An IMMF is relatively uncommon in the daily activity. A medial malleolar fracture is easily associated with a lateral malleolar fracture because a supination ankle injury is four times more common than a pronation ankle injury (80% versus 20%) [8, 12, 20, 21]. In normal human gait, the foot sustains the ground reaction forces over the lateral aspect of sole (the medial aspect of sole being vaulted without touching the ground). When a step is missed, the foot is generally in the supine position. The lateral malleolus becomes highly tensed and easily breaks before the medial malleolus [6, 7, 20].
When a patient with an ankle injury is examined, the whole lateral aspect of the ipsilateral leg should be evaluated concomitantly (Fig. 1) [20, 22]. If pain and tenderness are found, plain radiographs of the tibia must be taken to evaluate a combined fibular fracture. The lateral radiograph of ankle must be carefully inspected to search for a concomitant fracture of the posterior malleolus. If it exists, a stage 2 PA injury may be diagnosed when a medial malleolar fracture also occurs [14]. If it is absent, a stage 1 or 2 PE or PA injury may be possible. Because a stage 2 PA injury is unstable, using internal fixation to stabilize the medial malleolar fracture is imperative [16]. Three other injuries may be treated with non-surgical techniques, e.g. casting or bracing. Practically, differentiating a stage 2 PA injury from three other injuries should utilize clinical stress tests (Cotton and external rotation tests) [15]. Although a stage 2 SA injury without a lateral malleolar fracture (i.e. deltoid ligament tear) is also demonstrated as an IMMF on radiographs, the vertical fracture line is typical for diagnosis [7, 9, 23, 24]. Internal fixation is also imperative because it is unstable.
In the literature, a stage 2 PA injury has been reported to possibly exist at one of following situations: a sole tear of the AITF ligament [7, 10], a sole posterior malleolar fracture [6], or combined PITF ligament and these injuries [25]. Therefore, definite diagnosis of a stage 2 PA injury may be difficult and debatable. Theoretically, clinical stress tests should be used to assist decision-making whether conservative or internal fixation treatment should be chosen [11]. Practically, internal fixation should be performed immediately once the suspicion appears. The surgical risk is minimal but the cost-effectiveness is great. This is especially suitable for a stage 2 PA or a stage 2 SA ankle injury combined with an IMMF [16].
Currently, stabilization of a displaced medial malleolar fragment has achieved a great success [8, 16, 26]. Various stabilizing techniques generally can be used without problems. Because the medial malleolus is rich of cancellous bone, fracture healing is fast and effective. The possible problem is whether the syndesmosis should be stabilized with a cortical screw in a stage 2 PA injury [11, 26]. Despite that this doubt is still existed, external rotation stress and hook tests should be utilized after the medial malleolar fragment is stabilized [17]. Insertion of a trans-syndesmotic screw under imaging intensifier guidance may achieve the optimal treatment. In the present study, one of six stage 2 PA injuries had an unsatisfactory outcome. After internal fixation of the medial malleolar fracture, the syndesmosis diastasis occurred. Five other stage 2 PA injuries had no syndesmotic diastasis and had a satisfactory ankle function.
The outcomes of surgical or non-surgical treatment of an IMMF have been reported numerously. Generally, a satisfactory outcome is predictable [12, 27, 28]. In 2018, Hu et al. reported surgical treatment of medial malleolar fractures involving one, two or three malleoli. The nonunion rate was 3.7% [29]. In 2019, Lokerman et al. underwent a systematic review about a union rate in treatment of an IMMF [22]. The average nonunion rate after surgical treatment was 1.7% and after conservative treatment, 3.5%. The present study chose either treatment based on syndesmotic stability and achieved a 2% nonunion rate. To further upgrade the success rate, theoretically the stability should be authentically distinguished. Or, after conservative treatment is finished and the ankle is still unstable, progressive degeneration will be unavoidable [2, 30, 31].
An IMMF appears in the initial stage of pronation injuries and consequently either stable or unstable ankle may gradually be exposed. Until now, accurate prediction of the most possible pathway based on the first stage is nearly impossible. Practically, by way of various stress tests to assess stability on the spot may be the optimal approach in decision of treatment of an IMMF [12, 16].
Limitations of the present study may exist. First, an unstable IMMF cannot be studied with a randomized controlled trial. Therefore, whether an unstable IMMF can be treated conservatively cannot achieve a definite answer. After all, an unstable IMMF is relatively uncommon. In the present study, 12 unstable IMMFs occurred through the 10-year period. Therefore, internal fixation for an unstable IMMF may be the optimal choice. Second, differentiating stage 1 or stage 2 PA injury and differentiating stage 2 PA or stage 2 PE injury deeply base on stress tests. In the literature, various stress tests are considered to have large varieties in sensitivity and specificity [15]. A stable or unstable IMMF may not be reliable. At this moment, any doubt should force the orthopedic surgeon to perform internal fixation [16]. Third, the definite pathological structures in a stage 2 PA injury still cannot be clarified. A simple AITF ligament tear, a simple PITF ligament tear or tears of both ligaments cannot be confirmed [25]. It therefore causes the confusion of a stage 2 PE or a stage 2 PA. The influence is great because of different stability and optimal treatment. In the present study, if a stage 2 PA injury cannot be confirmed (with radiographs and stress tests), all are regarded as stage 2 PE injuries. The treatment techniques may consist of surgical or non-surgical approaches. In the present study, a stage 2 PE injury had a 63.6% satisfactory rate. Misguidance of treating a stage 2 PA injury as a stage 2 PE injury with negligence of syndesmotic instability may be a chief culprit.