An os trigonum is a small extra bone that sits behind the ankle, just off the back of the talus (the bone that connects your foot to your leg). It’s present in a notable portion of the population and is usually harmless. Most people never know they have one. But in certain situations, particularly in athletes and dancers, it can become a source of chronic posterior ankle pain known as os trigonum syndrome.
How the Os Trigonum Forms
Between the ages of 8 and 11, a secondary growth center appears behind the back edge of the talus. In most people, this small piece of developing bone fuses with the talus as skeletal maturity progresses. In some people, it never fuses. Instead, it remains as a separate, pea-sized bone connected to the talus by fibrous tissue. That unfused fragment is the os trigonum.
It sits near the lateral tubercle of the posterior talar process, right next to the ligament that connects the talus to the fibula. A tendon that helps control the big toe runs through a groove between the two tubercles of the posterior talus, which becomes relevant when the os trigonum causes problems.
Why Most People Never Have Symptoms
Having an os trigonum is an anatomical variation, not a disease. The bone sits quietly in the back of the ankle and causes no pain or functional limitation in the vast majority of people who have it. It only becomes a clinical issue when the connection between the os trigonum and the talus is stressed by injury or repetitive motion.
What Triggers Os Trigonum Syndrome
Os trigonum syndrome is a form of posterior ankle impingement, meaning tissue at the back of the ankle gets pinched or compressed during certain movements. The key motion is extreme downward pointing of the foot (plantarflexion). When the foot is forced into this position, the os trigonum gets caught between the back of the shinbone and the heel bone in what’s often described as a “nutcracker” mechanism.
This can happen in two ways. A single traumatic event, like a badly sprained ankle or a fall, can tear or stretch the fibrous tissue connecting the os trigonum to the talus, triggering sudden inflammation and pain. More commonly, it develops gradually through repetitive stress. Ballet dancers pointing their toes on pointe or demi-pointe, soccer players repeatedly kicking, runners pushing off the back of the ankle, and javelin throwers whose lead foot absorbs enormous force during the throw are all at elevated risk. In horizontal jumping events, upwards of 10 to 15 times body weight can pass through the lead leg during takeoff in maximum plantarflexion.
The syndrome is predominantly seen in ballet dancers and soccer players. People who regularly walk or run downhill are also more susceptible.
Symptoms to Recognize
The hallmark of os trigonum syndrome is pain deep in the back of the ankle, especially when you push off your big toe or point your toes downward. Other common signs include swelling at the back of the ankle, stiffness, tenderness when pressing the area, and reduced range of motion when trying to fully point or flex the foot. The pain typically worsens with activity and improves with rest, at least initially.
How It’s Diagnosed
A standard lateral (side-view) X-ray of the ankle can reveal the os trigonum as a small, well-rounded bone with smooth edges sitting behind the talus. The smooth, fully formed outer shell of the bone is the key distinguishing feature. Two related injuries can look similar on imaging but have very different implications.
A Shepherd fracture is an acute break of the lateral tubercle at the back of the talus, typically caused by an ankle sprain. On X-ray, the fractured fragment has irregular, jagged edges rather than the smooth cortex of an os trigonum. A Cedell fracture involves the inner (medial) tubercle and is rarer, usually resulting from more significant trauma. When X-rays aren’t conclusive, an MRI or CT scan can confirm whether the bone is an unfused os trigonum or a fresh fracture, and can also reveal inflammation in the surrounding soft tissues.
Non-Surgical Treatment
Most cases of os trigonum syndrome improve without surgery. The initial approach centers on rest, ice, and anti-inflammatory medications, combined with strict avoidance of the positions that compress the back of the ankle. That means no forced pointing of the toes, no relevés for dancers, and no kicking for soccer players until symptoms settle.
Physical therapy follows a phased approach. Early on, the focus is on controlling swelling and restoring pain-free range of motion while keeping the ankle in a neutral position. Exercises start simple: tracing the alphabet with the foot, then adding resistance with a band in all four directions. Standing balance work on both feet builds back gradually. Full plantarflexion is typically avoided for several weeks. Later phases introduce single-leg balance, squats, and sport-specific movements as the ankle tolerates them. Soft tissue therapy targeting the calf muscles and the tendons running behind the ankle can help relieve tightness that contributes to compression.
For people with persistent pain that doesn’t respond to therapy alone, a corticosteroid injection into the area behind the ankle can reduce inflammation. In one documented case, an ultrasound-guided injection brought pain levels down from 4 out of 10 to 2 out of 10 during the provocative movement.
When Surgery Is Needed
If several months of conservative treatment fail to resolve symptoms, surgical removal of the os trigonum is the next step. There are two approaches: open surgery and endoscopic (minimally invasive) surgery.
A randomized study comparing the two techniques in 52 athletes found clear advantages for the endoscopic approach. Athletes who had endoscopic removal returned to training in about 4.5 weeks, compared to roughly 9.5 weeks for those who had open surgery. Return to full competitive sport took about 7 weeks with the endoscopic method versus nearly 12 weeks with the open procedure. Complication rates were also notably lower with the endoscopic approach.
Both techniques produce good long-term results in terms of pain relief and function. In one study of arthroscopic excision, ankle function scores jumped from an average of about 61 out of 100 before surgery to 92 out of 100 afterward. The average return to full activity was 8 weeks, with a range of 4 to 16 weeks depending on the individual. For athletes who need to get back to competition quickly, the endoscopic approach offers a meaningful advantage in recovery time without sacrificing outcomes.
Os Trigonum vs. an Elongated Posterior Process
Not everyone with posterior ankle impingement has a separate os trigonum. Some people have a naturally elongated lateral tubercle on the back of the talus, sometimes called a Stieda process, that can cause the same nutcracker compression. The treatment principles are identical: reduce inflammation, avoid provocative movements, and remove the bony prominence surgically if conservative measures fail. The distinction matters mainly for imaging interpretation, since an elongated process that fractures (a Shepherd fracture) needs to be managed as an acute injury rather than a chronic impingement problem.