The Os Trigonum is an accessory bone—an extra, non-standard bone structure—located in the ankle. This small, isolated bone is present in 5% to 30% of the population. For most people, the Os Trigonum is asymptomatic. When this structure becomes a source of chronic pain and inflammation, it results in the clinical condition known as Os Trigonum Syndrome.
Anatomy and Developmental Origin of the Os Trigonum
The Os Trigonum is located at the back of the ankle joint, situated behind the talus and adjacent to the calcaneus (heel bone). It is typically connected to the talus by a synchondrosis, a band of fibrous or cartilaginous tissue. This small bone is a developmental remnant, originating from a secondary ossification center that appears during childhood.
Normally, this center of bone growth fuses completely with the main body of the talus during adolescence, usually between the ages of 8 and 13. The Os Trigonum exists when this fusion fails, leaving the bone fragment as a separate ossicle. The presence of the bone is considered a congenital anatomical variation.
The Clinical Condition: Os Trigonum Syndrome
The existence of an Os Trigonum is not a medical problem; the issue arises when the bone or surrounding soft tissue becomes irritated. Os Trigonum Syndrome (OTS) is the painful condition resulting from the inflammation of this accessory bone and its connecting tissues. It is a common cause of posterior ankle impingement, where structures at the back of the ankle are painfully pinched.
The primary mechanism of pain is compression, often described as a “nutcracker effect.” During forceful plantarflexion (pointing the foot downward), the Os Trigonum is squeezed between the lower end of the tibia and the calcaneus. This repetitive crushing irritates the bone and surrounding soft tissues, including the joint capsule and tendons, leading to inflammation and pain.
This condition is frequently observed in athletes requiring repeated or extreme plantarflexion, such as ballet dancers performing en pointe, soccer players, and runners. Symptoms include a deep, aching pain felt in the back of the ankle, which is exacerbated by activity. Patients also report tenderness and noticeable swelling at the back of the ankle joint.
Diagnosis and Treatment Pathways
Diagnosis begins with a thorough physical examination and detailed patient history focused on activity and pain triggers. A physician will palpate the area for tenderness and perform specific movement tests, such as forced plantarflexion, to reproduce the characteristic pain. Because symptoms can mimic other injuries, such as an Achilles tendon problem or a talus fracture, imaging is necessary to confirm the diagnosis.
Standard X-rays, particularly a lateral view, are the primary tool used to visualize the Os Trigonum and confirm its presence. MRI or CT scans may be employed to assess inflammation in the surrounding soft tissues or to rule out a stress fracture. The initial management approach is conservative, aiming to reduce inflammation and pain.
Conservative treatments include rest and activity modification to avoid movements that cause impingement. Anti-inflammatory medications (NSAIDs) are prescribed to manage pain and swelling. Physical therapy is also beneficial, focusing on gentle range-of-motion exercises and strengthening the surrounding muscles.
For persistent pain, a physician may recommend a targeted cortisone injection for potent anti-inflammatory relief. If symptoms continue despite several months of conservative care, surgical intervention may be considered. The procedure involves the removal of the Os Trigonum, often performed minimally invasively using an arthroscope. This procedure has a high success rate, allowing most patients to return to full activity levels after rehabilitation.