The os trigonum is an extra piece of bone at the back of the ankle. When the foot is forming, a separate bone center forms on the back of the talus (bone in the ankle). This occurs in children between the ages of 8 and 13 years.

This ossification (bone forming) center usually fuses with the remainder of the talus within a year. However, an extra bone, the os trigonum, forms if this ossification center fails to fuse after skeletal maturation.

About eight percent of the population has this condition. It's usually just on one side. Anyone with this condition may have pain in the back of the ankle that's worse when forcing the ankle and foot down into plantar flexion (toes pointed).

Treatment is usually rest, ice, antiinflammatories, and physical therapy. Steroid injections may be used along with taping. Rarely, surgery is done to remove the bone.