Advantage of pedicle screw fixation directed into the apex of the sacral promontory over bicortical fixation: a biomechanical analysis

Abstract

Study design: A biomechanical study of human cadaveric sacra using insertional torque and bone mineral density was conducted to determine the optimal sagittal trajectory of S1 pedicle screws.

Objective: To measure the maximal insertional torque of sacral promontory versus bicortical pedicle screw fixation.

Summary of background data: Fixation of instrumentation to the sacrum is commonly accomplished using S1 pedicle screws, with previous studies reporting biomechanical advantages of bicortical over unicortical S1 screws. The biomechanical effect of bicortical screws (paralleling the endplate) versus screws directed into the apex of the sacral promontory is unknown.

Methods: For this study, 10 fresh frozen cadaver sacra were harvested and evaluated with dual-energy radiograph absorptiometry to assess bone mineral density. Matched 7.5-mm monoaxial stainless steel pedicle screws then were randomly assigned by side (left versus right) and placed bicortically or into the apex of the sacral promontory under direct visualization. Maximum insertional torque was recorded for each screw revolution with a digital torque wrench (TQJE1500, Snap-On Tools, Kenosha, WI).

Results: Maximum bicortical S1 screw insertional torque averaged 5.22 +/- 0.83 inch-pounds, as compared with the maximum sacral promontory S1 screw insertional torque of 10.34 +/- 1.94 inch-pounds. This resulted in a 99% increase in maximum insertional torque (P = 0.005) using the "tricortical" technique, with the screw directed into the sacral promontory. Mean bone mineral density was 940 +/- 0.25 mg/cm2 (range, 507-1428 mg/cm2). The bone mineral density correlated with maximal insertional torque for the sacral promontory technique (r = 0.806; P = 0.005), but not for the bicortical technique (r = 0.48; P = 0.16).

Conclusions: The screws directed into the apex of the sacral promontory of the S1 pedicle resulted in an average 99% increase in peak insertional torque (P = 0.005), as compared with bicortical S1 pedicle screw fixation. Tricortical pedicle screw fixation correlates directly with bone mineral density.