A 25-year-old physiotherapist, who had previous trouble with anterior ankle impingement that was treated with arthroscopic surgery, presented with PAIS. He was a recreational football (soccer) player but did not report any single significant sporting injuries. He described chronic pain and had noted a bony swelling in the posteromedial aspect of his ankle. The plain radiograph of the ankle demonstrated an os trigonum; however, due to the orientation of the medial tubercle, this was not clearly seen and he was incorrectly diagnosed as having os trigonum syndrome (see Figure 1). The MRI scan reported mild inflammation associated with the os trigonum but also confirmed the presence of bone fragments posteromedially between FHL and the flexor digitorum longus (FDL) with marrow oedema affecting the medial tubercle of the talus (see Figure 2).
After failure of conservative treatment, he underwent surgery. Due to the position of the medial bony fragments, an open rather than an endoscopic excision was performed to avoid injury to the posterior tibial neurovascular bundle. The os trigonum was felt not to be contributary to the symptoms and was left alone. Surgery was uneventful and at final review reported resolution of the preoperative symptoms.