The Achilles or calcaneal tendon is the strongest and largest tendon of the human body. It is also one of the most common tendons to become injured due to its high biomechanical load but poor vascularity ².

Gross anatomy

The Achilles tendon forms by merging fibers of the gastrocnemius and soleus muscles, forming the tendon that inserts into the posterosuperior aspect of the calcaneus. A more precise description of the Achilles tendon allows us to recognize an internal structure; in fact, it is composed of three fascicles: the fascicles of the lateral and medial head of the gastrocnemius muscle and the fascicle of the soleus muscle ⁶.

The proximal fibers of the Achilles tendon have a rounded appearance that becomes relatively flat about four centimeters proximal to the insertion site. This alignment gives the tendon its spiral pattern, which helps with locomotion.

All the tendons that cross the ankle are enclosed within their synovial sheath except for the Achilles tendon, which has no covering surrounding synovial sheath but a posterior paratenon (so there is no calcaneal synovitis, but rather tendinitis or peritendinitis instead ¹).

Relations

Around the distal Achilles tendon are two similarly named bursae:

• subcutaneous calcaneal bursa: located between the Achilles tendon and the skin

• retrocalcaneal bursa: located between the Achilles tendon and the posterior calcaneus

Function

The Achilles tendon enables the person to rise up on their toes and push forward during walking or running, to jump and land, as well as to climb and descend stairs ².

Related pathology

• Achilles tendinopathy: clinical syndrome of pain, swelling, and impaired function; can occur in the mid-portion or insertional fibers ⁵

• Achilles paratendinopathy: acute or chronic inflammation in the structures surrounding the tendon ⁵

• Achilles tendinosis: gradual thickening of the tendon without visible inflammatory process; even though the tendon is thick, it is weak and more vulnerable to tear

• Achilles tendon tear: may be microtears or large, either complete (i.e. tendon rupture) or incomplete/parital

• Achilles tendon ossification

History and etymology 

The Achilles tendon is named after on the mythical Ancient Greek warrior Achilles, the hero of the Trojan War, whose whole body was invulnerable to all injuries, except for his heel. 

The oldest written record for the term Achilles being used to describe the Achilles tendon was in 1693 by Flemish/Dutch anatomist Philip Verheyen (1648-1710) in his text Corporis Humani Anatomia ⁴.