Preventing Instrumentation Failure in Three-Column Spinal Osteotomy: Biomechanical Analysis of Rod Configuration.

STUDY DESIGN

Biomechanical analysis.

OBJECTIVES

To show the role of additional rods and long-term fatigue strength to prevent the instrumentation failure on three-column osteotomies.

SUMMARY OF BACKGROUND DATA

Three-column osteotomy such as pedicle subtraction osteotomy (PSO) and vertebral column resections are surgical correction options for fixed spinal deformity. Posterior fixation for the PSO involves pedicle screw-and rod-based instrumentation, with the rods being contoured to accommodate the accentuated lordosis. Pseudarthrosis and instrumentation failure are known complications of PSO.

METHODS

Unilateral pedicle screw and rod constructs were mounted in ultra-high-molecular-weight polyethylene blocks using a vertebrectomy model with the rods contoured to simulate posterior fixation of a PSO. Each construct was cycled under a 200 N load at 5 Hz in simulated flexion and extension to rod failure. Three configurations (n = 5) of titanium alloy rods were tested: single rod (control), double rod, and bridging rod. Outcomes were total cycles to failure and location of rod failure.

RESULTS

Double-rod and bridging-rod constructs had a significantly higher number of cycles to failure compared with the single-rod construct (p < .05). Single-rod constructs failed at or near the rod bend apex, whereas the majority of double-rod and bridging-rod constructs failed at the screw-rod or rod-connector junction.

CONCLUSIONS

Double-rod and bridging-rod constructs are more resistant to fatigue failure compared with single-rod constructs in PSO instrumentation and could be considered to mitigate the risk of instrumentation failure.