The Effects of Fibular Malrotation on Tibiotalar Contact Area and Peak Pressure Distributions in Weber B Ankle Fractures

CATEGORY: Basic Sciences/Biologics; Ankle; Hindfoot; Trauma INTRODUCTION/PURPOSE: Decreased tibiotalar joint contact area (CA) and increased peak pressure (PP) following rotational ankle fractures may predispose the development of post-traumatic osteoarthritis. Previous studies have highlighted the effects of lateral talar translation on tibiotalar joint congruity. However, debate remains regarding surgical indications in minimally displaced (< 2mm of clear space widening), but potentially malrotated ankle fractures. Malrotation of the talus and fibula are poorly visualized on plain radiographs, thus their impact on ankle joint contact mechanics has not been determined. The aim of this project is to understand the effects of fibular malrotation on tibiotalar joint CA and PP distributions using an axially loaded cadaveric model. We hypothesized that fibular malrotation would result in decreased contact area and increased peak pressures within the tibiotalar joint. METHODS: Ten fresh frozen cadaveric lower extremity specimens transected mid-tibia were dissected free of soft tissues surrounding the ankle, sparing the ligaments. The proximal tibia and fibula were potted in quick drying cement for rigid mounting on a MTS machine. A pressure sensing element (TekScan model 5033) was inserted into the tibiotalar joint and used to measure CA (cm2) and PP (MPa). An axial load of 686 N was applied through the tibia and fibula, followed by a 147 N load via the Achillies tendon at mid-stance position, 15o dorsiflexion and 15o plantarflexion. The samples were first tested in the native condition, a Weber B ankle fracture was simulated and then re-tested in an anatomically fixed state, and a malrotated state. Malrotation was achieved by externally rotating the talus and shortening the fibula along the fracture by the maximal amount that would allow bony apposition along the fracture line (usually 5-10mm). RESULTS: In the six ankles tested thus far (Figure 1), we have observed small but statistically insignificant (P>0.05) increases in tibiotalar CA at all stance phases following malreduction. Significant (p>0.05) increases in tibiotalar PP were seen mid-stance following a simulated Weber B fracture, and these changes were shown to be greatest in the malreduced state versus the anatomically fixed state (7.21 MPa vs. 6.35 MPa respectively, p = 0.004). Interestingly, similar (p=0.84) decreases tibiotalar PP were shown during plantarflexion following a simulated Weber B fracture fixed in both the anatomically fixed and malreduced state. CONCLUSION: Our preliminary data supports the notion that significant changes in tibiotalar PP occur following ankle fractures even in an anatomically fixed state. Increases in tibiotalar PP seem to be further amplified following malreduction at specific stance phases. Further data collection is needed to validate these findings, and to determine the role of malrotation as a potential surgical indication for minimally laterally displaced ankle fractures.