LAUSR

RESEARCH DESIGN Finite element analysis based on CT images from the lumbar spine. OBJECTIVE Determined the pull-out strength of unsatisfactorily placed screws and repositioned screws after unsatisfactory place in lumbar spine surgery. BACKGROUND Pedicle screws are widely used to stabilize the spinal vertebral body. Unsatisfactory screws could lead to surgical complications, and may need to be repositioned. Screw removal and reposition, however, may decrease pull-out strength. METHODS We conducted a three-dimensional finite element analysis based on high-resolution CT images from a 39-years-old healthy woman. Pull-out strength was determined with the screw placed in different orientations at the same entry point (as selected by the Magerl method), as well as after removal and reposition. The material properties of the vertebral body and the screw were simulated by using grayscale values and verified data, respectively. A load along the screw axis was applied to the end of the screw to simulate the pullout. RESULTS The pull-out strength was 1840.0 N with the Magerl method. For unsatisfactorily placed screws, the pull-out strength was 1500.8 N at 20% overlap, 1609.6 N at 40% overlap, 1628.9 N at 60% overlap, and 1734.7 N at 80% overlap with the hypothetical screw path of the Magerl method. For repositioned screws, the pull-out strength was 1763.6 N, with 20% overlap, 1728.3 N at 40% overlap, 1544.0 N at 60% overlap, and 1491.1 N at 80% overlap with the original path. Comparison of repositioned screw with unsatisfactorily placed screw showed 14.04% decrease in pull-out strength at 80% overlap, 5.21% decrease at 60% overlap, 7.37% increase at 40% overlap, and 17.51% increase at 20% overlap with the screw path of the Magerl method. CONCLUSION Removal and reposition increased the pull-out strength at 20% and 40% overlap, but decreased the pull-out strength at 60% and 80% overlap. For clinical translation, we recommend removal and reposition of the screw when the overlap is in the range of 20%-40% or less. In vitro specimen studies are needed to verify these preliminary findings.