An inconspicuous stabilizer of the subtalar joint: MR arthrographic anatomy of the posterior talocalcaneal ligament

Objective The main function of the posterior talocalcaneal ligament (PTL) is to stabilize the posterior subtalar joint in the ankle. PTL is a potential source of pain in chronic subtalar instability. Our knowledge of the anatomy and function of PTL is limited and there are not many studies regarding its morphology. The aim of this study is to provide detailed information about imaging anatomy and morphology of PTL. Materials and methods This retrospective study included 197 ankle images of 184 patients (13 bilateral) obtained from MR arthrography (MRA) and conventional MRI between 2012 and 2019. The incidence of PTL was evaluated using both methods. The location of the ligament to the calcaneus, shape, and intraarticular extension was determined by MRA. In addition, thickness and lengths were measured in millimeters, and the presence of os trigonum, contrast agent extravasation into adjacent anatomical structures, was evaluated. The upper surface of the calcaneus was divided into nine regions in the axial view and three regions in the sagittal view. Results The incidence of PTL was 65.5% (n = 129). In axial view, the most common calcaneal attachment was in the 5th zone. The ligament was mostly fan-shaped (n = 104) and the extraarticular course was 87%. The average length was 15.9 mm and the average thickness was 1.1 mm. There were os trigonum in 18 cases. Conclusion Having knowledge of the morphology and variations of PTL and defining its relationship with adjacent anatomical structures can help evaluate subtalar instability.