Achilles Tendon and Ankle Ultrasound: Tendinopathy, Tears, and Plantar Fascia

Ankle and Achilles tendon ultrasound is a high-yield MSK study — the structures are superficial, dynamic imaging is easy, and ultrasound answers most clinical questions without radiation or MRI cost. The Achilles is the largest tendon in the body and one of the most commonly injured.

Achilles Tendon Anatomy

The Achilles tendon (calcaneal tendon) is formed by the confluence of the gastrocnemius and soleus muscles. It inserts on the posterior calcaneal tuberosity.

Key structures around the Achilles:

• Retrocalcaneal bursa: Between the Achilles and the calcaneus — normally a thin sliver of fluid (< 2 mm)
• Retroachilles bursa (superficial): Between the Achilles and the skin — normally trace or no fluid
• Kager's fat pad: Pre-Achilles fat triangle visible on sagittal view — a useful landmark

No tendon sheath: The Achilles lacks a true synovial sheath. It has a paratenon (thin connective tissue sleeve). Peritendinous fluid indicates paratenonitis.

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Scanning Technique

Probe: 12–15 MHz linear transducer.

Patient position: Prone, foot hanging off the edge of the table at 90° (neutral position) or slightly plantarflexed. A rolled towel under the ankle helps.

Approach: From the myotendinous junction proximally to the calcaneal insertion distally.

Long Axis (Sagittal) View — Primary View

Probe placed in line with the tendon, posterior aspect of the lower leg/ankle.

Normal appearance:

• Uniformly hyperechoic, fibrillar, striated tendon
• Parallel fiber bundles running the length of the tendon
• Width (AP diameter) measured at the thickest point
• Normal thickness: < 6 mm at midsubstance (most labs use 5–6 mm as upper normal)

Pitfall — anisotropy: The Achilles must be scanned perpendicular to its fibers. Angle the probe heel-to-toe and slightly tilt to maximize echogenicity. A slightly angulated probe will make a normal tendon look focally hypoechoic, mimicking a tear.

Short Axis (Transverse) View

Rotate 90° to view the tendon in cross-section. Oval to circular shape. Scan from the proximal gastrocnemius confluence down to the insertion.

Normal: Oval, uniform, hyperechoic cross-section. At the insertion, the tendon fans out as it meets the calcaneus.

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Measurement

Measure in long axis at the point of maximum thickness.

Normal: ≤ 6 mm AP diameter at midsubstance Tendinosis threshold: > 6 mm (with echogenic changes)

Always measure bilaterally and compare.

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Pathology: Tendinopathy

Non-insertional Tendinopathy (Mid-tendon, 2–6 cm from insertion)

Most common site of Achilles tendinopathy — the "critical zone" at 2–6 cm proximal to the insertion where vascularity is lowest.

Sonographic findings:

• Tendon thickening — AP diameter > 6–7 mm at the affected site
• Hypoechoic areas within the tendon (loss of fibrillar pattern) — focal or diffuse
• Loss of normal fibrillar echogenicity
• Neovascularization on power or color Doppler — abnormal vessels invading the tendon from the paratenon (neovascularization correlates with pain and chronicity)

Grades (descriptive, not formal):

• Tendinosis: Thickening + hypoechogenicity + no tear
• Partial tear: Focal hypoechoic defect that does not traverse the full tendon width
• Complete tear: Full-thickness disruption with retraction of the proximal stump

Insertional Tendinopathy

Pain and thickening at the calcaneal insertion. Associated with:

• Haglund's deformity (prominent posterosuperior calcaneal tuberosity)
• Retrocalcaneal bursitis
• Calcification within the distal tendon

Sonographic findings:

• Hypoechoic, thickened distal tendon at the insertion
• Calcifications within the tendon (echogenic foci ± shadowing)
• Enlarged retrocalcaneal bursa (> 2 mm)
• Enthesophyte at the calcaneal insertion

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Achilles Tendon Tears

Partial Tear

Focal hypoechoic defect within the tendon substance — does not extend full-thickness. May be articular-surface (deep) or superficial.

• Describe: location (distance from calcaneus), transverse extent (% of tendon width), AP depth
• Dynamic assessment: does the defect gape with dorsiflexion? (suggests more significant tear)

Complete Tear

Full-thickness tear with visible gap.

Classic history: Sudden "pop" and inability to plantarflex; palpable gap in the tendon.

Sonographic findings:

• Complete fibrillar discontinuity — a gap filled with fluid or heterogeneous hematoma
• Retracted proximal stump: the gastrocnemius-soleus pulls the proximal tendon proximally — measure the retraction distance
• Dynamic: significant gaping with passive dorsiflexion

Measure: Gap size at rest and with dorsiflexion. Distance of retraction. Length of tendon distal to the tear.

Thompson (Squeeze) Test: Squeeze the calf while the patient is prone — if the tendon is intact, the foot plantarflexes. A positive test (no plantar flexion) confirms complete tear. You may observe this clinically at the bedside.

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Retrocalcaneal Bursitis

Fluid in the retrocalcaneal bursa (between Achilles and calcaneus).

Normal: < 2 mm Bursitis: > 2 mm; anechoic or complex fluid; may be associated with Haglund's deformity or insertional tendinopathy

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Plantar Fascia / Plantar Fasciitis

The plantar fascia is best evaluated from the medial plantar foot.

Patient position: Prone, foot dorsiflexed (foot hanging off the table, toes toward the floor — passive dorsiflexion places the fascia under tension).

Probe placement: Medial longitudinal arch, heel to midfoot.

Normal plantar fascia:

• Hyperechoic, fibrillar band
• Thickness at the calcaneal origin: ≤ 4 mm (upper limit of normal)

Plantar fasciitis:

• Thickening ≥ 4 mm at the proximal origin (most meaningful at the calcaneal attachment)
• Loss of normal fibrillar echogenicity at the insertion (hypoechoic, edematous)
• Calcification at the origin (calcaneal spur — present in many normals too; the tendon thickness is more diagnostic than spur presence)
• Doppler: may show increased vascularity at the insertion in active plantar fasciitis

Measure: AP thickness at the calcaneal origin in long axis.

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Ankle Tendons (Additional)

For completeness, the ankle tendons are often included in the same scan:

Medial: Tibialis posterior, flexor digitorum longus, posterior tibial neurovascular bundle, flexor hallucis longus (Tom, Dick, ANd Harry — from anterior to posterior)

Lateral: Peroneus longus and brevis run posterior to the lateral malleolus. Peroneal tendon tears and subluxation are common after ankle sprains.

Anterior: Tibialis anterior, extensor hallucis longus, extensor digitorum longus — scan in long and short axis.

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Key Images to Capture

1. Achilles tendon — long axis, full length (myotendinous to insertion)
2. Achilles tendon — short axis at midsubstance with measurement
3. Retrocalcaneal bursa region
4. Any area of focal pathology — two planes with measurements
5. Doppler of any tendinopathy area (neovascularization)
6. Plantar fascia — long axis at calcaneal origin with measurement
7. Contralateral side for comparison

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Reporting Language

Normal: "Achilles tendon intact bilaterally. Uniform fibrillar echogenicity throughout. AP diameter 5.2 mm at midsubstance on the right, 5.0 mm on the left. Retrocalcaneal bursae: no significant fluid. Plantar fascia: 3.4 mm at the calcaneal origin bilaterally — within normal limits."

Tendinopathy: "Non-insertional Achilles tendinopathy. Right Achilles tendon thickened at 8.4 mm AP diameter at 4 cm from the calcaneal insertion with focal hypoechogenicity and loss of fibrillar pattern over a 2.5 cm segment. Increased color Doppler signal (neovascularization) at the involved segment. No complete tear. Contralateral left Achilles: 5.4 mm, normal appearance."

Complete tear: "Complete tear of the right Achilles tendon approximately 4.5 cm from the calcaneal insertion. A 2.8 cm gap is identified, filled with heterogeneous fluid and hematoma. The proximal stump is retracted 2.8 cm from the distal tendon. With passive dorsiflexion, the gap increases to 3.5 cm. Clinical correlation and orthopedic consultation are recommended."

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SonoBuddy's MSK protocols include shoulder, knee, ankle, and soft tissue ultrasound guides.