What is the anatomy of the Achilles tendon?

Anatomy of the Achilles Tendon

The Achilles tendon is the strongest and thickest tendon in the human body, formed by the conjoint tendons of the gastrocnemius and soleus muscles, with its structure characterized by a complex spiral arrangement of fibers that rotate approximately 90° internally from proximal to distal. 1

Structural Composition

• Origin and Formation:

  • Formed by the fusion of three parts: medial gastrocnemius, lateral gastrocnemius, and soleus muscle tendons 1
  • The relative contribution of these muscles to the tendon varies 2
  • May have a small contribution from the plantaris muscle 3

• Dimensions and Shape:

  • Narrowest approximately 5-7 cm above its calcaneal insertion, expanding again distally 1
  • The tendon fibers undergo a spiral rotation of approximately 90° from proximal to distal 1
  • This spiralization creates an area of concentrated stress but also confers a mechanical advantage 2

Insertion

• Calcaneal Attachment:

  • Crescent-shaped insertion with significant medial and lateral projections 2
  • Anterior (deep) fibers insert into the middle third of the posterior aspect of the calcaneal tuberosity 1
  • Posterior (superficial) fibers pass over the calcaneal tuberosity and fuse with the plantar aponeurosis 1
  • The specialized insertion is designed to aid in dissipation of stress from tendon to calcaneum 2

• Associated Structures:

  • Deep calcaneal bursa located between the calcaneal tuberosity and the anterior surface of the Achilles tendon 1
  • No synovial sheath but covered with sliding connective tissue (paratenon) along most of its length, except on its anterior surface 1

Vascular Supply

• Blood Supply:
  • Derives from three main sources: musculotendinous junction, vessels in surrounding connective tissue, and osteotendinous junction 2
  • Can be classified into three vascular territories:
    1. Proximal section: supplied by posterior tibial artery
    2. Midsection: supplied by peroneal artery
    3. Distal section: supplied by posterior tibial artery 2
  • A relatively hypovascular area exists in the mid-portion of the tendon, which is where most pathological problems occur 2

Innervation

• Nerve Supply:
  • Primary innervation from the sural nerve 1
  • Secondary innervation from the tibial nerve 2
  • The sural nerve crosses the Achilles tendon approximately 11 cm proximal to the calcaneal tuberosity 1
  • Motor innervation of the triceps surae muscle (gastrocnemius and soleus) is provided by fibers of the tibial nerve 1

Histology and Biomechanics

• Cellular Composition:

  • Tenocytes produce type I collagen, forming 90% of the cellular component of normal tendon 2
  • Pathological or ruptured tendons produce more type III collagen, which may affect tensile strength 2

• Mechanical Properties:

  • Can withstand forces up to 12.5 times body weight during activities like running 1, 2
  • Physiological stretching of collagen fibers ranges between 2-4% of tendon length 1
  • Stretching beyond 4% results in microscopic failure, while stretching beyond 8% causes macroscopic failure 1

Clinical Relevance

• Common Pathologies:

  • Most common tendon to rupture in the human body 1, 2
  • Insertional Achilles tendonitis presents with chronic posterior heel pain and swelling, aggravated by activity and shoe pressure 4
  • Bursitis associated with Haglund's deformity commonly presents with tenderness lateral to the Achilles tendon 4

• Diagnostic Considerations:

  • Radiographs may show spurring or erosion at the Achilles tendon insertion in cases of insertional tendonitis 4
  • MRI provides comprehensive evaluation of tendon structure and associated pathologies 5

Anatomical Pitfalls and Considerations

• The mid-portion's relative hypovascularity predisposes it to pathology and slower healing 2
• The complex spiral arrangement of fibers creates areas of concentrated stress that may be vulnerable to injury 1, 2
• Local corticosteroid injections are not recommended for insertional Achilles tendonitis due to risk of tendon weakening 4
• The specialized calcaneal insertion requires careful consideration during surgical approaches to avoid disruption of its biomechanical function 2