Guidance for dysmorphic sacrum fixation with upper sacroiliac screw based on imaging anatomy study: techniques and indications

Abstract Objective: This study aimed to investigate the techniques and indications of upper sacroiliac screw fixation for the dysmorphic sacrum. Methods: The dysmorphic sacra were selected from 267 three-dimensional pelvic models. The dysmorphic sacra which couldn’t accommodate a 7.3 mm upper trans ilio-sacroiliac screw were classified as the main dysmorphic sacra. Then, the size of the bone corridor, the length of the screw in the corridor, and the orientation of the screw were measured. The insertion point on the sacrum was identified by two bone landmarks. Results: totally, 30.3% of sacra were identified as the main dysmorphic sacra. The inclinations of the screw oriented from posterior to anterior were (21.80±3.56)° for males and (19.97±3.02)° for females (p<0.001), and from caudal to cranial were (29.97±5.38)° for males and (28.15±6.21)° for females (p=0.047). The min diameters of the corridor were (16.31±2.40) mm for males and (15.07±1.58) mm for females (p<0.001). The lengths of the screw in the Denis Ⅲ zone were (14.41±4.40) mm for males and (14.09±5.04) mm for females (p=0.665), and in the Denis Ⅱ+Ⅲ zones were (36.25±3.40) mm for males and (38.04±4.60) mm for females (p=0.005). The rates of LP-PSIS/LAIIS-PSIS were (0.36±0.04) for males and (0.32±0.03) for females (t=4.943, p<0.001). The lengths of LPM were (8.81±5.88) for males and (-4.13±6.33) for females (t=13.434, p<0.001). Conclusion: When the sacrum has the features of “sacrum not recessed” and/or “acute alar slope”, the conventional trans ilio-sacroiliac screw couldn’t be placed safely. The inclination oriented from posterior to anterior and from caudal to cranial are approximately 20° and 30°, respectively. The bone insertion point locates in the rear third of the anterior inferior iliac spine to the posterior superior iliac spine. The sacroiliac screw is not recommended to fix the fractures in Denis Ⅲ zone.