Six vs Four Screws: A Biomechanical Analysis of Short Segment Fixation of the Thoracolumbar Spine

Methods: An L1 corpectomy was performed on nine fresh human spines to remove all anterior column support. The specimens were instrumented with 6.25mm pedicle screws connected to quarter inch rods using solid slotted connectors. The spines were tested with and without L1 pedicle screws and then tested with and without a threaded transverse connector. The spines were cyclically loaded to 5 N-m in flexion / extension and axial rotation with an additional 400 N follower preload. Selspot LED's attached to T12 and L2 monitored the movements of the vertebral bodies. Paired t-test was utilized to analyze the differences between the constructs.

Results: The additional screws at L1 significantly stiffened the construct in flexion and torsion. With an additional screw, stiffness of the constructs increased by an average of 9% and 11%, respectively, for flexion (p=0.02) and torsion (p=0.03). With the addition of a transverse connector, the torsional rigidity of the constructs increased by an average of 16% for the 6 screw construct (p=0.006).

Conclusion/Significance: Adding pedicle screw fixation at the level of an L1 burst fracture significantly increases the rigidity of the construct. This 6-pedicle screw construct also benefits from the use of a transverse connector. This has clinical implications for cases in which short segment fixation is considered.