Anatomy and Causes of Os Trigonum Syndrome

The os trigonum is an accessory ossicle (extra bone) located posterior to the talus bone in the ankle. It forms as a separate ossification center during adolescence (typically ages 8-13) and normally fuses with the talus by age 15-17. In approximately 7-25% of the population, this secondary center never fuses, persisting as a separate bone connected to the talus by a fibrous or cartilaginous synchondrosis.

Os trigonum syndrome develops when this unfused ossicle becomes compressed between the posterior tibia and the calcaneus during maximal ankle plantarflexion — the pointed-toe position. This ‘nutcracker’ mechanism creates repetitive microtrauma at the synchondrosis, inflammation of the surrounding soft tissues, and irritation of the flexor hallucis longus (FHL) tendon that courses through the posterior ankle directly adjacent to the os trigonum.

The condition is most common in activities requiring repetitive or sustained plantarflexion: ballet (en pointe and relevé positions), soccer (kicking and shooting), gymnastics (pointed landings), downhill running (plantarflexed foot position), and swimming (flutter kick). Any activity that repeatedly forces the ankle into extreme plantarflexion can trigger symptoms in individuals with an unfused os trigonum.

Symptoms of Posterior Ankle Impingement

The hallmark symptom is deep, aching pain at the back of the ankle that worsens specifically during plantarflexion activities. Ballet dancers experience pain during relevé and en pointe work, soccer players during shooting and push-off, and runners during downhill terrain or speed work. The pain is typically well-localized to the posterolateral ankle, just lateral to the Achilles tendon.

A positive posterior impingement test — reproduction of pain when the examiner forces the ankle into maximal plantarflexion — is the key clinical finding. Dr. Tom Biernacki performs this test by stabilizing the lower leg and pressing the foot into pointed-toe position, which compresses the os trigonum between the tibia and calcaneus. Pain that reproduces the athlete’s typical symptoms strongly suggests posterior impingement.

Associated FHL tendon involvement causes additional symptoms including pain with great toe flexion (push-off), triggering or catching of the great toe, and tenderness along the FHL tendon behind the medial malleolus. This ‘dancer’s tendinitis’ occurs because the FHL tendon runs in a groove directly adjacent to the os trigonum, and inflammation from the compressed ossicle extends to involve the tendon within its sheath.

Diagnosis and Imaging

Lateral ankle X-rays clearly demonstrate the os trigonum as a separate bone posterior to the talus. However, the presence of an os trigonum on X-ray alone does not confirm the diagnosis — since 7-25% of the population has this variant, the ossicle must be correlated with clinical symptoms. Many os trigona are asymptomatic incidental findings discovered on X-rays obtained for other reasons.

MRI provides definitive diagnostic information by demonstrating bone marrow edema within the os trigonum and/or the posterior talus (indicating active stress at the synchondrosis), fluid around the FHL tendon (tenosynovitis), and soft tissue inflammation in the posterior ankle recess. These inflammatory changes confirm that the os trigonum is actively contributing to the patient’s symptoms rather than being an incidental finding.

Diagnostic injection of local anesthetic into the posterior ankle recess under ultrasound guidance serves dual diagnostic and therapeutic purposes. If the injection eliminates the patient’s pain temporarily, it confirms the posterior ankle as the pain source and predicts a favorable response to surgical excision. This injection also provides therapeutic relief and is often combined with corticosteroid for sustained anti-inflammatory effect.

Conservative Treatment

Initial conservative management includes activity modification to reduce plantarflexion-loading activities, anti-inflammatory medications, and physical therapy focused on posterior ankle flexibility and strength. For many recreational athletes, reducing the volume and intensity of aggravating activities while maintaining fitness through cross-training is sufficient to control symptoms without surgical intervention.

Corticosteroid injection into the posterior ankle recess provides targeted anti-inflammatory relief that can last weeks to months. Under ultrasound guidance, Dr. Biernacki delivers the medication precisely to the area of inflammation around the os trigonum and FHL tendon, maximizing therapeutic effect while minimizing systemic medication exposure. Some patients achieve lasting relief from one or two injections combined with activity modification.

Doctor Hoy’s Natural Pain Relief Gel applied to the posterior ankle before and after activity provides supplementary anti-inflammatory comfort. Physical therapy protocols emphasize posterior ankle and subtalar joint mobilization, FHL tendon gliding exercises, eccentric calf strengthening, and sport-specific movement modification to reduce impingement forces during athletic activities. PowerStep Pinnacle insoles with heel cushioning reduce ground reaction forces transmitted to the posterior ankle.

Surgical Excision of the Os Trigonum

Surgery is recommended when 3-6 months of comprehensive conservative treatment fails to provide adequate relief, particularly in competitive athletes whose performance is limited by persistent posterior ankle pain. The procedure involves removing the os trigonum and releasing any FHL tendon adhesions, eliminating both the mechanical impingement source and associated tendon pathology.

Dr. Tom Biernacki performs os trigonum excision using arthroscopic (endoscopic) technique through two small posterior ankle portals. The posterior ankle endoscopic approach provides excellent visualization of the os trigonum, posterior talus, and FHL tendon while avoiding the larger incision and more extensive tissue dissection required by open surgical approaches. The arthroscopic technique also allows complete clearance of the posterior ankle recess and release of FHL tendon tethering.

The procedure takes approximately 30-45 minutes under regional anesthesia as an outpatient surgery. The small portal incisions minimize soft tissue trauma, reduce postoperative pain, and allow faster return to activity compared to open excision. The os trigonum is removed in its entirety, and any associated posterior tibial or FHL pathology is addressed simultaneously through the same portals.

Recovery and Return to Sport

Recovery from arthroscopic os trigonum excision is significantly faster than open surgery. Weight bearing in a surgical shoe is allowed immediately, with transition to supportive athletic shoes by 1-2 weeks. Gentle ankle range of motion exercises begin within days of surgery to prevent stiffness and promote posterior ankle recess healing. Sutures are removed at 10-14 days.

Physical therapy begins at 2 weeks with focus on restoring full ankle plantarflexion range — the specific motion that was previously painful. Progressive strengthening, proprioceptive training, and sport-specific drills follow as comfort allows. Most recreational athletes return to full activity by 6-8 weeks. Competitive dancers and athletes typically require 8-12 weeks for full return due to the extreme plantarflexion demands of their sports.

Published outcomes for arthroscopic os trigonum excision report 85-95% excellent results with complete resolution of posterior ankle pain and full return to pre-injury activity levels. The arthroscopic approach achieves equivalent outcomes to open surgery with return to sport averaging 3-4 weeks earlier. CURREX RunPro insoles provide excellent dynamic support during the return-to-sport transition.

The Most Common Mistake We See

The most common mistake with os trigonum syndrome is treating posterior ankle pain as ‘just Achilles tendinitis’ without evaluating for posterior impingement. The Achilles tendon lies superficially while os trigonum pain is deep — a distinction easily missed without specific clinical testing. Athletes with posterior ankle pain that worsens specifically during plantarflexion rather than dorsiflexion should be evaluated for impingement with lateral X-rays.

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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

What is os trigonum syndrome?

Os trigonum syndrome occurs when an unfused extra bone at the back of the ankle gets compressed during pointed-toe activities, causing deep posterior ankle pain. It predominantly affects ballet dancers, soccer players, and gymnasts whose sports require repetitive ankle plantarflexion.

How is os trigonum syndrome diagnosed?

Diagnosis combines clinical examination (positive posterior impingement test), lateral ankle X-rays showing the os trigonum, and MRI demonstrating bone marrow edema and inflammation confirming the ossicle is symptomatic. Diagnostic injection may confirm the pain source.

Does os trigonum syndrome require surgery?

Not always. Many patients respond to activity modification, physical therapy, and corticosteroid injections. Surgery is recommended when 3-6 months of conservative treatment fails to provide adequate relief, particularly in competitive athletes whose performance is limited.

How long is recovery from os trigonum surgery?

With arthroscopic excision, most patients walk in regular shoes by 1-2 weeks, return to light training by 4-6 weeks, and achieve full return to sport by 6-12 weeks depending on the demands of their specific activity.

The Bottom Line

Os trigonum syndrome is a highly treatable cause of posterior ankle pain that responds well to both conservative and surgical management when properly diagnosed. At Balance Foot & Ankle, Dr. Tom Biernacki provides expert evaluation and arthroscopic treatment for posterior ankle impingement, helping athletes return to their sport pain-free.

Sources

1. Arthroscopy (2024) — Posterior ankle endoscopy outcomes for os trigonum excision
2. Foot & Ankle International (2025) — Comparative study of arthroscopic vs open os trigonum removal
3. Journal of Dance Medicine (2024) — Posterior impingement syndrome in professional ballet dancers
4. Sports Medicine (2024) — FHL tendinopathy associated with os trigonum in athletes