Variations in fibular position, talar length, and anterior talofibular ligament length

Background: Proprioceptive deficiencies, peroneal muscle weakness, varus hindfoot, and varus tibial plafond have each been implicated as a predisposing factor in recurrent ankle instability. Another predisposing factor for instability is a position of the fibula posterior to the transverse axis of the talus. This study examined the relationship of fibular position and ankle instability with anatomic dissection and magnetic resonance imaging (MRI). Methods: Data were obtained from 100 consecutive MRI scans of adult ankles: the malleolar index in degrees and the fibular position anterior or posterior to the anteromedial malleolar plane and relative to the transverse axis of the talus. Twenty cadaver ankles also were studied by MRI and then by anatomic dissection. The malleolar index, the talar length, and the length of the anterior talofibular (ATF) ligament were recorded. Results: The MRI study showed that variation of the malleolar index ranged from 2 to 30 degrees. There was no correlation between the malleolar index and the talar length or the length of the ATF ligament. Conclusions: There is a bell-shaped curve of variation of the position of the fibula relative to the transverse axis of the talus. An ankle mortise with a more posteriorly positioned fibula has less structural stability and is more susceptible to sprain. Neither talar length nor ATF ligament length correlated with fibular position.