The os trigonum is an accessory (extra) bone that sits in the back of the ankle near the heel bone. This occurs in 2.5–14% of normal feet. It is usually round, oval, or triangular, and varies in size. During growth, it may eventually fuse with the ankle bone (talus), or remain as a separate small bone connected to the talus by a fibrous band. The os or its fibrous connection can become painful when irritated of injured.
Os trigonum syndrome may occur with an injury, such as an ankle sprain. It is also frequently caused by repeated plantar flexion, in which the toes are pointed downward. This action has a “nutcracker effect” on the small os trigonum, compressing it between the ankle and heel bones of the foot. The fibrous tissue connecting the os may be torn or stretched, causing inflammation and pain. Os trigonum syndrome commonly occurs in athletes who perform repetitive plantar flexion (pointing of the toes and downward movement of the ankle). This movement repeatedly occurs in activities such as soccer, basketball and dance.
Dancers may notice a worsening of pain and discomfort when rising to the “en pointe” or “demi-pointe” position. Soccer players usually have pain with long kicks, and basketball players will complain of pain with jumping. Other symptoms include:
Other conditions that may be considered are an Achilles tendon injury, ankle sprain or fracture of the talus. Diagnosis of os trigonum syndrome begins with a thorough history and examination of the foot and ankle. To confirm the diagnosis, x-rays or MRI are usually ordered.
An x-ray will show a small, accessory bone with smooth edges distinguishes it from a fracture of the talus. MRI can identify a tear in the fibrous tissue, or a stress fracture of the os.
Treatment includes rest, ice, non-steroidal anti-inflammatory medications, physical therapy and taping. Depending on the severity of symptoms, a walking boot may be used to restrict painful ankle movement. Cortisone injections to the area are sometimes helpful for severe inflammation. Rarely, surgery is recommended for cases that do not resolve with any of these treatments, although this is not usually necessary in children.
Returning to activity and sports depends on patients' progress in physical therapy and having good pain control. Return to activity is possible when ankle movements are no longer painful, which usually occurs after 4–6 weeks of treatment.