Study design: The purpose of this study was to investigate the effects of implant selection and set-screw tightening technique on the loosening torques in long scoliosis constructs after long-term biaxial fatigue loading.
Summary of background data: Expanded use of pedicle screws in the correction of long scoliotic curves and the mechanical demands on segmental fixation systems requires surgeon awareness of revisiting set screws to ensure full screw/rod engagement and minimize the potential of set-screw loosening and/or rod slippage postoperatively.
Methods: Biomechanical tests were performed to evaluate the effect of set-screw tightening techniques and rod approximation on screw/rod interface strength.
Results: Rod reduction test shows the force required to approximate a rod to a pedicle screw is statistically lower with uniplanar or polyaxial screws, when compared with monoaxial screws. This ease of approximation in both polyaxial and uniplanar screws directly correlate to improvement in the axial slippage resistance. In the simulated spinal model construct, rod/screw securement can vary based on the number of tightening torques applied to the system.
Conclusions: Sequential revisiting of sets crews in long scoliosis constructs resulted in a statistically significant increase in loosening torque for monoaxial and polyaxial screw systems. Intraoperative securement assessment of set screws is recommended. The use of polyaxial and uniplanar screws at the distal ends in long constructs is recommended to increase the screw/rod interface strength.