In normal human gait, when one foot touches the ground, the center of body gravity will move to the contralateral side and introduces bending and rotational moments . Because the marked size disparity between the distal tibia and the lateral malleolus, the ankle contour becomes very unsymmetrical. Not only is the load transfer unsmooth, but also is the movement of talus greatly restricted . Once stresses from various directions are subjected to the ankle, various complex injuries will occur. Clinically, among various ankle injuries requiring treatment, supination-external rotation injuries are reported most common (around 60%) and PE injuries, around 15%. However, the severity of the latter is much higher and generally requires surgical intervention [6, 10, 12–14].
In advanced PE injuries (stage 3 or 4), the stability of medial aspect of tibia is lost first because of medial malleolar fractures or deltoid ligament tear. Consequently, the anterior inferior tibiofibular ligament tear follows. Then, the fibular shaft several centimeters proximal to the plafond (commonly > 6 cm) fractures spirally and interosseous membrane is disrupted from the syndesmosis upwards to the fractured fibula. This is the standard patho-mechanism of stage 3 PE injuries. If the damage continuously extends, the posterior malleolus will fracture or posterior inferior tibiofibular ligament is disrupted. So far, a stage 4 PE injury is developed [6, 13, 14, 25]. In the literature, either injury is regarded as frankly or potentially unstable and surgical treatment is normally necessary [7, 8].
Surgical techniques for treatment of advanced PE injuries are controversial. Some orthopedic surgeons declare that once the medial malleolar fracture is stabilized with screws and the fibular fracture is immobilized with a splint with or without plate fixation, the syndesmotic stability may be sufficient. Therefore, syndesmotic screw stabilization may be unnecessary [10, 26]. However, much more orthopedic surgeons object this concept. They believe that extensive destruction of ligaments and interosseous membrane is a potential risk for syndesmotic re-diastasis after walking. The talus will continuously push the lateral malleolus posterolaterally. A 1.5 months of splint immobilization is insufficient for healing of ligaments and interosseous membrane [8, 11]. Despite lack of statistical significance in the present study, the ratio of re-diastasis (30.6% versus 13.6%) may remind of potential danger of ignorance. The statistical insignificance may be due to insufficient sample sizes (post-hoc power = 0.31). Re-diastasis of syndesmosis had been reported to cause progressive osteoarthritis in the literature [1, 8, 11, 27, 28].
In the stance phase of gait, the talus will be pushed with 30-50 of external rotation and 1.5 mm of lateral shift . Consequently, the anteromedial aspect of lateral malleolus will bear intermittent stresses towards posterolateral direction. A non-stabilized syndesmosis should be very difficult to withstand this load when all surrounding ligaments and interosseous membrane are without resistant function. Theoretically, a stabilizing screw should be inserted [8, 30].
The outcomes of syndesmotic re-diastasis may require long-term observation. It is dissimilar to an acute SD, which is always combined with malleolar fractures. Patients are generally unable to weight bearing . However, syndesmotic re-diastasis is the only remaining sequela and the disasters will deteriorate insidiously. It may therefore cause the ankle function without statistical difference (86.4% versus 65.2% of satisfactory rate, p = 0.07; post-hoc power = 0.44) between the two groups in the present study. Increased sample sizes and longer follow-up may reflect the true clinical features.
The vascularity of syndesmosis has been enthusiastically studied. The perforating branch of peroneal artery mainly supplies the surrounding ligaments [24, 32]. In advanced PE injuries, the interosseous membrane tears from the upper edge of the syndesmosis upwards to the fractured fibula [6, 13, 14, 26, 33]. The blood supply is very possible to be severely compromised and hinders the ligament healing. Immobilization with a short leg splint for simply six weeks runs a high risk of re-diastasis. In the present study, 30.6% of the re-diastatic rate is found. Insertion of a stabilizing screw lowers the rate to 13.6% (p = 0.31).
Syndesmotic stabilization may be via a screw insertion or suture-button technique [31, 34]. Although the latter is gradually supported by some surgeons recently, the former still has merits of technical simplicity and a high success rate as long as screw removal is delayed beyond 3 months [7, 15].
Some limitations may exist in the present study. (1) Sample sizes are insufficient and follow-up periods are not long enough (average of 2 years). Therefore, although the ratios of re-diastasis and satisfactory ankle function are evidently different between the two groups, statistical comparison is still insignificant. After all, advanced PE injuries are not so common that enlarging case numbers may require much longer time. Clinically, comparison via a cohort study may be less possible. The sample sizes required for study of re-diastasis are 92 of each group, and for satisfactory ankle function, 63 of each group in the present study. (2) The optimal time for removal of syndesmotic screw or without removal cannot be guaranteed. Screw breakage or re-diastasis may let surgeons hesitate to insert a stabilizing screw. Recently, without removal of syndesmotic screws seem to achieve more supports [3, 15, 35].