COMPUTED TOMOGRAPHIC CONSIDERATIONS OF DORSAL SACRAL SCREW PLACEMENT

Axial computed tomographic scans were obtained from 40 sacrum specimen s. The best scans close to the inferior portion of S1 superior facet a nd the middle of S2 pedicle were chosen to evaluate various screw path s. Measurements of screw paths included the screw path lengths and ang ulations as well as the distances between the screw and the sacral can al. The results showed that no significant differences between male an d female specimens were found in any parameters, although the linear m easurements were greater for male specimens than for female specimens. In S1, the greatest value of the screw path lengths was noted in scre w path IV (anterolaterally directed) with an average of 36.9 +/- 7.3 m m. The mean value for screw path I (30 degrees anteromedially directed ) was 32.5 +/- 7.2 mm, and the mean distance between screw path I and the lateral cortex of the sacral canal was 6.0 +/- 3.1 mm. For S2, the mean value (31.9 +/- 7.6 mm) of screw path II (anterolaterally direct ed) was significantly greater than that (24.2 +/- 4.9 mm) of screw pat h I (30 degrees anteromedially directed) (p less than or equal to 0.00 1). The mean distance between screw path I and the lateral cortex of t he sacral canal was 4.8 +/- 3.2 mm. This study showed that computed to mography (CT) scans provide more accurate information of screw path le ngths. Preoperative CT evaluation of the sacral screw path angle and l ength is recommended.