The current study found that LC-1 PFCSFs could get benefits from the open reduction and internal fixation. The treatment algorithm should be separately made according to the degree of the sacral fractures displacement.
Stable LC-1 fractures can get good outcomes from conservative treatment because they will not displace under normal physiological weight bearing. Conversely, unstable patterns or displaced fractures are usually treated with surgical reduction and stabilization[23-25]
Determining whether or not an LC-1 fracture is stable remains challenging.  Magnetic resonance imaging and ultrasonography have been proposed as adjuncts, but their clinical utility has not yet been elucidated [26, 27]. Recently, the predicted value of pelvic ring instability by positive MUA was verified by several studies[14, 15]. By intraoperative MUA, Tosounidis et al.  demonstrated that the LC-1 injuries with a complete posterior sacral injury are inheritably rotationally unstable. Sagi et al reported that nearly 40% of LC-1 pelvic fracture was operatively treated with/without posterior ring fixation. Whereas, no more details of the long-term functional results were recorded. On the other hand, Whiting et al. reported that immediate weight-bearing as tolerated seems safe in patients with pelvic ring injuries who have had a negative MUA. Although the MUA may be the most reliable, it requires general anesthesia and may not be cost-effective, studies to evaluate this is warranted. Evaluation of pelvic ring stability in LC-1 type fractures, the X-ray or CT scan is widely used. According to Bruce et al. using CT scans, complete sacral fracture and bilateral rami fractures displace more prone to future displacement .
Previous clinical and biomechanical study [17, 28, 29] found that in partially unstable LC-1 fractures, fixation of the anterior pelvic ring only, which was similar to Group 1 in the current study, can provide some indirect compression along the sacroiliac joints, which can be beneficial for the functional outcomes. We compare the outcomes between the solitary anterior fixation and anterior-posterior pelvic ring fixation. Kanakaris  carried out a comparative study between the displacement of anterior or posterior pelvic fractures less 5 mm and more than 2 cm assessment during the EUA. However, the spectrum was broad, they did not mention the intermediate displacement of pelvic fractures. Although, Gaski et al.  investigated the intermediate displacement which was limited to the initial displacement less than 1cm by the measurement in the plain radiographs, however, the measurement has the inherently inaccurate defect, which has been confirmed by the previous study. Lin et al. reported that 45.5% of patients with bilateral ramus fractures and 42.0% of patients with dual-ramus fractures had concomitant sacral fractures not observed on plain radiographs yet ，let alone the accurate measurement. This issue can be solved by the CT scan. In the current study, we used 1 cm measured in the CT scan as the threshold to divide the displacement was much more reasonable.
Surgical intervention in these two groups was associated with significantly reduced pain post-operatively (3.05±0.98 in Group 1 and 4.06±1.32 in Group 2). The results were similar to the previous reports[15, 19]. These results support the view that surgical intervention provides significant pain relief and allow early ambulation. Simultaneously, in solitary anterior fixation (Group 1), the operation time was reduced significantly (117.6±46.0 vs. 158.9±28.1 mins), the saved time was similar to the previous reports[30, 31]. With the shortening of the operation time, the surgeons and patients also have less radiation exposure, which was essential for both patient’s and surgeon’s health. The current study showed that the excellent and good rate of functional outcomes (Majeed scores) was 88.9% and 87.5% in Group 1 and Group 2 respectively. There was no significant difference between Group 1 and Group 2 (p>0.05). These functional outcomes certified that patients with unstable LC-1 pelvic fracture would get benefits from surgical stabilization. The functional outcomes seemed better than the non-operation, which was performed by Gaski et al. in the complete sacral fracture with displacement less than 1cm.
The current study found that, in Group 1, after the anatomic reduction and internal fixation of the anterior ring, the displacement of the posterior ring was also reduced, there was no significant difference in the radiographic radiologic outcomes between this two groups（p>0.05). Meanwhile, there was also no significant difference between the fixation pattern of the anterior ring (p>0.05). The biomechanical study indicated that the retrograde screw could provide the comparable stability to reconstruction plate, meanwhile, the latest comparative study showed that the modified pedicle screw-rod fixation (infix) and anterior external fixation could provide similar satisfactory clinical outcomes for anterior pelvic ring fracture . Therefore, the selection of the implant for anterior ring fixation did not affect the outcomes in LC-1 pelvic fractures. The final follow-up of this group (Group 1) showed that all sacral fractures healed. We deduced that after the anatomic reduction of the anterior ring, the posterior ring restored to the correct position, the tension of the posterior ligament complex was reduced, which was beneficial to the healing of the posterior ring injury. Usually, the bilateral pubic ramus fractures were greater unstable than the unilateral pubic ramus fracture; however, there were no significant differences in functional outcomes after the ORIF(open reduction and internal fixation) of the anterior pelvic ring (p>0.05). We believe that, with the anterior ring fixed, the stability of the whole pelvic ring was significantly increased. The results have been approved by the finite element analysis.
The current study also found that after stable fixation, in the early stage, there was a significant difference of early weight-bearing between these two groups(p<0.05). It seemed to be that the patients were more aggressive in weight-bearing in Group 2, which might be attributed to pain relief.
The current study also found that the complications were no significant difference between Group 1 and Group2 (P>0.05). The most frequent complication was VTE, 20.6% of the patients developed VTE (14/68, six silent DVT in group 1, six silent DVT and one non-fatal PE in group 2), which may be attributed to the regularly VTE screening pre-operatively and post-operatively in our hospital. Although the thrombosis prophylaxis was routinely prescribed to the patients. Kim reported a similar incidence of DVT (20%). However, in Kim’s study, the rate of clinically significant VTE was much higher than the current study. In Group 1, one patient got INFIX-related LFCN injury, which gradually recovered after the removal of the implant 3 months later. In group 2, one patient got postoperative L5 nerve root injury, with some residual weakness of extensor hallucis longus, which may due to the iatrogenic injury. Surgical exploration and decompression were offered, but the patient refused; The preoperative L5 injury gradually recovered in both groups. In group 2, one patient got pedicle screw related skin irritation; the irritative symptoms recovered gradually after implant removed.
There are some limitations to the current study. First, as a retrospective control study, the patients were not randomly divided into two groups. Second, the current study did not compare the outcomes with the incomplete displacement of the sacral fracture of LC-1 pelvic fractures, which may potentially expand the surgical indication. Third, due to the inherent low incidence and small sample size, the study was not stratified according to the displacement of sacral fracture. Fourth, although the final functional outcomes seem good through solitary anterior fixation, this fixation need to be verified by the biomechanical study.