RELIABILITY OF AXIAL LANDMARKS FOR PEDICLE SCREW PLACEMENT IN THE LOWER LUMBAR SPINE

Study Design. Measurements were made on transverse-plan, computed tomo graphy scans from three different patient groups. Objective. To descri be the correlation between two previously described pedicle screw entr y points to the pedicle axis and the predicted frequency of pedicle br eakthrough from the use of a 6.5-mm screw placed parallel to the pedic le axis. Summary of Background Data. Fluoroscopic assistance improves the accuracy of pedicle screw placement. Whether this is a result of i mproved accuracy of the starting point or corrected directional guidan ce in unclear. No morphologic studies have been to assess the accuracy of previously described entry points. Methods. Computerized digitizin g and mathematic superimposition of the images from computed tomograph y scans of the low lumbar spine were used to quantify facet and pedicl e anatomy and the correlation between two previously described entry p oints and the pedicle axis. Results. The two previously described entr y points are significantly medial to the pedicle axis. They are most m edial at the L5 pedicle in patients with L4-degenerative spondylolisth esis. Conclusions. The two previously described entry points for pedic le screws in the low lumbar spine that were studied here are not relia ble and tend to direct screw placement medial to the pedicle axis enou gh to lead to a substantial frequency of pedicle breakthrough for scre ws parallel to this axis. Surgeons implanting screws should take this tendency into account and use alternative methods to obtain accurate e ntry to the pedicle.