Os trigonum.

There is a wide variation in the nomenclature for the posterior aspect of the talus (eg, os trigonum, trigonal process, Stieda process, posterior process). Injuries to these structures can be caused by chronic, repetitive impingement of the bony prominence or by acute hyperplantar flexion of the foot and ankle. After failing appropriate nonoperative treatment, surgical excision of the bony involvement is recommended. Surgical options include open or arthroscopic excision of the bony structure.