The os trigonum is an accessory ossicle or small bone that is a normal variant however may predispose to certain tendon injuries and may itself be a source of pain. The os trigonum comes off the posterior portion of the talus, one of the key bones in the ankle joint.
This condition represents an anatomic variant that you are born with.
Symptoms of os trigonum typically include posterior heel and ankle pain. There may be a loss of range of motion with stepping down as if on a gas pedal. The flexor hallucis longus tendon (FHL) may become irritated as it travels very closely in this region.
If the accessory ossicle is determined to be the true, singular source of pain, it may require surgical removal. If the accessory ossicle has led to irritation of a nearby tendon, then that tendinopathic condition may be treated with nonsurgical procedures, including tendon debridement with a TENEX procedure, needling under ultrasound, and/or injecting platelet-rich plasma (PRP).