Acetabular fracture surgery is one of the most challenging operations in trauma surgery, and because of the acetabulum’s complex anatomy and proximity to important nerves and blood vessels, it is easy to cause iatrogenic injuries during the operation. In order to avoid postoperative traumatic arthritis, anatomical reduction of the fracture ends is particularly important. According to the theory of Judet and Letournel, restoring the stability of the anterior and posterior columns and the integrity of the articular surface are the main objectives of surgical treatment of acetabular fractures . Biomechanical studies have shown that the stability of fracture fixation involving the anterior and posterior columns can be substantially improved by using an additional screw in the infra-acetabular region [11, 12]. However, in practice, due to the extremely limited volume of the bony corridor in the infra-acetabular region, it is difficult to place this screw, so it is rarely used.
The results of this study show no statistical difference between the tilt angle of the APP and 0°, which implies that the APP can be used as the pelvic reference plane for patients in the supine position, independent of the tilt angle of the pelvis and the patient's position, and that a uniform reference plane can be found for different patients according to their anatomical landmarks. The maximum diameter of 4.66 mm is obtained at a fluoroscopic angle of 35° in men, and the maximum diameter of 3.55 mm is obtained at a fluoroscopic angle of 15° in women, with the highest rate of screw insertion for both at 15° and 25° (73.94% vs. 46.77%). When manipulated on the 3D model, it is found that the pelvis is tilted too far at 45°, and part of the pelvic perspective shows the inner surface of the suprapubic ramus for corridor measurement. The traditional 45° pelvic inlet view (i.e., when the patient is lying flat and the radial tube of the C-arm machine tilts 45° towards the patient's head) does not provide a suitable perspective for IAS insertion. The axis of the male corridor is more parallel to the SMP than that of the female corridor, which means that during intraoperative fluoroscopy, the C-arm will need to be rotated more towards the contralateral side to obtain the maximum corridor path in female patients.
The results of a study by Kanezaki et al.  in 2020 showed that in 20% of 80 patients under fluoroscopy with a 25° pelvic inlet view, surgeons were unable to insert IAS because the corridor width was less than 3 mm. The mean diameter of the IAC was 4 mm, and the mean angle between the axis of the corridor and the SMP was 4.7°. Our results are close to the mean diameter of the IAC in this study but are quite different from the results of Gras et al. and Arlt et al, as follows. Gras et al.  analyzed 523 pelvises and found that 93% of pelvises had a corridor diameter ≥ 5 mm, with a mean diameter of 7.4 mm, and there was no statistical difference between the 94% male and 90% female screw insertion rates. Arlt et al.  studied the morphology of the IAC, and 97% of men and 91% of women had a corridor diameter of 3.5 mm or more. The large variation in the results may be due in large part to the ethnicity of the specimens examined, as Darling et al.  showed that Asian women have smaller skeletal dimensions and strength than Caucasian populations. In addition, differences in measurement methods and the choice of reference plane may have a certain effect on the results.
The central part of the IAC has a special biconical anatomical pattern due to the restriction of the acetabular fossa, so unlike the anterior and posterior column screw corridors, where the optimal access path is easier to find, the volume pattern of the IAC is more sensitive to the angle, so the largest screw channel can be found within a certain range . The effect of fluoroscopic angle on the parameters of the IAC has not been described to date, so this study investigated the variation of the parameters of the optimum IAC and the screw insertion rate under different fluoroscopic angles in order to provide guidance for surgical fluoroscopy in clinical practice. IAS insertion is usually performed with the aid of the pelvic inlet view, which is traditionally defined as imaging of the patient in the supine position with the C-arm tube tilted 45° to the cephalad side. However, Ricci et al.  recommended the use of a 25° pelvic inlet view based on the results of CT tomographic scans, and the conventional 45° fluoroscopy provides too large an angle of inclination to adequately expose the bony landmarks required for clinical manipulation. Gras et al.  showed that the optimal angle between the axis of the corridor to the APP is 54.8°, that is, the fluoroscopy is performed at 35°. Liu et al.  found that in contrast with the traditional IAS where the screw-out point is on the ischial tuberosity, placing the screw-out point between the ischial spine and the ischial tuberosity is more suitable for Asian people. At this time, the mean angle between the screw axis and the coronal plane is 75.2°, that is, the fluoroscopy is performed at 15°. The above findings provide a reference for setting the angle grouping in this study.
This study also has obvious limitations. Firstly, all steps are performed on a virtual model, which does not take into account the exposure of the surgical field and the presence of soft tissues around the acetabulum, and the surgical space is extremely limited in practice, which greatly increases the difficulty of screw insertion. Secondly, the screw insertion operation is performed on the complete pelvis model. The clinical situation of fracture reduction largely influences the choice of fixation method. The available bony volume for operation is much smaller than the theoretical value, and screw insertion should be considered only if the reduction is accurate. It is necessary to carry out preoperative planning and fully evaluate the feasibility of screw insertion to avoid iatrogenic injury during the operation.
Research on infra-acetabular screws has led to many recent advancements. When the concept of IAS was first introduced by Culemann, it was considered only applicable to fracture types handled by the ilio-inguinal approach (anterior column, anterior column with posterior semi-transverse, T-shape, and double-column fractures), which are driven from the second window, hence the name “second window screws” . In 2015, Gusic et al.  reported retrograde IAS insertion via the posterior K-L approach to fix T-shape with posterior wall fractures and hip dislocation, which first introduced the concept of retrograde IAS. In terms of surgical operation, three-dimensional navigation and electromagnetic navigation techniques have also been applied to screw insertion, and the development of new technologies has undoubtedly helped to shorten the operating time, improve the accuracy of screw insertion, and reduce the radiation dose to the doctor and patient [19, 20]. However, these techniques and hardware facilities are not available in general hospitals, and conventional X-ray fluoroscopy remains the dominant modality to assist with screw insertion, which requires a long learning curve for orthopedic surgeons to master the techniques needed in order to improve the safety and accuracy of access screws.