Posterior Ankle Impingement and Os Trigonum Syndrome: Causes, Diagnosis, and Treatment

What Is Posterior Ankle Impingement?

Posterior ankle impingement syndrome (PAIS) is a painful condition caused by compression of soft tissues or bony structures at the back of the ankle during plantarflexion (pointing the foot downward). It is particularly common in athletes who repeatedly perform extreme ankle plantarflexion — dancers, gymnasts, soccer players, and downhill runners.

The posterior ankle can be compressed by soft tissue (capsule, ligaments, synovium) — classified as soft tissue impingement — or by bony structures, most notably the os trigonum or a prominent lateral process of the talus (Stieda process).

The Os Trigonum

The os trigonum is an accessory ossicle (extra bone) located at the posterior aspect of the talus. It represents a secondary ossification center that fails to fuse with the main talus body, occurring in approximately 7–14% of the general population. In most people, the os trigonum is asymptomatic. However, in athletes who repeatedly drive into extreme plantarflexion, the os trigonum can be compressed between the posterior tibia and the calcaneus — producing pain, inflammation, and occasionally a stress fracture of the synchondrosis (cartilage junction) connecting it to the talus.

Symptoms

Posterior ankle impingement presents with pain localized to the back of the ankle, particularly between the Achilles tendon and the posterior malleolus. Symptoms characteristically worsen with:

• Full toe point (extreme plantarflexion)
• Push-off during walking and running
• Descending stairs or slopes
• Wearing high-heeled shoes

The pain is often dull and aching at baseline but sharp with provocative movements. Swelling may be present in the posterior ankle, and a “nutcracker” test — forcefully plantarflexing the ankle — reproduces posterior pain.

Diagnosis

Clinical history and examination suggest the diagnosis. Lateral weight-bearing X-rays can identify an os trigonum or prominent posterior talar process. MRI is the gold standard for diagnosing posterior impingement — it reveals bone marrow edema (stress response) in the os trigonum, fluid in the posterior ankle joint, and associated FHL (flexor hallucis longus) tendon pathology, which is commonly present simultaneously.

A diagnostic injection of local anesthetic into the posterior ankle provides important confirmatory evidence — if the injection eliminates pain during provocative testing, posterior impingement is confirmed as the pain source.

Non-Surgical Treatment

Initial management includes activity modification to avoid extreme plantarflexion, NSAIDs, and physical therapy focusing on posterior ankle flexibility and proprioception. A posterior ankle corticosteroid injection can reduce inflammation and provide 4–12 weeks of relief. Approximately 60–70% of cases improve with conservative management over 3–6 months.

Surgical Treatment: Os Trigonum Excision

When conservative treatment fails after 3–6 months, surgical excision of the os trigonum (and any prominent posterior talar process) is indicated. Arthroscopic excision is now the preferred approach, using a posterior two-portal technique with the patient prone. Arthroscopic surgery offers smaller incisions, lower complication rates, faster recovery, and equivalent outcomes to open excision.

During arthroscopy, the FHL tendon sheath is also inspected and released if tendinopathy is present — FHL pathology accompanies posterior impingement in 40–60% of cases.

Recovery After Surgery

Patients are typically weight-bearing in a boot within 2–3 days of arthroscopic os trigonum excision. Physical therapy begins at 2 weeks, focusing on range of motion and strength. Most athletes return to sport at 6–12 weeks post-operatively, significantly faster than the 3–6 month recovery typical of open procedures.

Outcomes after arthroscopic excision are excellent — studies report 85–95% good to excellent results with return to previous level of sport in the majority of athletes.

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Dr. Tom Biernacki, DPM

Dr. Tom Biernacki, DPM is a double board-certified podiatrist and foot & ankle surgeon at Balance Foot & Ankle Specialists in Southeast Michigan. With over a decade of clinical experience, he specializes in heel pain, bunions, diabetic foot care, sports injuries, and minimally invasive surgery. Dr. Biernacki is a member of the APMA and ACFAS, and his patient education content on MichiganFootDoctors.com and YouTube has reached over one million views.