Biomechanical study of lumbar pedicle screws in a corpectomy model assessing significance of screw height.

Abstract

We tested the hypothesis that a pedicle screw construct's height is an important factor in strengthening a screw-rod system. Six corpectomy constructs were made, each using two ultra-high-molecular-weight polyethylene blocks, 6.5-mm pedicle screws, and two 6.35-mm rods. Pedicle screws were placed at +10-, +5-, 0-, and -5-mm depths in relation to the dorsal surface of the corpectomy model. Nondestructive testing was performed in flexion/extension and in torsion. For all modes tested, the screw-rod constructs continued to increase in stiffness as the height of the construct was lowered, and this was statistically significant at all heights tested (P < 0.001). The stiffness increased 232% when comparing flexion at +10 and -5 mm and increased 231% in extension from +10 to -5 mm. The torsional stiffness increased 171% when comparing +10 and -5 mm. Thus, lower-profile instrumentation systems should be used to take advantage of this by decreasing the size and bulkiness of the implants while increasing the strength of the construct.