The Largest Secure Corridor of the Infra-acetabular Screw—a 3-D Axial Perspective Analysis

Background: The infra-acetabular screw is placed from the pubis to the ischium and can be used as a special lag screw of the posterior column of the acetabulum. This study was performed to simulate the surgical procedure and try to obtain the ideal insertion point, diameter, length and angle of the screw through the method of axial perspective in Chinese patients.Methods We randomly collected the pelvic computed tomography (CT) scans of 200 adults. DICOM-formatted CT-scan images were imported into Mimics software. The 3D digital model of the right semi-pelvic was established. A virtual cylinder representing the screw was placed from the pubis to the ischium to fix the posterior column. The largest secure diameter and length of the virtual screw were measured. The position of the insertion point and the directions of the screw were also researched.Results The screw insertion safe zone can exhibit an irregular “tear drop” from the reconstructed pelvic model. The mean maximum diameter of screws was 5.01±1.28 mm, and the mean maximum length of screws was 93.99±8.92 mm. The screw insertion corridor with a diameter of at least 3.5mm was found in 94 of 100 males (94%) and 86 of 100 females (86%). We found gender-dependent differences for the mean maximum diameter and the maximum length of the screw. The position of insertion point was statistically significant in different genders.Conclusions The study provides a valuable guideline for the largest secure corridor of infra-acetabular screw. We suggest an individual preoperative 3D reconstruction simulation for the ideal screw placement. Further biomechanical studies are needed to verify the function of the screw.