Achilles Tendon: Anatomy, Function, Clinical Significance, MRI & CT Imaging

Topic

Achilles tendon

The Achilles tendon, also known as the calcaneal tendon, is the strongest and thickest tendon in the human body. It connects the gastrocnemius, soleus, and plantaris muscles to the posterior surface of the calcaneus, forming the primary mechanism for plantarflexion of the ankle.

Its superficial location and heavy mechanical load during running, jumping, and walking make it highly susceptible to tendinopathy, partial tears, and complete ruptures. Clinically, rupture of the Achilles tendon is one of the most common tendon injuries in athletes and active individuals.

Synonyms

Calcaneal tendon
Tendo calcaneus

Origin, Course, and Insertion

Origin: Formed by the conjoined tendinous fibers of the gastrocnemius, soleus, and occasionally the plantaris muscle
Course: Descends vertically along the posterior aspect of the leg, forming a thick, fibrous band
Insertion: Posterior surface of the calcaneus (middle third), into the calcaneal tuberosity

Tendon Attachments

Merges with fibers of the gastrocnemius and soleus approximately halfway down the leg
Passes posterior to the ankle joint without a synovial sheath but surrounded by paratenon, which facilitates gliding
Inserts into the calcaneal tuberosity, sometimes blending with the plantar fascia

Relations

Anteriorly: Subtendinous bursa, posterior surface of the calcaneus
Posteriorly: Skin and subcutaneous tissue
Medially: Flexor hallucis longus tendon
Laterally: Fibular tendons (peroneal tendons)
Superiorly: Gastrocnemius and soleus muscle bellies
Inferiorly: Calcaneal insertion, continuous with plantar fascia

Function

Plantarflexion: Primary plantarflexor of the ankle, essential for walking, running, and jumping
Knee–ankle coordination: Acts across the ankle joint and indirectly across the knee via gastrocnemius
Propulsion: Provides explosive force during sprinting and leaping
Stability: Helps maintain posture and balance by resisting dorsiflexion forces

Clinical Significance

Tendinopathy: Chronic overuse leads to degeneration, thickening, and pain (common in runners)
Partial tears: Cause focal pain and weakness but may still allow limited function
Complete rupture: Sudden loss of plantarflexion strength, often accompanied by an audible “pop”
Bursitis: Inflammation of retrocalcaneal or subcutaneous bursa adjacent to tendon
Surgical relevance: Repair techniques vary; early diagnosis improves outcomes
Imaging role: MRI and CT are critical for diagnosis, staging, and preoperative planning

MRI Appearance

T1-weighted images:
- Normal tendon: uniformly low signal (dark band), surrounded by intermediate signal paratenon
- Partial tear: focal intermediate-to-bright signal within tendon fibers
- Complete rupture: gap in tendon continuity filled with intermediate signal hematoma
T2-weighted images:
- Normal tendon: low signal, darker than on T1
- Tendinopathy: focal or diffuse high signal intensity within thickened tendon
- Partial tears: hyperintense linear defects extending into tendon substance
- Complete rupture: high-signal fluid gap at site of discontinuity
STIR (Short Tau Inversion Recovery):
- Normal tendon: dark
- Pathology: bright hyperintense areas indicating edema, inflammation, or rupture-associated fluid
Proton Density Fat-Saturated (PD FS):
- Normal tendon: homogeneously dark
- Pathology: bright signal clefts or diffuse hyperintensity in tendinopathy, small partial tears, or peritendinous inflammation
T1 Fat-Sat Post-Contrast:
- Normal tendon: minimal or no enhancement
- Tendinopathy: mild enhancement of paratenon
- Postoperative or inflammatory changes: strong linear or diffuse enhancement

CT Appearance

Non-Contrast CT:

Tendon visible as a dense, thick band posterior to ankle
Useful in identifying calcifications, ossifications, or chronic degenerative changes
Indirect rupture signs: tendon discontinuity, surrounding soft-tissue swelling

Post-Contrast CT (standard):