CT-scan based anatomical study as a guidance for infra-acetabular screw placement

The infra-acetabular corridor is quite narrow, which makes a challenge for the orthopedists to insert the screw. This study aimed to explore the relationship between the infra-acetabular corridor diameter (IACD) and the minimum thickness of medial acetabular wall (MTMAW), and to clarify the way of screw placement.

The Computed tomography (CT) data of 100 normal adult pelvises (50 males and 50 females respectively) were collected and pelvis three-dimensional (3D) reconstruction was performed by using Mimics software and the 3D model was imported into Geomagic Studio software. The perspective of acetabulum was carried out orienting from iliopubic eminence to ischial tuberosity and the IACD was measured by placing virtual screws which was vertical to the corridor transverse section of "teardrop". The relationship between IACD and MTMAW was analyzed. When IACD was ≥5 mm, 3.5 mm all-in screws were placed. When IACD was < 5 mm, 3.5 mm in-out-in screws were placed.

The IACD of males and females were (6.15 ± 1.24) mm and (5.42 ± 1.01) mm and the MTMAW in males and females were (4.40 ± 1.23) mm and (3.60 ± 0.81) mm respectively. The IACD and MTMAW in males were significantly wider than those of females (P < 0.05), and IACD was positively correlated with MTMAW (r = 0.859), the regression equation was IACD = 2.111 + 0.917 MTMAW. In the all-in screw group, 38 cases (76%) were males and 33 cases (66%) were females respectively. The entry point was located at posteromedial of the apex of iliopubic eminence, and the posterior distance and medial distance were (8.03 ± 2.01) mm and (8.49 ± 2.68) mm respectively in males. As for females, those were (8.68 ± 2.35) mm and (8.87 ± 2.79) mm respectively. In the in-out-in screw group, 12 cases (24%) were males and 17 cases (34%) were females, respectively. The posterior distance and medial distance between the entry point and the apex of iliopubic eminence were (10.49 ± 2.58) mm and (6.17 ± 1.84) mm respectively in males. As for females, those were (10.10 ± 2.63) mm and (6.63 ± 1.49) mm respectively. The angle between the infra-acetabular screw and the sagittal plane was medial inclination (0.42 ± 6.49) °in males, lateral inclination (8.09 ± 6.33) °in females, and the angle between the infra-acetabular screw and the coronal plane was posterior inclination (54.06 ± 7.37) °.

The placement mode of the infra-acetabular screw (IAS) can be determined preoperatively by measuring the MTMAW in the CT axial layers. Compared with all-in screw, the in-out-in screw entry point was around 2 mm outwards and backwards, and closer to true pelvic rim.