of such techniques is the insertion of the screw through the lateral mass in the cervical spine.(2, 13-16). While several studies have looked at the methods for optimizing the efficacy of lateral mass fixation through modifications in the screw length and trajectory (17-20), no study to the author’s knowledge has investigated the effect of fixation rod on the biomechanics of the lateral mass construct. This laboratory-based research was intended to study the biomechanical effect of fixation rod diameter on the stability of the cervical spine and load sharing on the components of the fixation constructs in lateral mass fixation technique. To do this, first, a cadaveric experiment was conducted using thirteen human cervical spine specimens. The kinematics of the spine was evaluated and compared between the spine in intact condition and instrumented with screw, rod lateral mass fixation across C3-C6. The experiments were repeated for rods with diameters of 3.5mm, 4.0mm, and 4.5mm. Second, the cadaveric data were used to validate the finite element (FE) model of C2-C7 spine. The model was then used for simulation of the same surgical cases as in the cadaveric experiment and the load sharing on the hardware was calculated and compared among the cases.