In recent years, the trend in treatment of acetabular fractures is achievement of less invasive single ilioinguinal approach, especially in elderly patients [14,15,16,17,18]. Due to the complex characteristic of pelvic anatomy, the safe region of screw placement is far away from the acetabulum, which will reduce the peri-acetabular stability [7]. The common fixation methods for acetabular fractures are positional screw fixation and plate osteosynthesis [10]. Past researches of positional screw fixation have achieved good outcomes [19, 20]. The infra-acetabular screw can be applied via a single ilioinguinal approach to treat acetabular fractures involving a fracture line descending along the acetabular fossa and reaching the obturator formamen [1]. So far, there is no literature that identifies this screw as a positioning screw for the posterior column, and few digital anatomical studies are conducted on its properties.
Mimics software has been widely used in 3D reconstruction for the development of digital orthopedics technology. In our study, we applied the 3D method of axial perspective that described in previous studies were applied to simulate the surgical procedure [10, 11, 21]. We found the largest secure screw path along the longitudinal axis of the anterior part of posterior column after reducing the transparency of the 3D model. Compared with previous studies of computer-assisted determination or virtual three-dimensional model [12, 22], the method of axial perspective showed another osseous channel for positional screw of posterior column. We increased the diameter of virtual cylinder progressively and monitored the virtual screw in the views of coronal plane, sagittal plane and horizontal plane, without violating the cortices and articular surface. Compared with previous human cadaveric studies [23, 24], our method greatly saved manpower, material and financial resources, and can be repeated and verified through highly reliable test results.
In our research, the diameter and length of the infra-acetabular screw were significantly larger in males compared with females. In addition, the vertical distance from the insertion point to the arcuate line or pecten pubis and the distances from the insertion point to eminelntia iliopectinea and pubic tubercle were all measured in this study, which exhibited significant difference between genders. This was due to the obvious anatomic differences in pelvic bones between female and male. This study showed that the mean angle α and β between male and female had no statistical inference. From the Table 3, both angles were close to 0 degrees. This means that the screw is almost parallel to the medial surface of quadrilateral plate and perpendicular to the arcuate line.
Gras et al found that 93% pelves contained an infra-acetabular corridor with a diameter of at least 5 mm [13]. They also provided reference values for placement of a 3.5-mm cortical screw in the corridor. However, in our study, we found that the containable diameter of the screw was smaller in Chinese patients, especially in female. According to the data in our study, the maximum diameter to avoid cortical breaches is 5.54 ± 1.38 mm in male and 4.49 ± 0.90 mm in female. The screw insertion corridor with a diameter of at least 3.5 mm was found in 94 of 100 males (94%) and 86 of 100 females (86%). Only 77 males (77%) and 53 females (53%) possessed a corridor with diameter of at least 4.5 mm as shown in Fig 5. If a positional screw is to be used, a 3.5-mm cortical screw is the first choice and a 4.5 mm-hollow screw may be considered in males. Nevertheless, due to individual and sex differences, the use of preoperative measurements and calculations by digital tools is recommended. Technically, to avoid joint violation, it is crucial to save the subcondral bone while trying to place a juxta articular screw, which requires the use of screws with a smaller diameter than calculated by the preoperative CT scan. The screw diameter also dependents on some other variables like the quality of the reduction and the direction of the screw, so surgeons should prepare different size of screws before the surgery.
On the basis of mastering the diameter and length of screw, the insertion point and direction are important factors affecting the safe placement of infra-acetabular screw. Unlike the common posterior column screw, the infra-acetabular screw needs to be placed through the middle window of ilioinguinal approach. Culemann et al reported that the entry point for the infra-acetabular screw was 1 cm caudal of the eminelntia iliopectinea and in the middle of the pubic ramus [1]. Baumann et al found that the ideal entry point for the infra-acetabular screw was 10.2 mm caudal and 10.4 mm medial of the eminelntia iliopectinea [7]. Gras et al found that the optimized entry points of infra-acetabular screws were located in the mediocaudal region of the eminelntia iliopectinea [13]. Different from previous studies, we found that the optimized insertion point was 13.90 ± 2.58 mm away from the eminelntia iliopectinea in males and 15.00 ± 3.61 mm in females. This is due to the different pelvic shapes between female and male. The anatomic landmark of eminelntia iliopectinea and pubic tubercle can be used as effective references intraoperatively as they are large bony bumps that can be well palpable and identified. The parameters of the infra-acetabular screw may provide the surgeon appropriate information of safe positional screw placement for the treatment of acetabular fracture with separation of both columns. The large standard deviation of our results indicates great differences among individuals. As a result, preoperative planning should be implemented detailedly for each patient. 3D reconstruction and simulated screw placement technique with digital software before operation are valuable.
There are still some limitations to this study. We only analyzed the data based on genders, not different age groups. In addition, we did not collect data according to age, height, weight or body bone density. These factors may also affect the implantation of screws. We only studied the pelvises of Chinese people, who have different skeletal shapes with American and European populations. Thus, more biomechanical studies and related clinical research should be performed to compare the effect of the infra-acetabular screw with other acetabular screws.