The sacrum is an essential component of the pelvic ring, and unstable sacral fractures severely affect the integrity and stability of the posterior pelvic ring. It leads to traumatic spine-pelvis separation, and poor fracture repositioning can affect body weight-bearing and lower limb function. The treatment of unstable sacral fractures aims to rebuild the stability of the spine and pelvis, restore the biomechanical conduction of the lower extremity-pelvis-spine, and perform nerve decompression simultaneously when combined with nerve injury. The traditional posterior fixation methods commonly used in clinical practice include sacral rod fixation, posterior tension band plate fixation, and sacroiliac screw fixation. Sacroiliac screws and iliolumbar fixation were the most commonly used.
The advantage of iliolumbar fixation lies mainly in reconstructing the spine in the vertical direction. In 1994, Kach and Trentz first reported the successful treatment of longitudinally displaced sacral fractures using pedicle nailing and inter-iliac crest bracing, introducing the concept of the spine-pelvis bracing technique. We achieved the lumbar-pelvic fixation by connecting the L4 and L5 pedicle nails to the iliac crest screws with a nail rod. This technique is effective against vertical pelvic instability because it fixes the lumbar spine and pelvis with an arch nail system and has a bracing and closing effect, which is vertical. When there is concurrent sacral nerve injury and sacral canal occupancy, we can make posterior exploration for decompression and nerve repair simultaneously. This technique applies to all vertically unstable pelvic fractures. However, there are inherent disadvantages of this fixation method: sizeable surgical incision, which may cause complications such as infection and nonunion; slightly less effective fixation for unstable transverse fractures, which may cause fracture line separation; restriction of lower lumbar movement, which may cause scoliosis due to fixation on one side; the need to remove the internal fixation after fracture healing; and the need to bend the connecting rod, which increases the difficulty of fixation. Schildhauer et al. concluded that the iliolumbar fixation method does not maintain the rotational stability of the posterior pelvic ring. Because of its enhanced vertical stability, a 2-point fixation in the vertical direction cannot accomplish rotational stability. This type of fixation does not allow early weight-bearing.
Sacroiliac screw fixation is a significant advance in treating unstable sacral fractures and has become a minimally invasive technique commonly used to treat these fractures. These are the advantages of sacroiliac screws, such as minimal surgical injury, low rate of postoperative infection, and low incidence of heterotopic ossification. Compared with other posterior internal fixation techniques, the incidence of vascular and nerve injury caused by sacroiliac screws is higher, about 2%-15%. Kraemer et al. compared the extraction force of sacral body long screws, sacral body short screws, and sacral wing short screws, and the mean extraction force was 925 N, 327 N, and 71 N in order, and the difference was statistically significant. Sacroiliac screws that have been lengthened are utilized to strengthen the stability of the sacral fracture. Gardner and Routt proposed lengthened sacroiliac screws. The screws penetrate from the sacroiliac joint on one side to the sacroiliac joint on the other, achieving adequate stability.Jazini et al. concluded that vertical shear is the primary stress-causing instability of the posterior pelvic ring and confirmed by biomechanical tests that this stress is distributed over the entire screw. Therefore, one longer screw allows for a more reasonable distribution of stresses. The most extended screw that spans the entire sacroiliac complex is the lengthened sacroiliac screw, which is particularly suitable for bilateral sacral fractures. The number of cortical bones crossed medially and laterally by the lengthened sacroiliac screws is essentially the same at the fracture line, providing a balanced fixation. The lengthened sacroiliac screws used in this study are screws that penetrate the contralateral cortex. Sacroiliac screws have some shortcomings. ating et al. obtained an intraoperative rate of 84% anatomic repositioning or subatomic repositioning using the sacroiliac screw technique. However, the healing rate of the deformity was found to be as high as 44% at follow-up.
Griffin et al. concluded that sacroiliac screw fixation of vertical sacral fractures is more likely to result in internal fixation failure and loss of reduction.
The strength of iliolumbar fixation and sacroiliac screw fixation is not sufficient. Schildhauer et al. proposed Triangular osteosynthesis, a vertically oriented spinal one pelvic fixation combined with a transverse fixation device. The biomechanical study by Schildhauer et al. also showed that triangular fixation was stronger than sacroiliac screw fixation for unstable sacral fractures. There are still many questions about the biomechanical properties of triangular fixation that need to require attention. We, therefore, performed a finite element biomechanical study of Triangular osteosynthesis.
This study modeled a finite element model with a longitudinal cut through the right sacral foramen to create a unilateral vertical sacral fracture model (AO C3.1 DENISS II). Unilateral vertical sacral fractures involving the L5/S1 tuberosity are often exceedingly unstable; however, in this case, the budget was simplified, and the fracture line did not involve the L5/S1 tuberosity. In the fixation model, sacroiliac screws were used for trans-S1 segmental fixation, with normal sacroiliac screws and lengthened sacroiliac screws, respectively. Increasing the length of sacroiliac screws on the biomechanical properties of internal fixation with triangular fixation was evaluated. The design of the iliolumbar fixation model in the fixation model took into account that the fracture model was a unilateral sacral vertical fracture using a unilateral iliolumbar fixation model. This paper used two L4L5 segments or a single L5 segment for lumbar fixation. The evaluation of whether increasing the fixation segment affects the biomechanical properties of internal fixation was compared.
The sacral vertical displacement distance is an important index to assess the vertical stability of the sacrum. Under a vertical load of 600 N, the vertical displacement distance of the normal sacral model in this study was 0.159 mm. none of the four fixation models could achieve the stability of the sacrum in the normal state under fixation. L5LS1 sacrum had the best vertical stability among the four fixation models, followed by L4L5LS1. We found that the fixation model achieved the best state of sacral stability with lengthened sacroiliac screws. Therefore, increasing the length of sacroiliac screws can increase the vertical stability of the sacrum when applying the triangular fixation technique to treat unilateral vertical sacral fractures. Fixation model with L4L5 segment fixation versus L5 segment fixation only, With normal sacroiliac screws, the vertical displacement distance of the sacrum was increased by increasing the lumbar fixation segment. When lengthened sacroiliac screws were applied, the vertical displacement distance of the fixed L4 and L5 segments was smaller than that of the fixed L5 segment only, but the values were close to each other. This phenomenon may be because increasing the lumbar fixation segments alters the normal force transmission in the lumbar spine. The study that increasing the length of the sacroiliac screw increased the vertical stability of the sacrum is consistent with the findings in the literature. The vertical displacement of the sacrum was not significant at 100N follower load + 7NM torque.
The fracture separation distance represents the degree of stability of the fracture line in the fixed state. Under a vertical load of 600 N, the superior fracture line displacement distance was significantly more significant than the inferior fracture line displacement distance. This phenomenon is consistent with the biomechanical characteristics of the pelvis. The sacrum under vertical force, the force is transmitted along the sacroiliac joint-pelvis-acetabulum, so the closer the fracture line is to the mechanical transmission path, the greater the displacement. Comparing the a1-a2 distance in the four fixation models under 600N vertical load and 100N slave load + 7NM torque, the fracture separation distance with lengthened sacroiliac screws was significantly smaller than that in the model with normal sacroiliac screw fixation. However, in the same sacroiliac screw model, there was no significant difference in the a1-a2 distance between the models with the L4L5 lumbar fixation segment and L5 segment. Increasing the length of the sacroiliac screw when applying the triangular fixation technique to fix unilateral vertical sacral fractures increased the stability of the fracture end, and increasing the lumbar fixation segment had no significant effect on fracture stability.
The implant Von Mises stress represents the implant stress state in the finite element model. This study compared four groups of the implant model's maximum Von Mises stress. The maximum Von Mises stress value of L4L5LS1 is the smallest at 107.9 MPa under 600N vertical load. The maximum Von Mises stress value ofL4L5NS1 is the largest at 131 MPa. Under 100N + 7NM from the load, the maximum Von Mises value of the L5LS1 model is a minimum 40.8MPa. The maximum Von Mises value of L4L5NS1 is maximum 87.18MPa. Therefore, the internal fixation stress of the fixed L4L5 plus S1 lengthened sacroiliac screws combination is minor regardless of the standing condition or the lumbar rotation condition. The maximum Von Mises values of the four fixation models in this study did not differ significantly regardless of the motion except L4L5NS1. This result may alter the normal mechanical conduction of the lumbar spine after fixation of the L4L5 segment. Analyzing the Von Mises stress distribution of the four groups of internal fixation models, the triangular fixation under a vertical load of 600 N showed that the stresses were concentrated around the fracture ends of the linked pedicle screws and iliac screws, as well as the sacroiliac screws. Analyzing the Von Mises stress distribution of the four groups of internal fixation models, stress concentrations were observed at the pedicle screw and iliac screw attachment bar and around the sacroiliac screw fracture under a vertical load of 600N. Under a 100N slave load + 7Nm torque, stress concentrations were observed at the pedicle screw, the pedicle connecting rod, the iliopsoas screw connecting rod, and the sacroiliac screw fracture. This result is also consistent with clinical practice.
It is essential to point out that this study has some limitations. This study used a unilateral vertical sacral fracture (AO C3.1 Denis II) type, and the sacrum was cut longitudinally to create a vertical sacral fracture model. However, because of the variety of anterior pelvic ring injuries, their treatment methods are equally diverse, and the different treatment methods will undoubtedly impact the results of this study. The increase of influencing factors will inevitably increase the difficulty of data analysis in this study. Therefore, this study was not designed for anterior ring injuries, preserving the integrity of the anterior ring. This study is a finite element study based on pelvic CT data. Although finite element studies have made significant progress in recent years, there may be some differences between this study and human studies.
Using iliolumbar fixation combined with sacroiliac screws for unilateral vertical sacral fractures (AO C3.1 DENISII), the application of lengthened sacroiliac screws increased the vertical stability of the sacrum after internal fixation. It increased fracture stability when the sacroiliac screws were placed on the S1 segment. The use of triangular fixation with simultaneous fixation of L4 and L5 segments was not significantly effective in positively correlating the vertical stability of the sacrum with the stability of the fracture end. We should try to use the L5 segment for lumbar fixation.