Medically reviewed by Dr. Tom Biernacki, DPM — Board-Certified Podiatric Surgeon — Balance Foot & Ankle, Howell & Bloomfield Hills, MI. Last updated April 2026.

What Is the Os Trigonum?

The os trigonum is a small accessory bone that forms from a secondary ossification center of the talus during adolescence. In most people, this center fuses with the talus body by age 15, forming the Stieda process (posterior talar process). When fusion fails to occur, the separate os trigonum persists as a distinct bone connected to the talus by a fibrous or cartilaginous synchondrosis.

This accessory ossicle sits in the posterior ankle between the posterior tibia, the calcaneus, and the flexor hallucis longus tendon. During normal walking, it rarely causes problems. However, activities requiring extreme plantarflexion—pointing the foot downward—compress the os trigonum in a nutcracker-like mechanism between the tibia and calcaneus, producing pain and inflammation.

Posterior ankle impingement can also occur without an os trigonum when a prominent Stieda process or hypertrophied posterior intermalleolar ligament creates the same mechanical compression. The clinical presentation is identical regardless of the anatomic variant, and treatment approaches are similar. Accurate imaging distinguishes between these causes.

Who Gets Os Trigonum Syndrome?

Ballet dancers—particularly those performing en pointe—represent the highest-risk population. The extreme plantarflexion required for pointe work compresses the posterior ankle with every relevé and jump landing. Professional ballet dancers may perform thousands of plantarflexion cycles per rehearsal, creating cumulative microtrauma that eventually inflames the os trigonum synchondrosis.

Soccer players develop posterior ankle impingement from repetitive plantarflexion during kicking and from direct trauma during tackles. The instep kick requires maximal ankle plantarflexion at ball contact, reproducing the impingement mechanism with each strike. Goalkeepers are also affected from repetitive diving and push-off movements.

Downhill runners, swimmers performing flutter kick, and gymnasts who point their toes during routines also present with this condition. Any athlete whose sport demands repetitive or sustained plantarflexion beyond the normal 45-degree range is at risk. Acute onset can occur after a single forced plantarflexion event such as a missed step on stairs or an ankle hyperextension injury.

Symptoms and Clinical Presentation

The hallmark symptom is deep posterior ankle pain that worsens with plantarflexion—pointing the toes downward. Patients describe a pinching or catching sensation behind the ankle that intensifies during push-off, descending stairs, wearing high-heeled shoes, or performing sport-specific movements. Pain is typically localized to the posterolateral ankle, distinguishable from Achilles tendon pain which sits more midline.

Swelling in the posterior ankle may develop gradually with chronic cases or acutely following a triggering event. Some patients report clicking or popping behind the ankle during circular foot movements. The flexor hallucis longus tendon runs adjacent to the os trigonum, and concurrent FHL tenosynovitis produces pain with big toe flexion against resistance—a finding present in up to 60% of os trigonum syndrome cases.

Chronic cases develop activity-related posterior ankle stiffness that improves with warm-up but returns after prolonged activity. Some patients compensate by reducing their stride length or avoiding downhill terrain, developing secondary biomechanical issues in the knee or hip. Early recognition prevents these compensatory patterns from becoming established.

Diagnostic Evaluation

Physical examination includes the posterior impingement test—passive forced plantarflexion of the ankle that reproduces posterior pain. This test has 95% sensitivity for posterior ankle impingement when combined with tenderness on palpation of the posterolateral ankle. FHL provocative testing—resisted big toe flexion—identifies concurrent tendon involvement.

Lateral ankle X-rays reveal the os trigonum as a well-corticated oval bone posterior to the talus. The size ranges from 2mm to over 15mm. A prominent Stieda process without separation is distinguished from a true os trigonum by smooth cortical continuity with the talar body. Comparison views of the opposite ankle help identify bilateral os trigonum anatomy.

MRI is the gold standard for confirming active os trigonum syndrome versus incidental asymptomatic os trigonum. Bone marrow edema within the os trigonum and surrounding soft tissue inflammation confirm active pathology. MRI also identifies FHL tenosynovitis, posterior talofibular ligament pathology, and other sources of posterior ankle pain that may coexist or mimic os trigonum syndrome.

Conservative Treatment Options

Initial treatment includes activity modification to eliminate the provocative plantarflexion movements. For dancers, this means temporary restriction from pointe work and relevé. For athletes, eliminating downhill running, reducing kicking intensity, and avoiding forced plantarflexion during training. A short course of NSAIDs reduces acute inflammation while activity modification takes effect.

Immobilization in a walking boot for 2-4 weeks provides significant relief by preventing all ankle plantarflexion beyond neutral. This is combined with physical therapy focusing on posterior ankle mobilization, FHL stretching, and progressive strengthening. Ultrasound-guided corticosteroid injection into the posterior ankle recess delivers targeted anti-inflammatory medication and serves as both a diagnostic and therapeutic intervention.

Physical therapy progresses through phases: pain-free range of motion, eccentric strengthening of the posterior ankle musculature, proprioceptive training, and sport-specific rehabilitation. For dancers, a gradual return to demi-pointe precedes full pointe work. Conservative treatment succeeds in approximately 60% of cases within 3-6 months.

Surgical Treatment: Arthroscopic Os Trigonum Excision

Posterior ankle arthroscopy has revolutionized os trigonum surgery, offering smaller incisions, faster recovery, and lower complication rates compared to open surgery. The procedure uses two small portals on either side of the Achilles tendon to access the posterior ankle compartment. A 4mm camera and specialized instruments allow precise identification and removal of the os trigonum under direct visualization.

The surgeon removes the os trigonum while carefully protecting the flexor hallucis longus tendon, posterior tibial neurovascular bundle, and sural nerve. Concurrent FHL tenosynovitis is addressed by releasing the tendon sheath, and any hypertrophied posterior soft tissues contributing to impingement are debrided. The entire procedure typically takes 30-45 minutes.

Recovery after arthroscopic excision is significantly faster than open surgery. Patients typically weight bear in a walking boot within 1-2 days, transition to regular shoes at 2-3 weeks, and return to impact activities by 6-8 weeks. Ballet dancers generally return to full pointe work by 8-12 weeks post-surgery, though individual timelines vary based on conditioning level and tissue healing.

The Most Common Mistake We See

The biggest mistake patients make is attributing posterior ankle pain to the Achilles tendon and treating it with Achilles-focused stretching and strengthening exercises. Aggressive calf stretching actually worsens os trigonum symptoms by forcing the ankle into dorsiflexion rebound and stretching the inflamed posterior capsule. Accurate diagnosis is essential because Achilles tendinopathy and os trigonum syndrome require fundamentally different treatment approaches.

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In-Office Treatment at Balance Foot & Ankle

Our team provides sport-specific evaluation and treatment to get you back to your activity safely. We offer same-day X-ray, in-office ultrasound, and custom orthotic fabrication.

Same-day appointments available. Call (810) 206-1402 or book online.

Frequently Asked Questions

Can an os trigonum go away on its own?

The os trigonum bone itself does not resorb or disappear, but symptoms can resolve completely with conservative treatment in about 60% of cases. Activity modification, physical therapy, and anti-inflammatory treatment reduce inflammation around the bone, eliminating pain even though the anatomic variant remains. Symptom recurrence is possible with return to provocative activities.

Is os trigonum surgery painful?

Arthroscopic os trigonum excision produces less post-surgical pain than most patients expect. The small portal incisions heal quickly, and most patients manage discomfort with over-the-counter medication by day 3-4. The posterior ankle pain that prompted surgery typically improves immediately, which patients find encouraging during the recovery process.

Can os trigonum syndrome affect both ankles?

Yes. Bilateral os trigonum is present in approximately 50% of individuals who have the accessory bone. However, symptoms are usually worse on one side, often the dominant leg in athletes. If both ankles are symptomatic, staged surgical treatment with full recovery of the first side before operating on the second is the standard approach.

How do I know if my posterior ankle pain is os trigonum or Achilles tendinopathy?

Location is the key differentiator. Os trigonum pain is deep and posterolateral, worsening with plantarflexion (pointing toes down). Achilles pain is more superficial and midline, worsening with dorsiflexion (pulling toes up) and during push-off. A podiatrist can distinguish these through physical examination and confirm with imaging studies.

The Bottom Line

Os trigonum syndrome is a treatable cause of posterior ankle pain that responds well to both conservative and surgical management. Accurate diagnosis through clinical examination and MRI prevents misguided treatment directed at the Achilles tendon. When conservative measures fail, arthroscopic excision provides reliable symptom relief with a quick return to full activity, including demanding sports and dance.

Sources

1. Smyth NA, et al. Posterior Ankle Arthroscopy for Os Trigonum Excision: Outcomes and Return-to-Sport Analysis. Am J Sports Med. 2025;53(4):901-912.
2. Russell JA, et al. Posterior Ankle Impingement in Dancers: Prevalence, Biomechanics, and Management Strategies. Br J Sports Med. 2024;58(12):689-698.
3. Hayashi D, et al. MRI of the Posterior Ankle: Os Trigonum Syndrome Versus Other Causes of Posterior Impingement. Skeletal Radiol. 2025;54(3):345-358.
4. Willits K, et al. Conservative Versus Surgical Management of Os Trigonum Syndrome: A Prospective Comparative Study. Foot Ankle Int. 2024;45(7):756-765.

Os Trigonum & Posterior Ankle Impingement Treatment

An os trigonum is an extra bone behind the ankle that can cause chronic posterior ankle pain, especially in dancers and athletes who point their toes. At Balance Foot & Ankle, we offer both conservative management and minimally invasive surgical excision to eliminate impingement pain.

Learn About Our Ankle Pain Treatment Options → | Book Your Appointment | Call (810) 206-1402

Clinical References

1. Hedrick MR, McBryde AM. Posterior ankle impingement. Foot Ankle Int. 1994;15(1):2-8.
2. Abramowitz Y, et al. Outcome of resection of a symptomatic os trigonum. J Bone Joint Surg Am. 2003;85(6):1051-1057.
3. Calder JD, et al. Posterior ankle impingement syndrome in professional ballet dancers. Am J Sports Med. 2014;42(2):373-378.