Ankle Ultrasound

Reference: Musculoskeletal Ultrasound: Anatomy and Technique, Springer 2008

Setup

• High-frequency linear probe, hockey stick probe is preferred for smaller area especially in thin patients.
• Patient supine, leg exposed to the knee, you seated beside them.
• Common teaching is scanning in a fixed order (Anterior, medial, lateral, posterior). But for this note, I would go straight to the area of symptoms, as this approach is for MSK clinician rather than for radiologist or sonographer.
• Follow each tendon in short axis first, then long axis, from muscle to insertion.
• Tendons change direction around the malleoli, so keep adjusting the probe angle to avoid anisotropy.
• Compare with the other side. Do the same dynamic and stretch manoeuvres on both.

Scan sequence

1. Anterior

Position: knee flexed about 90 degrees, sole flat on the couch.

Short axis at the ankle joint. From medial to lateral: tibialis anterior (largest), extensor hallucis longus (about half the size of tibialis anterior), extensor digitorum longus, peroneus tertius. Dorsalis pedis vessels lie nearby.

Anterior joint recess: long axis at the ankle joint. Normal fluid is under 3 mm. Cartilage is a smooth dark layer 2 to 4 mm thick over bright cortex.

2. Posteromedial

Position: frog-leg (hip abducted, knee flexed to about 45 degrees, foot on its outer edge).

Place the probe transverse on the medial malleolus. From front to back: tibialis posterior, flexor digitorum longus, flexor hallucis longus. The posterior tibial nerve and vessels lie between the last two in the tarsal tunnel. (Tom, Dick And Very Nervous Harry)

Tibialis posterior is the biggest (4 to 6 mm). Up to 4 mm of fluid in its sheath below the malleolus can be normal. It often looks dark near the navicular because of anisotropy; do not call this tendinosis.

Small ossicles inside tendons are normal variants, not avulsion fragments.

3. Posterolateral

Position: knee flexed about 90 degrees, sole flat on the couch. or slightly lie on medial side.

Probe transverse at the posterior edge of the lateral malleolus. Peroneus brevis lies in front of peroneus longus, sharing one sheath (brevis is bigger).

Follow both tendons up to the muscles and down to attachment (Brevis runs to the base of the fifth metatarsal).

Dynamic check for subluxation: probe transverse at the posterior edge of the lateral malleolus, maintain without pressure. Evert and dorsiflex the foot.

An accessory peroneus quadratus is a normal variant, not a split tear.

4. Ligaments

Normal thickness is under 2 mm.

Anterior talofibular (ATFL, the commonly sprained one): plantar flex the foot. Probe horizontal oblique, back edge on the lateral malleolus, front edge on the talus.

Calcaneofibular (CFL): dorsiflex the foot. Probe angled from the tip of the lateral malleolus toward the calcaneus. It lies deep to the peroneal tendons.

Anterior tibiofibular (syndesmosis): probe transverse over the lateral distal tibia, medial edge on the anterolateral tibial cortex.

5. Posterior: Achilles tendon

Position: prone, foot hanging off the end of the couch.

Short axis first. The Achilles has a wavy contour and a thin bright paratenon, with Kager’s fat pad in front. Then long axis from heel to muscle.

Move the foot into dorsiflexion and plantar flexion to scan dynamically, especially if suspect of tear to see the distance of the gap.

Retrocalcaneal bursa: sits in front of the distal tendon. A visible retro-Achilles bursa behind the tendon is abnormal.

Plantar fascia (heel pain): prone, probe on the sole over the calcaneus. Normally under 4 mm thick.

6. Heel

Position: prone, foot hanging off the end of the couch.

Probe in long axis, start from the calcaneum to plantar fascia.

Thickening of the fibre in the case of plantar fasciitis usually happens at the proximal part of the fibre. Look for the ‘cliff edge’ for the measurement point.

Pitfalls

• Tibialis posterior and peroneal tendons look dark from anisotropy at the malleoli and insertions.
• Pressing hard can stop peroneal subluxation from showing.
• Unstretched ligaments look wavy or dark.
• Do not take sesamoid or accessory ossicles for fractures.

Checklist

• Correct position for each zone
• Anterior tendons and joint recess
• Tibialis posterior, FDL and FHL at the medial malleolus
• Posterior tibial nerve and vessels in the tarsal tunnel
• Peroneus brevis and longus in short and long axis
• Dynamic peroneal check for subluxation
• ATFL & CFL
• Achilles in both planes
• Plantar fascia thickness if heel pain