What is the most likely diagnosis and what other conditions should be considered in a 17‑year‑old female with bilateral internal tibial torsion who presents with acute right ankle pain in inversion, no trauma, normal neurovascular exam, no swelling, posterior ankle tenderness near the Achilles, and normal radiographs?

Most Likely Diagnosis: Peroneal Tendinopathy or Tenosynovitis

The most likely diagnosis in this 17-year-old with bilateral internal tibial torsion presenting with acute ankle pain in inversion, bilateral posterior ankle tenderness near the Achilles, and normal radiographs is peroneal tendinopathy or tenosynovitis, though you must also strongly consider syndesmotic injury given her anatomical predisposition.

Clinical Reasoning

Why Peroneal Pathology is Most Likely

• The inverted ankle position with refusal to straighten suggests peroneal muscle spasm or tendon irritation – the peroneal tendons (longus and brevis) run posterior to the lateral malleolus and are primary evertors of the foot 1

• Bilateral posterior ankle tenderness on either side of the Achilles tendon anatomically corresponds to the retromalleolar groove where peroneal tendons course, not just the Achilles itself 1

• No trauma or mechanism of injury makes this more consistent with overuse tendinopathy or anatomically-predisposed tendon irritation rather than acute ligamentous injury 1

• Normal radiographs exclude fracture per Ottawa Ankle Rules criteria (no bony tenderness at malleoli, able to bear weight) 2

Critical Consideration: Syndesmotic Injury Risk

• Internal tibial torsion significantly increases syndesmotic injury risk – research demonstrates that specimens with internal tibial torsion have 24.0% prevalence of syndesmotic injury versus 15.6% with external torsion (p=0.035) 3

• Her bilateral internal tibial torsion is NOT a benign anatomical variant in this context – it creates abnormal biomechanical stress at the distal tibiofibular joint 3

• Syndesmotic injuries can occur without external rotation mechanism in patients with internal tibial torsion, though classic mechanisms include external rotation and hyperdorsiflexion 1

Essential Additional Assessments

Immediate Physical Examination Maneuvers

• Squeeze test: Compress tibia and fibula at mid-calf – pain at the ankle indicates syndesmotic injury 1

• External rotation stress test: With knee flexed 90°, externally rotate the foot – pain at anterior/posterior tibiofibular ligaments suggests syndesmosis injury 1

• Palpate the entire fibular length: Tenderness along the proximal fibula or interosseous membrane suggests high ankle sprain 2

• Anterior drawer test: Though optimally performed 4-5 days post-injury for lateral ligament assessment (sensitivity 84%, specificity 96%), perform now to establish baseline 2

• Specific peroneal tendon palpation: Palpate posterior to lateral malleolus along the retromalleolar groove, assess for subluxation with active eversion 1

Imaging Considerations

• MRI is indicated if syndesmotic injury is suspected – it has 93-96% sensitivity and 100% specificity for syndesmotic injuries, osteochondral defects, and occult fractures 2

• Ultrasound can evaluate peroneal tendons if available with experienced technician (92% sensitivity for ligament injuries, though only 64% specificity) 2

• Do NOT order stress radiographs – they are obsolete due to limited diagnostic value and pain during stressing 2

Differential Diagnosis to Consider

High Priority (Must Rule Out)

1. Syndesmotic (high ankle) sprain – given internal tibial torsion predisposition 3
2. Peroneal tendinopathy/tenosynovitis – most consistent with presentation
3. Peroneal tendon subluxation – can occur with inversion positioning
4. Occult fracture – though Ottawa Ankle Rules are negative, consider if symptoms persist 2

Moderate Priority

1. Posterior tibial tendinopathy – though less likely with inversion positioning
2. Flexor hallucis longus tendinopathy – runs posteromedially near Achilles
3. Tarsal coalition – can present in adolescence with pain and limited motion

Lower Priority (Less Likely Given Presentation)

1. Lateral ankle sprain – unlikely without trauma and with bilateral posterior tenderness 1, 2
2. Achilles tendinopathy – tenderness is bilateral and lateral to Achilles, not within it

Management Algorithm

Immediate Management

• If squeeze test or external rotation test is positive: Treat as syndesmotic injury with strict non-weight-bearing, immobilization, and urgent orthopedic referral 1

• If peroneal pathology suspected: Immobilize in neutral position (not inverted), ice, compression, elevation – though evidence for RICE is limited 2

• Avoid forced correction of inverted position until neurovascular status confirmed stable and specific diagnosis established 1

Follow-Up Strategy

• Re-examine in 4-5 days for optimal ligamentous assessment if initial exam equivocal 2

• Order MRI if: Pain persists beyond 7-10 days, syndesmotic injury suspected, or inability to return to normal function 2

• Physical therapy referral once acute phase resolves, focusing on peroneal strengthening and addressing biomechanical issues from tibial torsion 1

Critical Pitfalls to Avoid

• Do not dismiss internal tibial torsion as incidental – it creates real biomechanical vulnerability to syndesmotic injury 3

• Do not assume lateral ankle sprain without appropriate mechanism – the inversion and internal rotation mechanism must be present 1

• Do not rely solely on initial physical exam for ligamentous injury – sensitivity and specificity are optimized at 4-5 days post-injury 2

• Do not overlook bilateral symptoms – her bilateral posterior ankle tenderness suggests systemic or anatomical predisposition rather than isolated traumatic injury 1