My Patient Has Os Trigonum Syndrome

Not her legs

My 11 year old patient came in complaining of pain at the back part of her ankle. She's an avid ballet dancer and tennis player and trains up to 4 times a week.

After examining her thoroughly, I found that her pain was reproduced at a very specific location, especially when she was pointing her foot. I explained to her mum that she probably has Os Trigonum syndrome.

The Os Trigonum is a small, extra accessory bone that is sitting at the back of the ankle joint. This extra bone may be present in up to 20 percent of the general population. It forms when one area of the bone fails to fuse with the rest of the bone (the talus in this case). Often people do not know if they have an Os Trigonum if it has not caused any problems. 

This is similar to the accessory navicular bone in the foot, which usually does not cause any problems. However, it can cause pain after twisting your ankle (just like it can cause pain in the accessory navicular bone in the foot). This happens when the Os Trigonum moves or breaks leading to pinching at the back of the ankle joint, causing what is known as a posterior impingement. 

It can also be caused by repeated downward pointing of the toes, which is especially common in ballet dancers (when assuming an en pointe position) and football players (when shooting).

Sometimes it can be mistaken for pain originating from the Achilles tendon. An x-ray is usually ordered by the doctor to show the Os Trigonum at the back of the ankle. A MRI scan is used occasionally to exclude articular cartilage damage in the ankle.

Reference

Skwiot M, Sliwinski Z, Zurawski W et al (2021). Effectiveness Of Physiotherapy Interventions For Injury In Ballet Dancers. PLoS one. 24(6): eo253437. DOI: 10.1371/journal.pone.0253437.

Thanks to Byron for the picture