According to Burgess[26] and Manson[27], pelvic ring injuries can be classified into three major types based on the mechanism of injury: anterior-posterior compression (APC), lateral compression (LC), and vertical shear (VS). Crescent-shaped pelvic fractures belong to the LC-II type of injury. In the AO/OTA classification, they are classified as 61-B2.2 or 61-B2.3[11]. In previous studies, it was believed that CFDP was caused by lateral violence rather than damage to the pelvic floor ligaments, resulting in rotational instability but stability in the vertical direction[3, 28]. However, Zong et al.[4] proposed that with the increasing incidence of high-energy injuries such as motor vehicle accidents, vertical instability can still occur in patients with CFDP. Therefore, when treating such patients, it is important to assess whether there is significant displacement in the vertical direction and take appropriate reduction measures[29]. In this study, we found a total of 5 cases of crescent fractures with vertical instability, so we should to pay attention to the vertical displacement of the sacroiliac joint in the treatment of such patients, and take appropriate measures to reset and fixation, for example, we can take two modified SIS for fixation or take the modified SIS combined with the sacral 2 of the penetrating screws to strengthen the fixation.
Ma et al.[30] conducted anatomical studies on a large number of normal 3D printed pelvises in a 1:1 ratio and proposed the existence of anterior-posterior iliosacral screw channels. The inner wall of the channel is formed by the pelvic arcuate line, while the outer wall is composed of the posterior-lateral aspect of the sacral wing and the lateral aspect of the ilium. The superior wall is the SAS and the bottom of the greater pelvis, while the inferior wall is the line between the sacral foramen and the greater sciatic notch. Our selected entry point is located at the posterior superior edge of the acetabulum, and the exit point is on the medial aspect of the superior articular eminence of the S1 vertebrae. Postoperative CT scans of all patients confirmed that the modified SIS was located within the bony channel without any signs of penetration into the acetabulum or sacral canal. Wang et al.[31] analyzed pelvic CT data from 80 normal individuals and concluded that the anterior-posterior iliosacral screw channel has a sufficient width and length to accommodate one or two screws with a diameter of 7.0–8 mm and 90–130 mm in length and that the longer length of the iliac segment of the channel allows the placement of longer channel screws with better biomechanical properties than traditional SIS.
Advances in technology has led to the introduction of robot-assisted artificial intelligence surgery, and TiRobot-assisted technology has been widely used in trauma, joint and spine surgery[32, 33]. In this study, we used TiRobot-assisted modified SIS combined with LC-II screws for the treatment of Day type II CFDP. The differences in the cause of injury, fracture classification, time from injury to surgery, intraoperative bleeding volume and functional score at the last follow-up were not statistically significant between the two groups when compared to traditional placement of SIS and posterior iliac screws. Our results show that the iliac segment of the modified SIS is longer than that of the traditional SIS and that anatomically, the iliac segment of the modified SIS is encapsulated medially and laterally by cortical bone, therefore we believe this provides better stability even in patients with severe osteoporosis. Although the sacral segment of the modified SIS is slightly shorter than that of the traditional SIS, the difference is not statistically significant. At the same time, we found that two patients in group B had postoperative CT suggesting that the entry point of the SIS was in the fracture line, leading to failure of the internal fixation, while modified SIS placement adopts an anterolateral to posterior medial approach, and the distance of the SIS entry point from the iliac fracture line is much longer than that of traditional SIS, which effectively avoids the occurrence of internal fixation failure due to the screw entry point being located at the fracture line. The intraoperative and postoperative fluoroscopic results showed that the quality of the reduction of the sacroiliac dislocation was better in Group A than in Group B. We believe that the trajectory of the modified SIS is closer to the most obvious point of the sacroiliac dislocation and has a better effect on the reduction of the dislocated sacroiliac joint with compression. In addition, the use of the TiRobot navigation device intraoperatively helped us greatly by improving the accuracy of screw placement, greatly reducing the number of intraoperative fluoroscopies and shortening the operative time.
In clinical application, we also found several problems: 1. The sacral segment of the modified SIS is slightly shorter, and when the vertical pelvic shift is obvious, the S2 penetrating screw can be used to enhance fixation. 2. Fluoroscopy of the iliac wing front view is very important during intraoperative screw placement, as this image can not only prevent the LC-II screw from penetrating the inner or outer iliac plate but also clearly show whether the modified SIS breaks through the medial edge of the iliac bone and enters the pelvic cavity to avoid the "in-out-in" phenomenon with screws[34]. 3. When planning the screw path, the angle of inclination of the screw at the outlet view should not be too large; otherwise, the entry point may be obstructed by the greater trochanter of the femur. 4. The screw is inclined at a large angle to the iliac bone surface at the point of entry, so it is necessary to stop at the bone surface during entry and rotate the drill hole before continuing to enter to avoid slipping on the lateral wall of the iliac bone and resulting in a deviation of the screw trajectory. 5. An modified SIS should not be used in patients with bucket handle fractures; otherwise, the compression effect of the screw may aggravate the inversion, internal rotation and upward deformation of the pelvic ring on the injured side. 6. Intraoperative reduction of the fracture is crucial, and the surgeon needs to have extensive experience in the closed and incisional reduction of pelvic fractures. When a satisfactory reduction under closure cannot be achieved, a small incision can be made to assist in reduction[35, 36].
There are certain limitations to this study: 1. This is a retrospective study, so selection bias is inevitable. 2. The sample size of this paper is small. 3. There was no biomechanical stability testing. In future studies, we will conduct a finite element analysis and biomechanical studies of modified SIS combined with LC-II screws to provide an adequate biomechanical rationale for this treatment modality and to expand the clinical sample size and observe the clinical prognosis of follow-up patients.