Posteromedial Fracture Fragment of the Posterior Malleolar Fracture: A Closer Look

CATEGORY: Ankle, Trauma INTRODUCTION/PURPOSE: To date, there have been no studies describing the characteristics of posteromedial fragment in the posterior malleolus fracture. The aim of this study was to investigate the variability of posteromedial fracture fragments in trimalleolar fractures to enable better surgical planning. METHODS: In our department, data on all ankle fractures treated surgically are prospectively collected on our internal database. We collated data from August 2014 to October 2018 for this study. All Mason and Molloy type 2B fractures from our database were identified to analyse the preoperative computed tomography scan. The morphology of the fracture fragment was categorized on whether the fracture appeared to be an intraarticular pilon fragment (i.e. caused by impaction of the talus) or an avulsion extraarticular fracture (i.e. caused by the pull of a ligament). The fracture fragment characteristics were analysed for both the posteromedial and posterolateral fragments. RESULTS: The fracture patterns of the posteromedial fragment were investigated in 47 cases (mean age, 46.6; 11 male, 36 female). Morphologically, the fracture could be divided into 2 subtypes, 1) a large pilon type intra-articular fragment (mean of X axis: 32.97 mm, Y: 30.69 mm, Z: 31.74 mm) present in 29 cases with mean Interfragmentary angle of 32.09 and back of tibia angle of 32.66 degrees. This was seen in 25 out of 27 cases with supination injury pattern. 2) A small extra-articular avulsion fragment (mean of X axis: 9.56 mm, Y: 13.22 mm, Z: 11.53 mm) present in 18 cases with mean Interfragmentary angle of 10.97 and back tibia angle of 10.06 degrees. It was seen in 80% of pronation injuries. CONCLUSION: The posteromedial fragment of posterior malleolus fracture can be morphologically subtyped into an avulsion type and pilon type variants. The avulsion type is more common in pronation injuries, likely the result of the pull of the inter-malleolar ligament, and the pilon type is more common in supination injuries, likely the result of the rotating talus impaction. Due to the intra-articular involvement, the pilon type should undergo fixation to achieve articular congruity, unlike the avulsion type whose function is only a secondary syndesmotic stabiliser.