To evaluate the significance of S1 posterior edge inlet view for placement of percutaneous sacroiliac screws.

1. CT data of the pelvis were collected from 134 normal adults and introduced into Mimics Medical 21.0 system. Anatomical parameters of sacral vertebrae were measured and analyzed to observe the anatomical disparities between the anterior and posterior edges of S1 vertebral body. A mathematical model was established using the data acquired. 2. Manual placement of sacroiliac screws was performed using a conventional S1 posterior edge inlet view on the pelvic specimens from 5 adult cadavers in simulation of actual surgical situations. After placement, the inlet views from both the S1 anterior and posterior edges were taken to observe the imaging differences and to check if the screws had pierced the sacral canal. 3. A retrospective study was conducted of the 11 patients with posterior pelvic ring fracture who had been treated at Department of Orthopaedics, Tongji Hospital from January 2019 to October 2020. Their fractures were fixated by percutaneous sacroiliac screws under the guidance of a C-arm X-ray machine. The manual placement of the screws was guided intraoperatively by the inlet views from both the S1 anterior and posterior edges to secure a safe placement. Pelvic CT examinations were performed to check any screw dislocation.

1. CT measurements in the normal adults showed that the angle of S1 anterior edge inlet view (20.71°±11.89°) was smaller than that of S1 posterior edge inlet view (41.99°±11.67°) and the width of S1 upper end plate [(32.22±3.41) mm] greater than that of S1 lower end plate [(20.10±3.28) mm], showing significant disparities in anatomy between the anterior and posterior edges of S1 vertebral body (P<0.05). 2. In 2 of the 5 cadaveric specimens, imaging differences were observed between the inlet views of the anterior and posterior edges of S1 and the screws pierced out of the sacral canal. 3. Satisfactory closed reduction was achieved in all the 11 patients. A total of 17 screws were placed, with 12 ones into S1 and 5 ones into S2. Operation time ranged from 84 to 141 min (average, 114.4 min), fluoroscopy frequency from 69 to 101 times (average, 89.6 times), and intraoperative blood loss from 110 to 463 mL(average, 296.6 mL). No screw dislocation was observed on postoperative CT.

As there is a difference between the inlet views of the anterior and posterior edges of S1 vertebral body, the inlet view of the posterior edge of S1 can display the posterior edge of S1 more clearly so as to improve the safety of placement of percutaneous sacroiliac screws.

Fracture fixation, internal; Fluoroscopy; Surgical procedures, minimally invasive; Inlet view of pelvic