Sacral Dysmorphism in the Indian Population: Anatomic Determinants for Safe Surgical Angle in Iliosacral Screw Fixation

Abstract Background: Iliosacral screw fixation is a standard method for posterior pelvic ring stabilization. However patients with sacral dysmorphism have a higher risk of cortical perforations and thereby of developing complications. However, no study has described the prevalence of sacral dysmorphism in the Indian population, and measurements regarding safe zone and safe surgical angles for iliosacral screw fixation are lacking. Materials and Methods: A total of 107 computed tomography (CT) scans and pelvic X-rays were analyzed for the features of dysmorphic sacral vertebrae. We reformatted the CT scans to true axial and true coronal sections by aligning with sagittal sections, and we measured the “safe zone distance” (shortest distance between the anterior alar cortex and neural foramina), “safe surgical angle” (angle from the ilium to safe zone distance through which screw passes) calculations for both S1 and S2 vertebrae in axial and coronal sections. We also calculated the sagittal body heights of S1 and S2 vertebrae. Results: Incidence of sacral dysmorphism in the Indian population was 26%. Average axial safe zone distance in dysmorphic S1 vertebrae (16.61 ± 2.73 mm) is 11% smaller compared to that in normal (18.71 ± 2.21 mm, P value < 0.001). Average coronal safe zone distance in dysmorphic S1 vertebrae (15.13 ± 2.77 mm) is 7.75% smaller compared to that in normal (16.40 ± 2.72 mm, P value 0.037). Mean safe surgical angle (18.23 ± 3.27 mm) in true axial sections was 16.57% smaller in dysmorphic individuals compared to normal ( P value < 0.001). The mean safe surgical angle in coronal sections (18.78 ± 3.55 mm) was 7.72% smaller in dysmorphic individuals compared to normal ( P value 0.030). In S2 vertebrae axial sections, average safe zone distance and safe surgical angle are smaller compared to normal S2. However, interestingly coronal safe zone distance and safe surgical angle were found to be bigger compared to normal. The mean sagittal height of dysmorphic S1 is 3.05% higher compared to normal S1 vertebrae and dysmorphic S2 is 9.63% taller than normal S2. Conclusion: Safe zone distance and safe surgical angle in first dysmorphic sacral vertebrae are much smaller compared to those of normal vertebrae. It precludes transverse screw placement in first dysmorphic sacral vertebrae; however, screws can be placed in oblique direction. S2 vertebrae have more or less the same safe zone distance in axial sections compared to normal S2 vertebrae. We found no difference between phenotypes and even significant more coronal safe surgical angle in dysmorphic S2 vertebrae compared to normal. It needs careful preoperative planning considering the higher prevalence and significant short safe zone distance and short safe surgical angle measurements in dysmorphic vertebrae.