To analyze surgical alternatives to cement augmentation, the present study sought to quantify the impact of a distal expansion mechanism added to a standard pedicle screw on an axial pullout resistance.
A biomechanical laboratory study on the uniaxial pullout resistance of a standard pedicle screw versus a customized pedicle screw with a distal expansion mechanism.
A total of 40 vertebrae from seven fresh-frozen human specimens were harvested and subjected to a computed tomography scanning and an analysis of the bone mineral density (BMD). The vertebrae were instrumented with a standard 6.0-mm pedicle screw and a modified 6.0-mm pedicle screw with a distal expansion mechanism added. The actual working length of both screws inside the vertebrae was identical. The distal expansion mechanism made up one-fifth of the shaft length. The accuracy of the screw insertion was assessed using biplanar radiographs and by inspection. Analysis of resistance to pullout was performed by a coaxial alignment of the pedicle screws and attachment to an electromechanical testing machine. The pullout rate was 5 mm/min, and the load-displacement curve was recorded until the force of the pullout resistance peaked. The peak load-to-failure was measured in Newtons and reported as the ultimate failure load. With each test, the mode of failure was noted and analyzed descriptively.
A total of 17 vertebrae with matched pairs of standard and expansion pedicle screws were eligible for the final statistical analysis. The BMD of the vertebrae tested was 0.67¡À0.19 g/cm3. The screw length was 50 mm, and the actual working length of both screws was 40.3¡À4.2 mm. The ultimate failure load of the standard screw was 773.8¡À529.4 N and that of the expansion screw was 910.3¡À488.3 N. Statistical analysis revealed a strong trend toward an increased failure load with the expansion screw (p=.06). The mean increase of the ultimate failure load was 136.5¡À350.4 N. Abrupt vertebral fracture at the vertebral body-pedicle junction and the pedicle occurred seven times with the expansion screw and only five times with the standard screw (p=.16).
Our study indicates that adding a distal expansion mechanism to a standard pedicle screw increases the failure load by one-fifth. Modern expansion screws might offer an intermediate solution for the augmentation of screw-rod constructs in osteoporotic bone while reducing the need for cement-augmented screws and avoiding the related risks.