Ideal insertion point and projection of the infra-acetabular screw in acetabular fracture surgery

•Although the morphological characteristics, shape, and size of the corridor are possible reasons for the difficulties in properly placing the IAS, the intraoperative detection of the ideal insertion point and angle of the IAS is also a source of failure. This study aimed to detect the ideal insertion point and angle with respect to anatomical landmarks that could be directly identified intraoperatively for accurate insertion of the IAS.•We considered that using PIP would provide a better indicator for IAS insertion, which contains the anterior border of both the sacroiliac joints and the posterior superior border of the pubic symphysis. As the PIP is close to the insertion point of the IAS, other index points contained in this plane can be confirmed under direct vision during surgery.•The mean IAS distance was 61.0 ± 5.7 mm with women having a greater distance than men (p<0.01). The insertion angle formed with the x-axis (α-angle) in the outlet view was 8.4 ± 6.6 °, and women exhibited greater angles than men (p<0.01). The angle formed with the y-axis in the sagittal plane (β-angle) was 86.5 ± 10.6 °; there was no significant difference between men and women (p = 0.467). Length of IAS was 97.1 ± 4.7 mm in men and 89.2 ± 3.6 mm in women (p < 0.01).