Posterior Ankle Impingement (Os Trigonum Syndrome)

Quick answer

Posterior ankle impingement is pain at the back of the ankle that develops when the foot is forcefully pointed downward (plantarflexion). It happens because tissue or bone gets pinched between the back of the tibia and the heel bone (calcaneus). The most common culprit is a small extra bone called an os trigonum — a normal anatomical variant that becomes a problem only when activities push the ankle into extreme plantarflexion repeatedly. Ballet dancers and soccer players are the classic patients.

What’s actually getting pinched

Several structures can be impinged at the back of the ankle:

• Os trigonum — a small accessory bone behind the talus, present in 5–15% of people. Most have it without ever knowing
• Stieda process — an enlarged posterior process of the talus (the same bone shape, just attached rather than separate)
• Posterior capsule and ligaments — can become inflamed and thickened
• Flexor hallucis longus tendon — runs through this region and can develop tenosynovitis

The classic mechanism is forced plantarflexion (pointing the foot downward) compressing these structures. Less commonly an acute injury (like a sprain or kicker’s fracture) creates the impingement.

Activities that drive it

• Ballet — en pointe and demi-pointe positions are extreme plantarflexion. Posterior ankle impingement is one of the most common ankle problems in dancers
• Soccer — kicking with the front of the foot puts the ankle in extreme plantarflexion
• Gymnastics — landing in deep plantarflexion
• Running downhill — repetitive end-range plantarflexion
• High heels — chronic plantarflexion can aggravate

How to recognize it

• Pain at the back of the ankle, behind the lateral malleolus or in the midline
• Worse with plantarflexion — pointing the foot down reproduces the pain
• Worse with running, jumping, or kicking
• Tenderness when pressing along the back of the ankle, particularly behind the lateral malleolus
• Sometimes a sense of fullness or swelling at the back of the ankle
• Often confused with Achilles issues, but the location of pain is more focal and deeper than Achilles tendinitis

A useful exam test: passive forced plantarflexion (the examiner points the patient’s foot down) reproduces the pain.

Diagnosis

• History and physical exam — pattern recognition plus the forced plantarflexion test
• X-rays — show os trigonum or Stieda process; standard lateral view, sometimes with the ankle in plantarflexion
• MRI — gold standard. Shows bone marrow edema, soft tissue inflammation, FHL tenosynovitis, and rules out other causes
• Diagnostic injection — a small amount of local anesthetic into the impingement area can confirm the source of pain

Treatment

Conservative care (first-line)

Most cases respond to non-operative care:

• Activity modification — temporary reduction in dance, soccer, or other plantarflexion-heavy activities
• NSAIDs for inflammation
• Physical therapy focused on flexibility, posterior chain strengthening, and modification of dance/sport technique
• Ice after activity
• Cortisone injection — into the posterior ankle joint or os trigonum region. Often dramatically helpful, both diagnostic and therapeutic
• Heel lift — slightly reduces end-range plantarflexion in non-dance settings

Many dancers and athletes can manage symptoms with technique modification and selective injections, returning to full activity.

Surgery

For pain that fails conservative care or in elite athletes who can’t reduce activity:

• Os trigonum excision (arthroscopic or open) — removes the offending bone. Highly effective for pain relief
• FHL tendon release — added when tendon irritation contributes
• Recovery — typically a few weeks in a boot followed by progressive return to activity. Full return to dance/sport at 3–6 months
• Outcomes — generally excellent; recurrence rates are low

Arthroscopic excision is the preferred technique in most centers, with smaller scars and faster recovery.

Bottom line

Posterior ankle impingement is the classic dancer’s injury but affects anyone who repetitively plantarflexes the ankle. Diagnosis combines a careful exam, imaging, and often a diagnostic injection. Conservative care helps most people; arthroscopic excision is reliable for those who fail. The presence of an os trigonum on X-ray alone doesn’t make the diagnosis — many people have one without symptoms. Match imaging findings to clinical exam.