Reliability of axial landmarks for pedicle screw placement in the lower lumbar spine

Measurements were made on transverse-plan, computed tomography scans from three different patient groups. To describe the correlation between two previously described pedicle screw entry points to the pedicle axis and the predicted frequency of pedicle breakthrough from the use of a 6.5-mm screw placed parallel to the pedicle axis. Fluoroscopic assistance improves the accuracy of pedicle screw placement. Whether this is a result of improved accuracy of the starting point or correct directional guidance is unclear. No morphologic studies have been done to assess the accuracy of previously described entry points. Computerized digitizing and mathematic superimposition of the images from computed tomography scans of the low lumbar spine were used to quantify facet and pedicle anatomy and the correlation between two previously described entry points and the pedicle axis. The two previously described entry points are significantly medial to the pedicle axis. They are most medial at the L5 pedicle in patients with L4 degenerative spondylolisthesis. The two previously described entry points for pedicle screws in the low lumbar spine that were studied here are not reliable and tend to direct screw placement medial to the pedicle axis enough to lead to a substantial frequency of pedicle breakthrough for screws parallel to this axis. Surgeons implanting screws should take this tendency into account and use alternative methods to obtain accurate entry to the pedicle.