Radiological Evaluation of SI Posterior Screw Placement

Axial computed tomographic scans were obtained from 40 normal sacrum for evaluation of screw length and angle in the posterior lumbosacral screw fixation. The best scans close to the inferior portion of S1 superior facet and the middle of S2 pedicle were chosen to evaluate various screw paths. Measurements of screw paths included the screw path lengths and angulations as well as the distances between the screw and the sacral canal. The results showed that no significant differences between male and female were found in any parameters, although the linear measurements were greater for male than for female. In Sl, the greatest value of screw path lengths was noted in screw path VI(anterolaterally directed) with an average of 34.8±2.97mm. The mean distance between screw path I and the lateral cortex of the sacral canal was 5.6±0.4mm. For S2, the mean value(30.6±2.08mm) of screw path II (anterolaterally directed) was significantly greater than that(23.8±1.93mm) of screw path I(30° anteromedially directed) (p≤0.001). The mean distance between screw path I and the lateral cortex of the sacral canal was 4.0±0.36mm. This study showed that computed tomography(CT) scans provide more accurate information of screw path lengths. Preoperative CT evaluation of the sacral screw path angle and length is recommended.