Burgess[16] and Man-son[17] classified pelvic ring injuries into 3 major categories based on the mechanism of injury: anterior-posterior compression (APC), lateral compression (LC), and vertical shear (VS). Day type II CFDP belong to the LC-II type of injury. Previous studies have mostly concluded that the ligaments in the lower part of the sacrum are not damaged and therefore only rotational instability exists and are stable in the vertical direction[3, 18]. However, Burgess[16] suggested that when the pelvis is subjected to vertically orientated violence, it is also capable of producing injuries similar to crescentic fractures, and on pelvic front view, the anterior half of the ilium can show significant vertical displacement. Zong et al.[4] used whether the sacroiliac joints were displaced in the vertical direction by more than 1 cm as a basis for determining whether the pelvis was stable in the vertical direction, and their study of 31 patients with CFDP found that 27 of them had rotational instability of the pelvis, and 4 of them had pelvic instability in the vertical direction. In the two groups of Day type II CFDP patients in this study, a total of 7 patients had significant vertical displacement of the sacroiliac joints, and we found that all 7 patients originated from traffic injuries, we believe this is due to the multidirectional, high-energy nature of the violence of traffic injuries, so the patients with Day II CFDP can also have vertical displacement of the pelvis. We performed preoperative supracondylar femoral traction in all of these patients, which had a certain effect on the resetting of sacroiliac joint subluxation, and intraoperatively, intraoperative attention should also be paid to the need for vertical traction repositioning when correcting rotational displacement[19].
Xiang et al.[20] have been using percutaneous posterior iliac screws in combination with traditional sacroiliac screws to treat Day type II CFDP patients since 2005, and have achieved satisfactory results. In this study, a total of 35 patients' iliac fractures were fixed with LC-II screws, and we found that through appropriate reduction methods, most of the patients' iliac could be fixed with closed-replacement screws, thus reducing the trauma caused to the patients and promoting their early rehabilitation exercises. Sacroiliac screws are widely used for fixation of the posterior pelvic ring with its advantages of minimally invasive and biomechanical stability, but when we took traditional sacroiliac screws to treat Day type II CFDP patients, we found that the entry point of sacroiliac screws was located in the iliac fracture line in three patients, which resulted in our inability to use the screws to pressurize and fix the sacroiliac joints, and even internal fixation failure occurred. Moreover, from an anatomical point of view, the traditional sacroiliac screw has a short iliac segment channel, and the cancellous bone on both sides of the screw, which makes the screw holding force insufficient for elderly osteoporosis patients, and is prone to the "pendulum effect"[8, 21]. To address these shortcomings, we changed the trajectory of the traditional sacroiliac screw and proposed a modified sacroiliac screw channel. The inlet of this channel is located at the posterior superior margin of the acetabulum, and there are no important blood vessels or nerves in the surrounding area, which is a safer position for nail placement. The outlet is located at the medial side of the S1 superior articular process, which is similar to the entry point of the pedicle screw proposed by Roy-Camille, and it has been proved to be a safe position for pedicle screw fixation[5].
Ma et al.[21] conducted anatomical studies on a large number of normal 3D printed pelvises in a 1:1 ratio and proposed the existence of anterior-posterior sacroiliac screw channel. 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. In this study, postoperative CT of all patients in group A suggested that the modified sacroiliac screws were located in the bony channel, and none of them entered the acetabulum or sacral canal, which confirmed the safety of the modified sacroiliac screw channel. In this study, although the difference between the total length of the sacroiliac screws and the length of the sacral segment between the two groups was not statistically significant, the length of the iliac segment of the modified sacroiliac screws was longer than that of the traditional sacroiliac screws. Also, from an anatomical point of view, the iliac segments of the modified sacroiliac screws were encapsulated medially and laterally by cortical bone, which better provided stability of the posterior pelvic ring. Therefore, we believe that the biomechanical stability of the modified sacroiliac screw is superior to that of the traditional sacroiliac screw, and it is suitable for the fixation of the posterior pelvic ring in elderly osteoporotic patients. Meanwhile, the shortest distance from the entry point of the modified sacroiliac screw to the iliac fracture line in the ilium is much longer than that of the traditional sacroiliac screw, which effectively avoids the impossibility of pressurized fixation with the screw due to the fact that the entry point is located in the iliac fracture line.
From the comparison of the quality of reduction in the two groups, we found that the modified sacroiliac screw had better compression reduction effect on the dislocated sacroiliac joints under the premise of using the same reduction method. We believe that, on the one hand, the limited safe channel of conventional sacroiliac screws, especially in variant sacrum, where the safe channel of the screws is close to the posterior aspect of the sacroiliac joints, makes it difficult to provide good compression reset of anteriorly dislocated sacroiliac joints, on the other hand, the point of action of the modified sacroiliac screw is close to the most obvious position of sacroiliac joint dislocation, and although the screw trajectory is not completely perpendicular to the bone surface, it still has a better compression resetting effect on the dislocated sacroiliac joints(Fig. 9). Although the difference in Majeed's functional scores at the final follow-up between the two groups was not statistically significant, the VAS scores of sacroiliac joint pain still showed that the modified sacroiliac screws were more effective than the conventional sacroiliac screws for fixation of sacroiliac joint dislocation.
In addition, the use of the intraoperative TiRobot navigation device was of great help, as it improved the accuracy of nail placement, greatly reduced the number of intraoperative fluoroscopies, and shortened 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; 3.Intraoperative repositioning of the fracture is crucial, and the operator needs to have rich surgical experience in closed and incisional repositioning of pelvic fractures; 4.The sample size of the current study is limited, and there is a need for further expansion of the sample size and regular observation and follow-up in follow-up work to ensure the maturity and reliability of this technique.