Os trigonum syndrome

Quick overview

• Definition: Painful posterior ankle joint impingement caused by an accessory piece of bone (os trigonum) or an elongated posterior process of the talus.
• Commonly affected: Dancers, footballers, gymnasts, runners - activities with repeated strong plantar flexion.
• Key symptoms: Stabbing pain at the back of the ankle joint, swelling, increased pain with equinus position, possibly snapping of the big toe flexor tendon (FHL).
• Diagnostics: Clinical examination, X-ray (lateral view), if necessary ultrasound and MRI to assess bone edema, synchondrosis and tendon involvement.
• Therapy initially conservative: load adjustment, physio, anti-inflammatory measures, short-term immobilization; targeted injection if necessary. Surgery only if restrictions persist.

Anatomy: What is the Os trigonum?

The os trigonum is a small, additional piece of bone behind the talus. It forms as an accessory bone core that does not completely fuse with the talus during puberty. It often remains attached via a cartilaginous connection (synchondrosis).

• Location: Posterior edge of the talus, between the heel bone (calcaneus) and the shin bone (tibia).
• Proximity: Proximity to the flexor tendon of the big toe (Flexor hallucis longus, FHL) and the capsule of the posterior ankle joint.
• Occurrence: Estimated to occur in 5–10% of the population – often symptom-free.

Only with repeated or extreme equinus position can a painful pinching mechanism occur between the tibia, talus and heel bone (posterior impingement).

Causes and triggers

The syndrome arises from mechanical entrapment of the os trigonum and surrounding soft tissues with strong plantar flexion. Microtrauma leads to inflammation, edema, and irritation of the synchondrosis or FHL tendon.

• Repetitive strain in the equinus position: ballet (en pointe, demi-pointe), soccer ball, gymnastics, sprinting, running downhill.
• Sprains/supination trauma: Irritates the posterior ankle and can cause discomfort.
• Anatomical variants: Large os trigonum or extended posterior talar processes increase the tendency to entrapment.
• Technique and training factors: rapid increase in load, inadequate trunk/foot muscles, limited dorsal ankle mobility.

Symptoms: How do you recognize Os trigonum syndrome?

• Stabbing or pressing pain at the back of the ankle, especially a. with equinus position or push-off movement.
• Stress-related swelling and tenderness behind the lateral or medial malleolus (often posterolaterally).
• Pain when standing on toes, dancing or shooting with the instep.
• Occasional snapping/rubbing of the big toe flexor tendon (FHL), pain when actively flexing the big toe in equinus position.
• Improvement at rest, increase when intensive activities are resumed.

After an acute sprain, symptoms can develop gradually. In adolescents, symptoms are more common during growth spurts.

Diagnostics in practice

The diagnosis results from anamnesis, targeted examination and imaging procedures. We check triggering movements, painful pressure points and tendon involvement.

• Clinic: pain provocation in maximum plantar flexion (posterior impingement test); tenderness behind the talus; Testing the FHL tendon (pain/snapping when flexing the big toe).
• X-ray (side view): Depiction of an os trigonum or an extended talar process; Exclusion of bony injuries.
• Ultrasound: Dynamic assessment of soft tissues, FHL tendon and, if necessary, effusion.
• MRI: evidence of bone marrow edema, irritation of the synchondrosis, capsular thickening, tenosynovitis of the FHL; important for unclear findings.
• Differential diagnosis: Differentiation from Achilles tendon problems, retrocalcaneal bursitis, osteochondral talus lesions, etc.

Conservative therapy: Our approach first

The goal is to relieve pain, calm inflammation and gradually increase stress. In most cases, os trigonum syndrome can be controlled without surgery.

• Load adjustment: Temporary avoidance of extreme equinus position, reduction of jumping and shooting-intensive activities.
• Short-term immobilization: Depending on the level of irritation, use a walker/splint for 1-2 weeks, then gradual mobilization.
• Anti-inflammatory: Cooling, if necessary short-term non-steroidal anti-inflammatory drugs (NSAIDs) - according to individual consideration and tolerability.
• Physiotherapy: swelling management, pain-adapted mobilization, improvement of dorsiflexion, FHL gliding exercises, foot muscle and calf strengthening, proprioception.
• Technical and shoe advice: Adjustment of the scope of training, technique coaching (e.g. for dancers), heel wedge if necessary, taping to limit the end position.
• Return-to-Activity Plan: Step-by-step structure with clear stress criteria and symptom monitoring.

Targeted injections: when does it make sense?

If symptoms persist despite consistent basic therapy, an ultrasound-targeted injection can be considered. It is used to reduce inflammation or for diagnostic limitation.

• Periarticular/Peri-Os infiltration: Low-dose corticosteroid with local anesthetic for short-term calming of inflammation.
• FHL tendon sheath: Injection for tenosynovitis/snapping symptoms.
• Information: risk-benefit assessment, possible side effects (e.g. temporary increase in blood sugar, tendon irritation), limiting the frequency of injections.

Depending on the findings, regenerative procedures (e.g. PRP) are used cautiously and after informed consent; the evidence is heterogeneous.

Surgery: indications and procedures

Surgical removal of the os trigonum is considered if, despite adequate conservative therapy, there is relevant functional limitation over several months or if there is recurrent performance limitation in athletes.

• Procedure: Minimally invasive posterior ankle arthroscopy (hindfoot endoscopy) or open excision; If necessary, accompanying FHL tenosynovectomy.
• Goals: Elimination of mechanical entrapment, reduction of inflammation, restoration of resilient mobility.
• Risks: infection, nerve irritation (e.g. sural nerve), impaired wound healing, thrombosis, persistent symptoms; Careful explanation is essential.
• Follow-up treatment: early functional mobilization, temporary partial weight-bearing/protection in shoes/boots; Return to sport individually, often after 6-12 weeks.

There is no guarantee that there will be no symptoms. The decision is made individually, taking into account goals, findings and stress profile.

Rehabilitation and return to sport

Course and prognosis

Many sufferers achieve significant improvement with consistent conservative therapy. If there is a severe mechanical conflict situation, a surgical approach may be necessary. The medium-term results are usually good if the indication is appropriate, but requires patience and structured rehabilitation.

Self-help: What you can do yourself

• Activity adjustment: temporarily avoid movements in the maximum equinus position, plan breaks.
• Cooling after exertion and elevating if there is swelling.
• Footwear: Slightly raised heel/heel wedge can reduce final position; sufficiently stable heel support.
• Targeted exercises: dorsiflexion mobilization, gentle calf and plantar fascia stretching (without forced equinus end position), arch and toe strength.
• Warm-up and technique: Structured warm-up routine and clean technique reduce impingement peaks.

When should you see a doctor?

• Persistent posterior ankle pain for several weeks despite rest.
• Pain/blockage due to equinus position, snapping of the big toe flexor tendon.
• Significant swelling, restricted movement or repeated twisting events.
• Warning signs: Acute trauma with severe pain and inability to exercise, sensory disturbances, fever or redness - please clarify promptly.

Special features for athletes

In competitive and stage sports, the focus is on early symptom control and precise, sport-specific rehabilitation. A close exchange between orthopedics, physiotherapy, the trainer team and the patient is crucial.

• Dance/ballet: technique training for en pointe/demi-pointe, measured transition back to pointe training.
• Football: Progression from running and passing exercises to shooting stress, starting outside instep/instep shots pain-free.
• Gymnastics/Running: Pay attention to the measured jumping and landing technique, the choice of surface and shoe soles.

Differential diagnoses

• Achilles tendinopathy or partial tear
• Retrocalcaneal/retroachillary bursitis
• Stieda process (elongated posterior process of the talus without a separate os)
• Osteochondral lesions of the talus
• Posterior capsulitis/synovitis without os trigonum
• Stress reactions/fractures (posterior talus, calcaneus)
• Tarsal coalition
• Peroneal tendon and tibialis posterior complaints

Common mistakes and misunderstandings

• “It always requires surgery” – that’s not true. Conservative measures are often sufficient.
• Re-entry too quickly without criteria – increases the risk of relapse.
• Aggressive stretches in the final equinus position – can increase the entrapment.
• Imaging alone without functional testing – easily leads to over- or under-treatment.

Your appointment in Hamburg

We take the time for a careful diagnosis and discuss the treatment options in an understandable and transparent manner. Our practice is located at Dorotheenstraße 48, 22301 Hamburg (Winterhude). You can easily receive appointments via Doctolib or by email.