The advantage of iliolumbar fixation lies mainly in reestablishing spinopelvic stability in the vertical direction. Kach and Trentz [22]first applied the pedicle screw system of the spine to the ilium and called it Spinal-pelvic fixation or iliolumbar fixation. The disadvantages of this fixation are obvious firstly, limited resistance to pelvic rotation, and secondly, difficulty in the surgical repositioning of the rotational deformity of the pelvis with the help of iliolumbar fixation. Schildhaner al. [11] Showed that this type of fixation only enhances vertical stability. In contrast, rotational stability cannot be accomplished by 2-point fixation in the vertical direction, so the posterior pelvic ring still has rotational instability that does not allow early weight-bearing.
Sacroiliac screws are tension screws that are inserted into the sacrum through both cortices of the ilium and one side of the sacrum. There is also a lengthened sacroiliac screw that goes from the iliac bone on one side through the sacrum to the opposite iliac bone. Zhao al. [15–16] conducted a biomechanical study on lengthened sacroiliac screws and concluded that the increased length of lengthened sacroiliac screws would better divide the load in the vertical direction than normal sacroiliac screws, reduce stress concentrations, and counteract displacement. These long sacroiliac screws have more threads than the short sacroiliac screws and can be anchored to the bilateral iliac cortical bone, increasing the holding power of the screws. This method has disadvantages, such as higher technical requirements, higher risk of neurovascular damage, longer operation time, and more bleeding.
TOS is a lumbopelvic osteotomy combined with lateral fixation of sacral fractures, allowing immediate entire weight-bearing functional exercise for patients with C-shaped pelvic fractures. Lumbar pelvic fixation can be performed with L4 or L5 pedicle nails combined with sacral lateral blocks or iliac pedicle nails. Transverse fixation of sacral fractures can be with trans-sacral plates, sacroiliac screws, or both for all vertically unstable sacral fractures. Schildhauer et al. [12] compared biomechanical experiments on early weight-bearing of unstable sacral fractures. It was concluded that the stability of the posterior pelvic ring was significantly higher with TOS fixation than with sacroiliac screws [23]. The use of iliolumbar fixation combined with sacroiliac screws is now a more common mode of clinical application for TOS. There are some complications of TOS, such as nerve injury, vascular injury, and soft tissue injury. The application of orthopedic robots and orthopedic navigation has reduced the complications of TOS surgery. Furthermore, some scholars have accomplished the application of TOS in a minimally invasive way to reduce soft tissue injury during surgery and achieved good results [24].
A literature review shows that few biomechanical studies are addressing the triangular osteosynthesis. There are no relevant studies on the effect of sacroiliac screw fixation position and length on the overall mechanical properties of the implants in triangular osteosynthesis.We designed six fixation models for biomechanical studies. The sacral fracture model simulated a unilateral vertical fracture of the sacrum through the sacral foramen. Unilateral L5 + iliac fixation was used to simulate unilateral iliolumbar fixation for all fixation models. The fracture in the model did not involve the L5/S1 tuberosity, so the L5 vertebral body was intact, and a single L5 segmental fixation was more appropriate to the actual situation. In using sacroiliac screws, we chose S1 and S2 alone, and S1 + S2 combined to compare the effect of different fixed segment fixation on the stability of sacral fixation. We chose normal sacroiliac screws and lengthened sacroiliac screws to be applied separately to compare the effect of different sacroiliac screw lengths on the stability of sacral fixation.
The vertical stability of the sacrum after fixation was represented by the displacement distance of the upper surface of the sacrum under a vertical load of 600 N. None of the six fixation models could achieve the normal model of sacral stability. The best stability was obtained by simultaneous fixation of the S1 segment + S2 segment with lengthened sacroiliac screws, followed by the model with the S1 segment with lengthened sacroiliac screws. In order to obtain the best vertical stability of the sacrum with the triangular osteosynthesis, we recommend simultaneous fixation of the S1 segment + S2 segment with lengthened sacroiliac screws. If only a single-segment screw fixation is available, we choose the S1-segment lengthened sacroiliac screw fixation. When the triangular osteosynthesis is used, the sacrum's vertical stability is good with the S1 segment normal screw fixation. Secondly, S1 segment + S2 segment double segment normal screw fixation, but both values are close. Therefore, we suggest that if only normal sacroiliac screws can be applied as a transverse fixation device for triangular osteosynthesis, we suggest choosing S1 segment fixation.
The fracture separation value represents the stability of the fracture contact surface after sacral fixation. Unlike the vertical stability of the sacrum, the fracture separation value represents the stability in multiple directions.In this study, point a was marked on the fracture line as the marker point, and when the fracture line moved to produce two points a1a2, the distance between the two points was the fracture separation value.Point a is selected closer to the sacroiliac joint on the fracture line. This location is on the spinal-pelvic mechanical conduction axis, and the displacement of point a is more indicative of the stability of the fracture contact surface.Under a vertical load of 600N, the fracture separation value was the smallest for simultaneous fixation of the S1 segment + S2 segment with lengthened sacroiliac screws, followed by fixation of the S1 segment with lengthened sacroiliac screws. At 100N slave load + 7Nm torque, the fracture separation value was the smallest with the lengthened sacroiliac screw fixation of the S1 segment + S2 segment simultaneously and the same with the lengthened sacroiliac screw fixation of the S1 segment.
The maximum von Miles stress of the endophyte represents the stress concentration of the endophyte under mechanical action. Under a vertical load of 600 N, the maximum von Miles stress of L5LS12 was the smallest at 89.5 MPa. This indicates that when the triangular osteosynthesis was done using lengthened sacroiliac screws while fixing the S1S2 segment, the stress distribution was more uniform, the maximum stress on the endophyte was minor, and the risk of fracture of the endophyte loosening screws was low. Under the action of 100N slave load + 7Nm, the maximum von Miles stress of L5LS1 was the smallest at 40.80Mpa, followed by L5LS12 at 42.25MPa; both values were close.
In our study, we found poor vertical stability of the sacrum, large fracture separation values, high maximum von Miles stress of the endophyte, and a high risk of fracture of the internal fixation loosening screws when the S2 segment was chosen as the fixed segment. This phenomenon was more pronounced with the application of normal screws than with lengthened screws. This is because the S2 segment is farther away from the spinal-pelvic mechanical conduction axis than the S1 segment and has a weaker counteracting effect on mechanical conduction. We also found this feature in the internal fixation distribution cloud, where the stresses on the S1 segment screws were significantly more concentrated than those on the S2 segment screws. This indicates that the S1 segment screws are more effective than the S2 segment screws in resisting the forces at the fracture end.
In this study, the following points must be kept in mind. First, whereas anterior ring instability is a hallmark of C-shaped pelvic ring injuries, numerous treatment strategies for anterior ring fractures may have an impact on the stability of the posterior pelvic ring. The anterior pelvic ring was solely kept in its normal state during the current study rather than simulating its injury and fixation. Our comparison of various study models was unaffected by the minimal impact of the anterior pelvic ring on the stability of the posterior pelvic ring. Second, we kept numerous significant pelvic ligaments in our analysis to better mimic the stability of the pelvis. Additionally, we did not simulate the muscles to mimic the additional stability they could cause to remove any unanticipated forces that could alter the measurements. Although joint flexibility settings, muscle parameters, and joint data factors had a qualitative impact on the outcomes, the computations were too complex to complete the experiment。