Posterior ankle impingement syndrome due to os trigonum

[...] if it fails to fuse, an os trigonum is formed (in 7% to 14% of the cases) that articulates with the talus via a synchondrosis.3,5,6 Although common in ballet dancers, os trigonum syndrome is also encountered in those who participate in other sporting activities that involve forced plantar flexion of the foot, such as soccer, basketball, and volleyball, as well as in those who participate in nonsport- related activities.1,3 Patients typically present with recurrent posterolateral ankle pain, especially on plantar flexion, and sometimes on weight bearing, along with swelling and stiffness of the posterior ankle. [...] soft-tissue abnormalities and bone contusions of the lateral talar tubercle and os trigonum are findings of PAI syndrome, which can be clearly depicted on MR imaging.