Alar slope angle —an alternative perspective for measuring the transsacral screw bone corridor

Objective: Transsacral screws are commonly utilized in clinical settings for addressing vertical instability injuries of the posterior pelvic ring and fragility fractures in elderly patients. Nevertheless, the potential stenosis of the bone corridor resulting from dysmorphism in the upper sacral segment poses challenges in the insertion of S1 through the transsacral screw, increasing the risk of significant complications such as screw misalignment and neurovascular injury. The purpose of this study was to more effectively identify whether the sacrum was deformed by reformatting CT scan, as well as search for novel indications for predicting the possibility of the S1 segment being fixed with transsacral screws and guiding the selection of appropriate internal fixations. Questions/purposes: (1) which imaging features have a greater correlation with sacral dysmorphism? (2) whether the sacral wing slope angle can be utilized as a new imaging feature to aid in the determination of sacral dysmorphism? (3) whether the S2 transsacral screw-bone corridor can be used as an alternative when the S1 transsacral screw-bone corridor is restricted? Patients and Methods: Pelvic Computed Tomography (CT) data from 106 normal individuals (65 males and 41 females) was imported into Mimics software, which generated a virtual representation of the pelvic outlet view in order to identify seven qualitative markers of sacral dysmorphism. The distance between the bilateral iliac spine line and the upper endplate of S1(D-IS) as well as the left and right alar slope angles (LASA/RASA) on the coronal plane, were measured as new measurement indicators after the CT scan plane parallel to the posterior border of S1 was reformatted. On the basis of a standard sacrum lateral view and a three-dimensional virtual model of the pelvis, the maximal circle diameter of the transverse bone corridor of S1 and S2 (MCD-S1/MCD-S2) has been calculated. Sacral dysmorphism was classified as an MCD-S1 measurement of less than 8 mm. Results: Out of the total cases, 46 (43.40%) were classified as having sacral dysmorphism. The sacral dysmorphism group had a significantly higher angle than the normal sacral group, and there was a statistical difference between the two groups (p<0.001), as well as different LASA/RASA values (12.71±5.57 vs. 27.00±3.86, 13.95±5.36 vs. 27.93±3.82, respectively). The study of the receiver operating characteristic (ROC) curve revealed that the LASA/RASA values for sacral dysmorphism were 21.08 (with a sensitivity of 0.93 and specificity of 0.98) and 21.09 (with a sensitivity of 0.98 and specificity of 0.92), respectively. Conclusion: When the angle of the alar slope exceeds 21 degrees, it is suggested to consider oblique iliosacral screw fixation or S2 transsacral screws fixation due to the narrowing of the bone corridor.