Acetabular anatomy is complex and surrounded by important tissues and organs. Acetabular fracture surgery has been the most complicated and challenging surgery in the field of traumatic orthopedics because of the difficulty of exposure and massive bleeding during the operation. Furthermore, patients of acetabular fracture also face a series of complications such as traumatic arthritis, heterotopic ossification, thromboembolism, and so on. Anatomical reduction and strong internal fixation can reduce the incidence of acetabular fracture complications and improve the clinical prognosis of the patients [1, 2].
In 2010, Culemann et al.  proposed that an additional infra-acetabular screw can close the incomplete periacetabular fixation “frame”, consisting of both osseous columns, the ilioinguinal plate, and supra-acetabular screws fixation. Meanwhile, they developed the indications for infra-acetabular screw fixation, including all fracture patterns addressed via an ilioinguinal approach and involving a fracture line descending along the fovea acetabuli and reaching the foramen obturatorium, resulting in a separation of both columns. The acetabular fracture types involved are anterior column fracture, anterior column and posterior hemitransverse fracture, T-type fracture, and both column fracture. Marintschev et al.  found in a biomechanical study that the locking plate modality did not reduce the maximum fracture displacement, but the additional infra-acetabular screw placement doubles the fracture fixation strength independent of the used plate system. Marintsche, Gras et al.  used the same fracture pattern for a biomechanical study, in which three groups of different screws (group II, titanium; group III, stainless steel; and group IV, biodegradable Poly-L-Lactid) were compared with the standard plate fixation (group I). The results demonstrated that screw fixation is a promising alternative approach for the stabilization of noncomminuted acetabular fractures of the anterior column with equivalent fixation strength to the standard plate fixation, and the additional placement of an infra-acetabular screw significantly increases the fracture fixation independent of the used implant. Clinical applications showed that the infra-acetabular screw can get a good prognosis lead to a high rate of fine reduction, limited complications, a low displacement risk, and good clinical efficacy [6, 7].
Even with good biomechanical performance and clinical outcome, the placement of the infra-acetabular screw is difficult, for the infra-acetabular screw corridor is irregular and narrow. It has been reported that placement of a 3.5 mm infra-acetabular screw in some patients is impossible . Therefore, to provide references for the successful placement of the infra-acetabular screw, many scholars have studied the diameter, length, entry point, spatial position and spatial shape of IAC. Gras et al.  presented a mean-shape analysis of 523 healthy pelves and found the existence of an IAC with a minimum corridor diameter of 5 mm in 93% by a biomorphometric CT-based analysis. Arlt et al.  demonstrated that the IAC consistently showed a double-cone shape with the isthmus located at the acetabular fovea by a 3D radiomorphometric analysis. What’s more, they found that congenital hip dysplasia does not affect secure infra-acetabular screw insertion. Baumann et al.  found the entry point of an ideal infra-acetabular was of high constancy, with an average distance of 10.2 mm caudally and 10.4 mm medially to the IPE, which provided a guideline for placement of an infra-acetabular screw via an intra-pelvic approach. The findings of these scholars could help surgeons safely place an infra-acetabular screw. However, the specimens of these studies were all Westerners and the results of which may not be suitable for Asians. At present, the exit point of the infra-acetabular screw reported was all located at the ischial tuberosity [6, 8–11].
As a trauma center in southern China, we have great experience in placing the infra-acetabular screw. We realized that when the exit point was located between the ischial tuberosity and the ischial spine, the placement of a 3.5 mm infra-acetabular screw is possible and easier in some patients. This provides an alternative for the infra-acetabular screw placement, besides the traditional ischial tuberosity exited infra-acetabular screw. However, data describing the new infra-acetabular screw corridor are lacking. Therefore, this study aims to explore the anatomy differences of the two infra-acetabular screw corridors. We hypothesized that the infra-acetabular screw corridor with an exit point located between the ischial tuberosity and the ischial spine has a larger maximum diameter than the traditional ischial tuberosity exited corridor.
In recent years, the application of computer-aided technology has enabled the rapid development of digital orthopedics. Bio-morphological study of screw corridors around the acetabulum can be performed easily and quickly by computer-aided techniques [12–14]. Our team has proposed the use of "axial perspective" to study the largest anterior and posterior column screw corridors of the acetabulum[13, 15]. Therefore, in this study, “axial perspective” was also used to study the anatomical parameters of the two different infra-acetabular screw corridors.