Morphology of occult posterior malleolar fractures associated with tibial shaft fractures

D. Myatt, H. Stringer, L. Mason, B. Fischer

Introduction: Diaphyseal tibial fractures account for approximately 1.9% of all adult fractures. Several studies have demonstrated a high proportion of diaphyseal tibial fractures have ipsilateral occult posterior malleolus fractures, this ranges from 22-92.3%. We hypothesize a rotational element will be highlighted in the Mason & Molloy Classification of occult posterior malleolus fractures associated to tibial shaft fractures.

Methods: Our primary outcome was to identify any extension of tibial fractures to the posterior malleolus and describe its morphology.
A retrospective review of a prospectively collected database was performed at Liverpool University Hospitals NHS Foundation Trust between 1/1/2013 and 9/11/2020. The inclusion criteria was age over 16, with a diaphyseal tibial fracture and who underwent a CT. The Mason and Molloy posterior malleolus fracture classification system was used to describe the morphology of the fracture.  

Results: 764 diaphysial tibial fractures were analysed, of these 300 had a CT and could be included. There were 127 intra-articular fractures. A total of 83 (27.7%) cases were classifiable using the Mason and Molloy classification.  There were 8 type 1 (9.6%), 43 type 2 (51.8%), 5 type 2B (6.0%) and 27 type 3 (32.5%). The majority of the posterior malleolar fractures (n=75, 90.4%) were undisplaced pre-surgical intervention.  The majority of PM fractures occurred in type 42A1 (65 of 142 tibia fractures) and 42B1 (11 of 16).  
PM fractures associated with tibial fractures occur as a rotational mechanism. Unlike, the PM fractures of the ankle, the majority of PM fractures were undisplaced.

Conclusions: We theorise 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 fractures of the tibia, the rotational force in the axial plane progresses from proximal-distal. Therefore, the force transmission which exits posteriorly, finally dissipates and thus unlikely to displace.