Unstable pelvic ring injuries typically result from high-energy trauma and are considered by orthopaedic surgeons as one of the most clinically challenging lesions. Minimally invasive fixation of pelvic fractures as an adjunct to posterior fixation is becoming more common and can effectively improve anterior pelvic stability and restore function in the pelvis, while being associated with minimal traumatic invasion and fewer complications . INFIX is a technique in which pedicle screws are placed on the supra-acetabular corridor with a crossrod in the bikini area . INFIX has recently emerged as an alternative to traditional surgical methods with the advantages of few soft tissue complications, improved biomechanical construction, and no disruption of sitting posture or screwdriver function. MPRSF combines the advantages of INFIX and pelvic bridging. The safety and effectiveness of minimally invasive fixation of pelvic fractures have been demonstrated by several anatomic and clinical studies, although fixation stability remains a concern [20, 21]. Additionally, few studies have investigated whether these 2 fixation techniques lead to sufficiently strong implants and adequate biomechanical reduction.
The FEA results showed that displacement distribution and VM stress were similar in the injured pelvis models treated by MPRSF and INFIX compared to the intact pelvis, indicating that both fixation techniques can effectively treat anterior pelvic ring fractures. However, maximum displacements of the pelvis and implant were lower in the MPRSF group than in the INFIX group in the single- (left/right) and dual-leg standing and sitting postures, indicating greater stability. On the other hand, the injury model treated by MPSRF showed slightly higher maximum implant stress compared to the INFIX group in all 4 postures, which may be attributable to part of the stress being concentrated on the third screw and obstructing implant micromovement.
Although the MPRSF group had a longer operation time and greater intraoperative blood loss, there were no significant differences in hospital stay and postoperative complications between the 2 groups. Thus, MPRSF only expanded the local skin incision without causing permanent neurovascular injury and did not increase the risk of postoperative complications. However, radiographic (Tornetta and Matta grade) and functional (Majeed score) outcomes of the MPRSF group were improved compared to the INFIX group, especially at 6 months after surgery. Thus, while both fixation methods can restore stability to the anterior pelvic ring and pelvic function, MPRSF promotes the latter at an earlier time point following operation.
LFCN irritation is the most common postoperative iatrogenic complication . In a multicentre review, 30% (21/91) of patients had LFCN irritation although in most cases it was self-limiting and improved once the implant was removed . In line with this study, in our investigation the rate of LFCN injury was 33.3% (10/30) in the MPRSF group and 24.2% (4/33) in the INFIX group. A case series of LFCN irritation suggested that screws that are too deeply or insufficiently embedded in the bone and inadequate pre-bending of the rod can lead to irritation of the LFCN and sartorius muscle. To prevent this, a rod-to-bone distance of 20–25 mm (30–40 mm for obese patients) but < 40 mm is recommended . We first locked the screws at bilateral AIIS so that the pullout strength of the screw was mainly concentrated in the supra-acetabular region, where bone density is high. The screw at the pubic tubercle—which has relatively sparse bone—was then locked, thus providing auxiliary support. Furthermore, additional screws should not be placed too close to the lateral pubis to avoid damaging the spermatic cord or round ligament. For unilateral pubic rami fractures, the screw was fixed into the fracture side if the fracture line was far away from the pubic symphysis; otherwise, it was inserted on the uninjured side. For bilateral pubic rami fractures, the screw was fixed into the side with less injury.
Recent studies using the INFIX or MPRSF technique have reported potentially devastating complications, especially femoral nerve palsy. There was one such case in the MPRSF group, but symptoms gradually disappeared once emergency screw adjustment was performed, and there was no permanent nerve damage after implant removal. In a case series of iatrogenic femoral nerve palsy, it was suggested that femoral nerve compression occurs as a result of impingement of the implant on the psoas sheath; meanwhile, delayed palsy may be caused by engorgement of the psoas with blood and/or a change in pressure . The authors noted that this could be avoided by placing the interconnecting rod in such a way that it does not limit the space for the psoas and femoral nerve. Our solution for reducing neurovascular compression was to bend the connecting rod outward in the horizontal direction (Fig. 4). Although follow-up is ongoing, the patient’s neurovascular compression symptoms have been significantly alleviated.
Despite the positive clinical outcomes and FEA results, our study had several limitations. Firstly, this was a single-centre retrospective study with a relatively small sample size; more cases should be examined in a multicentre investigation, with long-term functional assessment. Secondly, the characteristics of the implant materials are not consistently reported; as such, our results depend on input parameters. Additionally, the FEA model was patient-specific and ignored the effects of ligaments and muscles on the stability of the pelvis and implant devices. Finally, the angle of applied force was constant although variations in the angle may have influenced VM stress and displacement, as reported in a previous study .