Posterior ankle pain can be frustrating because the source is often deep behind the ankle rather than obvious from the outside. We see this particularly in athletes and dancers who repeatedly point their toes, push off, or place the ankle into deep plantar flexion. An X-ray may show an os trigonum, a small accessory bone behind the talus, but that finding alone does not mean the bone is causing the pain. Many people have an os trigonum without symptoms.
Our job is to determine whether the accessory bone is actually being compressed, whether the flexor hallucis longus tendon or another structure is involved, and whether the symptoms fit posterior ankle impingement.
Anatomy
The talus sits between the tibia, fibula, and calcaneus and plays a central role in ankle motion. Behind the talus is a narrow space containing several important structures, including the flexor hallucis longus tendon, which helps bend the big toe.
An os trigonum is an accessory bone that develops behind the talus when a secondary ossification center does not completely fuse with the rest of the talus during development.
The presence of this extra bone is not necessarily abnormal or painful. Problems arise when the os trigonum and surrounding soft tissues are repeatedly compressed during plantar flexion.
This is sometimes described as a nutcracker mechanism, with the structures at the back of the ankle becoming pinched during forceful pointing of the foot.
Why Os Trigonum Syndrome Develops
Symptoms can develop gradually from repetitive loading or suddenly after an ankle injury.
Dancers who repeatedly work en pointe or in relevé place substantial demands on the back of the ankle. Soccer players, runners, and other athletes may also develop symptoms when their activities repeatedly require forceful plantar flexion.
An acute ankle injury can irritate the connection between the os trigonum and talus and turn an otherwise asymptomatic accessory bone into a painful one.
The flexor hallucis longus tendon can become involved because it passes close to the posterior ankle. In some patients, the tendon is an important part of the pain rather than the os trigonum itself.
Symptoms
Patients typically describe pain deep at the back of the ankle.
The symptoms may include:
- Pain when pointing the toes downward
- Pain with push-off, running, jumping, or kicking
- Tenderness behind the ankle
- Swelling after activity
- Stiffness or a feeling of blockage during plantar flexion
- Pain when performing ballet positions that require the ankle to point
- Pain along the course of the flexor hallucis longus tendon
Some patients develop symptoms gradually as activity increases. Others can identify a specific ankle injury after which the posterior pain began.
What We Look for at the First Visit
We start by observing the ankle while you stand and walk. We then examine the back of the ankle and identify exactly where the pain is reproduced.
Forced plantar flexion is particularly useful because it can reproduce the compression responsible for posterior ankle impingement. We also assess ankle motion and examine the Achilles tendon, peroneal tendons, and surrounding structures.
Because the flexor hallucis longus passes through the same region, we assess the tendon as well. Pain with resisted big-toe flexion or along the tendon can provide an important clue that FHL irritation is contributing to the symptoms.
Plain X-rays can demonstrate an os trigonum or an elongated posterior talar process. When the diagnosis is uncertain, MRI can show bone marrow edema, inflammation, synovitis, tendon pathology, or other soft-tissue abnormalities.
CT can provide a more detailed view of the bone and can be particularly useful when we need to distinguish an os trigonum from another posterior talar fragment or define the anatomy before surgery.
Other Diagnoses
One of the most important parts of evaluating this condition is making sure the visible bone is actually responsible for the symptoms.
Several conditions can produce similar posterior ankle pain. Achilles tendinopathy and retrocalcaneal bursitis usually produce pain around the Achilles insertion rather than deep within the posterior ankle. Flexor hallucis longus tendinopathy can mimic os trigonum syndrome. A posterior talar process injury can also look similar on imaging.
Nerve irritation, ankle injury, stress injury, and other tendon disorders may need to be considered depending on the examination.
When the symptoms and imaging do not match, we do not assume that the os trigonum is the problem simply because it appears on an X-ray.
Nonsurgical Treatment
When symptoms are mild or have developed recently, our first goal is to remove the repetitive irritation.
This may mean temporarily reducing activities that repeatedly force the ankle into deep plantar flexion. For a dancer, that may mean modifying pointe work. For an athlete, it may mean temporarily reducing running, jumping, kicking, or other movements that reproduce the pain.
Ice, appropriate medication, and footwear modification may help control symptoms. In more symptomatic cases, a walking boot can temporarily limit ankle motion and allow irritated tissues to settle.
Physical therapy can address calf and ankle flexibility, strength, and mechanics while gradually restoring the movements required for the patient’s activities.
In selected patients, an injection may be useful diagnostically or therapeutically when inflammation around the posterior ankle or FHL tendon is contributing to symptoms.
When Surgery Becomes Reasonable
We consider surgery when posterior ankle pain continues to interfere with activity despite appropriate nonsurgical treatment and the examination and imaging consistently point to the os trigonum or associated posterior impingement as the pain generator.
The goal is not simply to remove an accessory bone that happens to appear on an X-ray. The goal is to decompress the painful area and address any associated pathology.
For appropriate patients, the os trigonum can be removed through a posterior endoscopic or arthroscopic approach. These techniques use small incisions and allow the surgeon to work directly around the posterior ankle.
Open excision remains useful when the os trigonum is large, the anatomy is complex, or additional structures require direct treatment. If the FHL tendon is significantly involved, it may need to be released or treated during the same procedure.
Recovery Depends on What We Find
Recovery varies according to the procedure and the amount of pathology that needs to be addressed.
Early recovery generally involves protecting the surgical area and gradually increasing weight-bearing according to the specific procedure. A boot may be used initially, followed by progressive restoration of ankle motion and strength.
Physical therapy can help restore range of motion, calf strength, balance, and sport-specific mechanics.
The final stage is returning to the movements that originally caused the symptoms. For an athlete or dancer, that means progressively reintroducing running, jumping, kicking, pointe work, or other high-demand activities rather than returning to them all at once.
What We Watch for After Surgery
Potential complications include persistent pain, stiffness, infection, scar sensitivity, nerve irritation, and continued tendon symptoms.
The sural nerve and other structures around the posterior ankle require careful attention because of their proximity to the surgical area. Chronic inflammation can also make recovery slower than expected.
A successful operation still requires rehabilitation. Removing the source of impingement does not immediately restore strength, flexibility, or sport-specific conditioning.
What Happens Without Treatment
An asymptomatic os trigonum does not require treatment simply because it is visible on an X-ray.
A symptomatic condition can, however, continue to limit activities that require plantar flexion. Repeated irritation may also involve the FHL tendon or surrounding soft tissues.
If symptoms persist despite treatment, we reassess the diagnosis rather than assuming that the accessory bone is the only possible explanation.
What a Good Outcome Means
A good result means more than removing the os trigonum on imaging.
We want the patient to be able to point the foot, push off, walk, run, jump, or perform the activities that matter to them without the posterior ankle pain that brought them to us.
For dancers and athletes, the goal is a return to the specific demands of their sport or performance—not simply being able to walk comfortably.
The most important step is identifying the actual source of pain. When the clinical examination, imaging, and symptoms all point toward posterior impingement from the os trigonum, treatment can be directed at the problem rather than the incidental X-ray finding.
When We Recommend an Evaluation
Persistent pain at the back of the ankle, particularly pain that occurs when pointing the toes or during push-off, deserves evaluation when it begins interfering with sports, dance, work, or daily activities.
We also recommend evaluation when posterior ankle pain develops after an injury or when symptoms continue despite reducing the activity that initially caused them.
Severe pain after significant trauma, inability to bear weight, major swelling, or an open wound requires more urgent assessment.
Our approach is to determine whether the os trigonum is actually symptomatic, identify any associated tendon or joint pathology, and choose the least invasive treatment that addresses the underlying problem.
The content on this page has been authored, edited, or approved by the doctors below, and was last reviewed for accuracy on September 15, 2026.
