Morphology of occult posterior malleolus fractures associated with tibial shaft fractures.

Abstract

Aims: Occult posterior malleolar fractures (PMFs) associated with tibial shaft fractures are thought to occur most commonly in spiral fracture types of the tibia. We hypothesize that tibial diaphyseal fracture patterns would be associated with certain PMFs, highlighting the pathomechanics of the injury.

Methods: A retrospective review was performed on data collected between 1 January 2013 and 9 November 2020. The inclusion criteria were patients aged over 16 years with a diaphyseal tibial fracture who had undergone a CT of the affected lower limb. The Mason and Molloy posterior malleolar fracture classification system was used to describe the morphology of the PMFs.

Results: There were 764 diaphyseal fractures identified. Of these, 442 met the inclusion criteria. A total of 107patients (24.21%) had PMF extensions. The classification of the PMFs according to Mason and Molloy revealed eight type 1 fractures (7.48%), 60 type 2A (56.07%), six type 2B (5.61%), and 33 type 3 fractures (30.84%). The most common PMF seen in this study was the minor rotational pilon (type 2A). PMFs generally occur in combination with spiral diaphyseal fractures (42A1, 42B1, 42C1, and 43A1). The majority of PMFs were undisplaced pre-surgical intervention. Only the 2B subtype (major rotational pilon) had a significant association with fracture displacement.

Conclusion: This study highlighted an association between spiral tibial shaft fractures and type 2A posterior malleolus fractures. Unlike the PM fractures of the ankle, the majority of PM fractures associated with tibia fractures are undisplaced. We theorize that unlike the force transmission in ankle fractures, where the rotational force is in the axial plane in a distal-proximal direction, in the PM fractures related to tibia fractures, the rotational force in the axial plane progresses from proximal-distal. Therefore, the force transmission which exits posteriorly, finally dissipates the force and is thus unlikely to displace.