Biomechanical Assessment of Conventional Unit Rod Fixation Versus a Unit Rod Pedicle Screw Construct: A Human Cadaver Study

Study Design. Fresh frozen human cadaver specimens were used to perform a biomechanical assessment focusing on spinal-pelvic fixation. Unit rod fixation alone was compared with that of unit rod fixation supplemented with L5 pedicle screws. Objectives. To investigate the biomechanical effects of adding L5 pedicle screw fixation to a unit rod construct. Summary of Background Data. The treatment of neuromuscular scoliosis associated with pelvic obliquity continues to evolve. Spinal fusion using fixation extending from the thoracic spine to the pelvis supplemented with segmental instrumentation has become the standard of care. The instrumentation of choice used at this institution has been a unit rod construct. Traditional Luque-Galveston technique with sublaminar wiring from T2 to L5 has been used. A noted complication has been a loss of distal fixation, the emphasis for this study. We have subsequently modified this technique, replacing wires at L5 with pedicle screws. This was done to improve the strength of the distal fixation, with the goal of decreasing the rate of instrumentation failure before fusion. Methods. Twelve fresh-frozen human cadavers, between the ages of 45 and 60 years, were instrumented from T4 to the pelvis with a Unit Rod (Sofamor Danek). The specimens were randomized into two groups. Segmental sublaminar wiring was performed from T4 to L5 in one group of six spines. The remaining six spines underwent sublaminar wiring from T4 to L4, with pedicle screws used for L5 fixation. The spines were then potted from T4 to T7 in preparation for biomechanical testing. The spines were tested in a randomized fashion for subfailure stiffness in anteroposterior, oblique, and lateral planes with an Instron 1321 servo-hydraulic load frame (Instron Corp., Canton, MA). Each spine was then loaded to failure in forward flexion. Results. Our results demonstrate a statistically significant increase in lateral stiffness (P = 0.0006) and oblique stiffness (P = 0.003) associated with the use of L5 pedicle screws. Statistical significance was also achieved with load to failure testing (P = 0.0007). The mean load to failure was 1377 Newtons for the pedicle screw construct and 467 Newtons for the wire construct. Conclusions. We conclude that the addition of L5 pedicle screws increases the stiffness and strength of the unit rod construct. Clinically, this should help to avoid complications associated with loss of fixation in this area.