The usefulness of reformatting CT scanning plane to distinguish sacral dysmorphism and introducing the variable of elevated height for predicting the possibility of trans-sacral screw fixation.

Introduction: Ilio-sacral screw fixation has been used for sacral fractures and sacroiliac joint dislocations. However, it is associated with significant complications including screw loosening with backing-out and loss of reduction. Trans-sacral screw fixation is indicated for rigid fixation in specific circumstances including bilateral posterior ring injuries, osteoporotic bone, or to supplement other types of posterior ring fixation.

Hypothesis: The reformatting the CT scanning plane may distinguish sacral dysmorphism and predict the possibility of trans-sacral screw fixation by introducing the new indicator.

Material And Methods: The CT data of 112 adult cadavers (61 males and 51 females) were imported into Mimics® software and a 7.0 mm-sized trans-sacral screw was virtually placed in the ideal position of S. The osseous widths around screw in the axial images (AxW) were measured. By reformatting the CT scanning plane parallel to the superior endplate of S, the AxWR was measured and the height of elevated segment in the upper sacrum (elevated height) was assessed as a new preoperative indicator.

Results: Cortical violation around screw was detected in 26 models and was considered as sacral dysmorphism. The average AxW was measured as 13.70±2.76mm in the non-dysmorphism group, and 5.81±2.19mm in the dysmorphism group, with statistical difference (p<0.001). By reformatting the CT scanning plane, the average AxWR increased to 16.61±2.79mm in the non-dysmorphism group, and 8.04±2.62 in the dysmorphism group, and the difference was statistically significant (p<0.001). The elevated height was 3.29±4.19mm in the non-dysmorphism group, and 17.52±3.09mm in the dysmorphism group, and the differences were statistically significant (p<0.001). Based on the ROC curve analysis, the cut-off value of elevated height was 12.90mm (sensitivity 1.0 and specificity 1.0).

Discussion: By reformatting the CT scanning plane, the osseous width around screw was widened, and the new preoperative indicator of elevated height could be introduced to predict the possibility of trans-sacral screw fixation into S. If the elevated height exceeded 13mm, the pelvis was assigned to sacral dysmorphism and thus, could not apply the trans-sacral screw fixation into S.

Level Of Evidence: III, controlled laboratory study.