The goal of the surgical treatment of ankle fractures is to restore the normal anatomical morphology of the ankle, maintain joint stability, and reach maximum functional recovery. Unlike the treatment for medial and lateral malleolar fractures, the surgical indications and surgical fixation methods for PMF are inconclusive . At present, the use of surgical treatments often depends on the size of the PMF. Previous studies have confirmed that PMFs involving more than 25% of the articular surface may lead to an increased incidence of post-traumatic osteoarthritis, therefore, open reduction and internal fixation (ORIF) is indicated if the fragment percentage is greater than 25% of the tibial articular surface [14, 15]. However, Ferries et al.  believed that it is inaccurate to estimate the percentage of PMFs using simple X-rays. Haraguchi et al.  suggested the use of CT to reliably estimate the fragmentation percentage. Therefore, in the present study, patients with PMF were divided into ≥ 15% and < 15% fragmentation groups according to the FAR measured from CT cross-section images. After reduction of the fracture, patients in the two groups were treated with PA screws, AP screws, or a PA plate.
In previous studies, PMFs with small displacement were often reduced indirectly and fixed with screws using the anterior to posterior approach [18, 19, 20], which can avoid delayed wound healing, soft tissue adhesion and iatrogenic sural nerve damage caused by soft tissue dissection. However, due to the interposition of soft tissue or loose osseous fragments in the PMF gaps, it is difficult for indirect reduction to achieve anatomical reduction, and it is technically difficult to fix small or comminuted fragments [21, 22]. In recent years, more attention has been paid to the importance of anatomical reduction and internal fixation of PMF [23–25]. Anatomical reduction and fixation of posterior ankle fractures can be achieved under direct vision using a posterolateral approach, which is highly accepted [10, 21, 26, 27]. In this study, we chose the posterolateral approach to reduce posterior ankle fractures under direct vision. When the fragment size was ≥ 15%, there was no statistical difference in the AOFAS score, with limited dorsiflexion ROM between the PA screw and PA plate fixations, which is similar to the results of a previous study . Our findings suggested that the fixation of PMF with a fragment size of ≥ 15% using either a PA screw or PA plate can both achieve satisfactory outcomes. Unlike previous studies of fixation of PMF using AP screws with indirect reduction [18, 19, 20], in this study, when fixation of PMF was with an AP screw, we also performed anatomical reduction of PMF under direct vision, and placed the AP screw through an incision on the anteromedial aspect of the medial malleolus to fix the fracture. Our results demonstrated that with a fragment size ≥ 15%, no statistical differences in the outcomes after AP plate were observed compared with the AP plate or PA screw fixation. Anwar et al.  performed a biomechanical study of different fixation methods for fixation of PMF using finite element analysis and found that fixation of a posterior ankle fracture using a PA plate or PA screw can achieve better clinical results than AP screw fixation, and the plate had higher fixation strength. The reason for the difference between our results and the above previous study may be that in our study, the posterolateral approach is selected, fracture reduction is more accurate under direct vision, and the fixation is also performed under direct vision, which can avoid the poor outcomes caused by a lag screw that is too short or in a poor position, and improve the efficacy of the fixation. Additionally, AP screw fixation does not increase secondary injuries and can be removed easily. Therefore, although the fixation strength of the AP screw is not as high as the PA plate, the AP screw fixation with direct reduction via a posterolateral approach may also be effective for fixation of PMF with a fragment size of ≥ 15%.
Whether there is any need to perform surgical fixation for PMF with low fragmentation remains inconclusive. The posterior malleolar is the posterior edge of the distal tibia that extends downward, and the attached inferior transverse tibiofibular ligament further deepens this extension, which can restrict the backward movement of the talus in the ankle and help maintain the weight-bearing function and stability of the ankle joints . In addition, the posterior-inferior tibiofibular ligament is attached to the lateral tubercle of the posterior malleolus, which can provide up to 42% of the strength of the syndesmosis. In the case of PMF, the posterior-inferior tibiofibular ligament is usually not ruptured completely . Langenhuijsen et al.  emphasized the importance of the quality of the posterior malleolus reduction, and advocated that the joint congruity, not the fragment size, is the main factor determining the need for fracture fixation. Jaskulka et al.  performed a 5–7 year follow-up study of 142 patients with ankle fractures, and suggested that if fixation was not performed, long-term results were significantly poor, even in patients with a small marginal fragment of the lip of the tibia. Tosun et al.  showed that the fixation of PMF is closely related to the successful radiological and functional outcomes of trimalleolar fracture. After PMF is fixed, transyndesmal screw fixation may not be required. For these reasons, the authors believe that PMF with all fragment sizes should be fixed. Gardner et al.  performed a study investigating the role of the fixation of PMF in the stability of the distal tibiofibular syndesmosis, they concluded that the fixation of PMF provided greater syndesmotic stability than the fixation of the distal tibiofibular syndesmosis alone. Therefore, in this study, the same surgical treatments were performed for patients who had a fragment size of < 15% and ≥ 15%.
Interestingly, our results showed that when the fragment size was < 15%, there were statistically significant differences in the AOFAS scores and limited ankle-dorsiflexion ROM between the posterior plate fixation and screw fixations. The results indicated that the PA screw or AP screw fixation may be more effective than PA plate fixation for PMF with a fragment size of < 15%. The reason may be that when the fragments are small and located at the distal end of the tibia, the plate needs to be placed at the distal end of the tibia to obtain effective fixation. However, when the plate is placed close to the distal end of the tibia, the very distal part of the plate may protrude, and unlike the screw, the plate cannot be countersunk, which can stimulate flexor hallucis longus, cause the formation of serious adhesions , and affect postoperative outcomes. These problems may also exist when treating PMFs with a fragment size of ≥ 15%, but when the fragments are larger, the plate can be placed slightly more proximal to the tibia, which can reduce the protrusion of the very distal part of the plate, thereby reducing the postoperative adhesions of the flexor hallucis longus. Therefore, different outcomes were obtained during treatment of PMF with fragment size of ≥ 15% and < 15% using the PA plate. However, further investigations are needed to verify these results.
There are some limitations in our study that need to be considered. In this study, the fragment size was assessed according to the ratio of the fragment area to the total cross-sectional area of the tibial plafond measured based on axial CT scans, which is not assessed according to the anteroposterior (sagittal) diameter of the tibia measured using X-ray. The indication of ORIF for PMF is based on the fragment sizes measured on X-ray. The fragment sizes can be determined more accurately based on axial CT scans, but it also affects the evaluation of the indication of surgical treatment. In addition, we did not grade and compare the status of radiographic osteoarthritis. Theoretically, anatomical reduction and rigid fixation of PMF can restrict the talus, stabilize the syndesmosis, and reduce the risk of post-traumatic osteoarthritis . Furthermore, we did not compare intraoperative indicators, such as surgical time, surgical bleeding volume, and radiation time between the three fixation methods for treatment of PMF with fragment size of ≥ 15% and < 15%. Further prospective randomized controlled studies with larger sample sizes are needed to confirm these findings.