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Computed tomographic considerations of dorsal sacral screw placement.

Axial computed tomographic scans were obtained from 40 sacrum specimens. 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 specimens were found in any parameters, although the linear measurements were greater for male specimens than for female specimens. In S1, the greatest value of the screw path lengths was noted in screw path IV (anterolaterally directed) with an average of 36.9 +/- 7.3 mm. The mean value for screw path 1 (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 directed) was significantly greater than that (24.2 +/- 4.9 mm) of screw path I (30 degrees anteromedially directed) (p < or = 0.001). The mean distance between screw path I and the lateral cortex of the sacral canal was 4.8 +/- 3.2 mm. 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.