With the acceleration of the rhythm of life and the different travel modes in modern society, various high-energy injuries such as car accident injuries, high-energy blunt injuries and fracture injuries caused by falling from a height have become more common. In particular, pelvic fracture and traumatic spinopelvic dissociation cause the most serious consequences, ranging from physical dysfunction or disability to life-threatening conditions[16, 17]. Although the design of medical equipment and the development of various technical methods for spinal and pelvic surgery internal fixation have made many new advances, achieving stable sacral pelvic fixation is still one of the most difficult challenges in orthopaedic surgery[15, 18]. There are two reasons for this challenge. First, the lumbosacral region is characterized by a unique complex anatomical structure, and, second, current devices for the treatment of fractures do not perfectly solve the problem of fracture internal fixation. At present, there are various fixation methods for the posterior pelvic ring and fracture of the local region, such as the traditional sacroiliac screw, anterior or posterior sacroiliac joint plate fixation, posterior tension band plate fixation of the sacrum, and sacropelvic fixation based on screw-rod, etc. In numerous fixation devices, sacroiliac screws have been applied by clinicians, because of their advantages of minimal invasiveness, a short learning curve and easy placement. Hence, instead of being phased out, these screws have been greatly developed in recent years, with advancements ranging from simple single sacroiliac screw fixation to percutaneous cement sacroiliac screw fixation[19–21] and the recent emergence of lengthening sacroiliac screws through the bilateral sacroiliac joint. Despite the development of sacroiliac screws, many problems caused by the sacroiliac screw itself cannot be changed, such as screw fracture, insufficient internal control force, poor stability, and important neurovascular injury in the screw placement process. However, the development of sacroiliac screws reflects the idea that clinicians want to explore and implement a reliable, fixed and effective screw.
Lumbar-sacral fusion has been utilized in many clinical scenarios, such as flat-back syndrome and kyphosis, pelvic obliquity, high-grade spondylolisthesis, and extensive sacropelvic tumour resection[7, 9–13]. Lumbar-sacral fusion surgery has experienced approximately three generations of development. The first-generation technique was the Galveston technique in 1984, the second-generation technique involved iliac screws with connectors, and the third-generation technique was the SAI (sacral alar-iliac) screw technique. The first S2AI screw was successfully performed in paediatric spinal surgery by Dr. Sponseller and Dr. Kebaish in 2007[23, 24]. Dr. Mattei et al. reported the combined use of S1AI and S2AI as a remedy and expanded the scope of application of the SAI technique in 2013. S3AI was first reported as a new long-stage spinal fusion anchor point in 2020. The SAI technique had several strengths: improved construct stability and biomechanical torsion and reduced complications, including implant prominence, wound healing problems, and pain of the sacroiliac joint. Changing the position and orientation of the fixing screws is key to the success of the SAI technique; therefore, inspired by the SAI screw trajectory, we changed the traditional sacroiliac screw trajectory as follows: the sacroiliac screw started at the entry point (as shown in Fig. 1) on the dorsal side of the sacrum, passed through the sacral alar and sacroiliac joint, and extended into the iliac wing, thus we called it a “modified sacroiliac screw”.
The advantages of the modified sacroiliac screw over the traditional sacroiliac screw are reflected in the following aspects. First, after comparing screw trajectories, we concluded that the modified sacroiliac screws had a greater increase in screw length in the sacrum and iliac bone than traditional sacroiliac screws, thus the stability would be increased. Second, there were M2SI and M3SI screws in four modified sacroiliac screws passing through the intraosseous bone above the greater sciatic notch, thus the thread purchase force in the intraosseous of iliac will be further increased, and the stability and pullout resistance will be increased again (Note: The presence of the iliac grooves that need special attention can affect the length of M2SI and M3SI screw placement, so preoperative three-dimensional pelvic model reconstruction is still necessary). Third, the direction of all modified sacroiliac screws is at an acute angle with the direction of the broken screw force, which will reduce the breakage rate of the modified sacroiliac screw. Fourth, based on the parameters of the modified sacroiliac screw that we measured, we chose the screw with the maximum radius, because the lateral bending strength of the screw is proportional to the radius to the 4th power. Fifth, modified sacroiliac screws can further stabilize the sacroiliac joint, thus significantly reducing lumbosacral pain caused by sacroiliac joint instability.
Due to the theoretical advantages of these modified sacroiliac screws, we conducted this study by measuring the parameters and angles of modified sacroiliac screws in order to instruct clinical application.
In view of the results, we found that each channel could be fitted with a screw of maximum radius and length. The screw radius decreased sequentially from M1SI to M4SI, and their mean radius ranged from 4.96 ± 0.38 mm to 3.4 ± 0.39 mm, while the gradual decrease in the volume of the sacrum from proximal to distal was responsible for tapering of the screw radius. The order of screw length was M4SI ༜ M1SI ༜ M2SI ༜ M3SI, and the range of mean length of the screw was 89.05±9.48 mm to 124.24±12.64 mm. The trajectory of the M3SI screw started from the PSIS and extended to the ASIS (anterior superior iliac spine), and therefore, it was the longest screw. The range of the distance between the entry point and the PSIS was 25.29±4.52 mm to 49.49±7.43 mm. These data illustrated that the entry point for M2SI was closest to the PSIS, while M4SI was furthest from the entry point to the PSIS. As was summarized in the PSIS and α table, a number of statistically significant differences (P <0.05) appeared in the genders, such as the PSIS length, while M4SI was furthest from the entry point to the PSIS. The mean screw radius of M3SI and M4SI screws were 4.26±0.44 mm and 3.52±0.32 mm for males and 3.91±0.08 mm and 3.29±0.42 mm for females. The mean screw length of M3SI and M4SI screws were 128.05±8.15 mm and 91.39±8.69 mm for males and 120.56±15.45 mm and 88.63±8.25 mm for females. The mean screw distance from the entry point to the PSIS of M3SI and M4SI screws were 34.58±6.39 mm and 50.90±7.49 mm for males and 33.39±6.27 mm and 48.08±7.18 mm for females. These data (P <0.05) indicated that the modified sacroiliac screws used in men were thicker and longer than those used in women, and confirmed that the male pelvic wall was thicker and higher than the female pelvis. However, the α angle was greater in women than in men, since the mean screw α angle range of the M4SI screw was 32.99°±5.03° to 34.21°±4.98° for males and 34.98°±5.44° to 36.13°±4.56° for females. These data reminded us of the difference in the α angle in males and females during intraoperative screw placement, and were also consistent with the characteristics of the female pelvis. There was no significant difference in the other parameters of the MSI screws, so we did not analyse and compare them, but they were equally important in guiding our clinical work.
Although we performed much work in this study, there were some limitations. First, the measurements of all data were manually measured on the real data pelvis simulated by the software, and there were artificial errors. Second, the pelvis model simulated by the software occurred after smooth operation, so there was a model distortion problem caused by the software, resulting in measurement error.