Os trigonum is a relatively common accessory (extra) bone that can occur in the foot.
This bone is like an extension on the posterior aspect (behind) the talus bone.
Sometimes, with plantarflexion of the ankle (pointing the toe downward), this accessory bone can be pinched between the back of the tibia and the calcaneus.
When impingement of the os trigonum causes pain, though anti-inflammatory medications and activity modifications can be helpful, usually the most effective cure is to have this extra bone removed surgically with the surgery done on an outpatient basis.