An unstable pelvic fracture is a severe injury that is the cause of high mortality and morbidity . Due to complex local anatomy, unique biomechanics, and poor bone quality, the fixation of sacral fractures remains a challenge . A key aspect of sacral fracture repair is sufficient stability to counterbalance translational and rotational forces in the vertical and horizontal directions . In addition, to improve the quality of life of patients following surgery, sacral implants should also minimize adverse effects on patients. In the present study, static FE analysis methodology that is widely accepted by the community was used to evaluate the stability of five internal fixation methods on sacral fracture. The results of simulation show that the new internal fixation methods for the treatment of Denis type I sacral fractures can meet safety and stability requirements.
In a biomechanical study, Acklin et al. found that under Denis II sacral fracture model with vertical loading, the average axial stiffness of SIS was greater than that of unilateral LPF . Although a Denis type I fracture model was used in the present study, the results of the simulation were consistent with those in the literature. In an FE analysis study, Song et al. found that unilateral LPF could not provide sufficient horizontal and rotational stability for patients with unilateral sacral fractures . In the present study, the change in relative displacement of the unilateral LPF fluctuated greatly in flexion compared with standing, indicating that unilateral LPF is potentially unstable. In another study, Vigdorchik et al. found that the stiffness of the SIS was greater than the pedicle screw construct (namely TIFI) under the sacroiliac joint injury model . Although a Denis type I fracture model was used, the stiffness of SIS which is greater than TIFI in our study was consistent with the result of Vigdorchik et al.
A higher maximum von Mises stress indicates that the model has a higher risk of implant failure . According to the results of the simulation (except SIS), the LPF group displayed a greater risk of implant failure, occurring at the connection between the iliac screw and rod. This confirms previous clinical studies which found that implant failure usually manifests as disengagement of the screw to rod connector . The TIFI and S2AI-CS1 groups displayed a moderate risk of implant failure at the shaft and junction of the head and main screw. The S2AI-S1 group exhibited a lower risk of implant failure which also occurred at the shaft and the junction of the head and secondary screw.
Higher stresses on the bone around the screw may lead to screw loosening and secondary fractures . Based on the results of the simulation, the SIS group has a higher risk of screw loosening than the LPF group. In a cyclic loading experiment by Acklin et al., SIS was found to be more prone to loosening than LPF , consistent with our prediction. Furthermore, the main screws were predicted to have a greater risk of loosening than the secondary screws. For the main screws, the S2AI-CS1 and SIS groups had a higher risk of screw loosening, the S2AI-S1 and LPF groups displayed a moderate risk of screw loosening, while the TIFI group had a lower risk of screw loosening. For the secondary screws, the S1 pedicle screw in the S2AI-S1 group has a higher risk of loosening than other screws. Therefore, it is believed that the disadvantage of S2AI-S1 is a relatively increased risk of loosening of the secondary screw. When comparing the S2AI-S1 group with the S2AI-CS1 group, the ipsilateral S1 pedicle screw had a greater influence in reducing the maximum von Mises stress for the S2AI screw than the contralateral S1 pedicle screw.
According to the mean RD values, the overall stability of the five internal fixation methods in the FE models, in descending order are: S2AI-S1, S2AI-CS1, SIS, LPF, and TIFI. Considering that unilateral LPF and TIFI are insufficient for stability of the posterior pelvic ring, it is considered necessary to add a sacroiliac screw to unilateral LPF or TIFI to enhance stability, as demonstrated in previous studies [29, 34]. The RD of different points reveals that a fracture is more stable the closer it is to a screw (S2AI screw/Sacroiliac screw). This indicates that a single-segment S1 sacroiliac screw provides limited stability for the lower sacrum. A number of researchers have suggested adding an S2 sacroiliac screw for biplanar stability [19, 35]. Since the S2AI-CS1 group had greater RD at point 1 and point 4 than the S2AI-S1 group, the former was slightly less stable when used in sacral fractures than the latter. In addition, although the overall stability of the S2AI-CS1 group for sacral fractures was superior to that of SIS, the stability of the upper sacrum was slightly worse.
The change in relative displacement reveals the principal reason for the difference in stability. TIFI and unilateral LPF are methods that can achieve indirect stabilization of sacral fractures through a screw-rod system, while SIS, S2AI-S1, and S2AI-CS1 can provide direct fixation of sacral fractures using screws. According to the AO principles of fracture management, fixation of the fracture interface using screws could create a preload . This preload would compress the fracture and prevent separation, while friction between the fracture surfaces and between the screw and bone would oppose displacement due to shear . By comparing the relative changes in displacement for three types of motion, the TIFI and LPF groups displayed greater fluctuations during motion, while the SIS, S2AI-S1, and S2AI-CS1 groups exhibited only small fluctuations. This indicates that the SIS, S2AI-S1, and S2AI-CS1 groups can achieve absolute stability for the fracture, while the TIFI and LPF groups can only achieve relative stability.
This study confirmed the role of the new internal fixation in the stability of Denis I sacral fractures. The invasiveness of LPF and problems with the fixation range  (limitation of mobility of lumbar vertebrae, adjacent segment disease, and the need for implant removal) suggest that S2AI-S1 may be a beneficial method for internal fixation. Compared with TIFI, S2AI-S1 is not only expected to improve in biomechanical stability, but also eliminate the problem of screw protrusion. Although S2AI-S1 may not be as minimally invasive as SIS , the former has better biomechanical stability and a lower risk of screw loosening than the latter. Considering that S2AI-CS1 is slightly less stable in the upper sacrum and that the main screw has a higher risk of loosening, such fixation is not recommend as the first choice. When the S1 pedicle screw insertion point on the affected side is damaged, S2AI-CS1 can be used as a good alternative to S2AI-S1. Although the new internal fixation method can meet the safety and stability requirements, patients are advised to avoid premature weight bearing to prevent implant loosening and loss of reduction.
Nevertheless, some potential limitations related to this study should be noted. First, the biomechanical results obtained through finite element analysis are inadequate to fully confirm the stability of S2AI-S1, which needs to be further verified by cyclic loading experiments on cadaver bone models. Second, the screw in this experiment used simplified processing to reduce the calculation time and analysis error due to stress concentration. However, to obtain accurate stress distribution on the screw and the bone around the screw, the screw thread should be considered in the FE model . Third, in this study, only the main ligaments were simulated, but in practice, some other ligaments and muscles may still play a role . Therefore, there may be some differences between our simulation data and cadaver bone model data. Finally, the present study used CT data from a normal male pelvis for development of the FE model, and no anatomical differences between individuals were considered.