The Achilles tendon, or the heel cord, also known as the calcaneal tendon, is a tendon at the back of the lower leg, and is the thickest in the human body. It serves to attach the plantaris, gastrocnemius (calf) and soleus muscles to the calcaneus (heel) bone. These muscles, acting via the tendon, cause plantar flexion of the foot at the ankle joint, and (except the soleus) flexion at the knee.

Abnormalities of the Achilles tendon include inflammation (Achilles tendinitis), degeneration, rupture, and becoming embedded with cholesterol deposits (xanthomas).

The Achilles tendon was named in 1693 after the Greek hero Achilles.

History

The oldest known record of the tendon being named after Achilles was written in 1693 by the Flemish anatomist Philip Verheyen. In his widely used text he described the tendon's location and said that it was commonly called "the cord of Achilles." The tendon has been described as early as the time of Hippocrates, who described it as the "" (Latin for "great tendon") and by subsequent anatomists prior to Verheyen as "" (Latin for "Hippocrates' string"). The tendon begins near the middle of the calf, and receives muscle fibers on its inner surface, particularly from the soleus muscle, almost to its lower end. Gradually thinning below, it inserts into the middle part of the back of the calcaneus bone. The tendon spreads out somewhat at its lower end so that its narrowest part is about above its insertion. The tendon is the thickest tendon in the human body. It can receive a load stress 3.9 times body weight during walking and 7.7 times body weight when running.

The blood supply to the Achilles tendon is poor, and mostly via a recurrent branch of the posterior tibial artery, and some through arterial branches passing through surrounding muscles. Because the fibres of the tendon spiral about 90 degrees, fibres from the gastrocnemius tend to attach to the outer part of the bone, whereas fibres from the soleus tend to attach closer to the midline. Vibration of the tendon causes movement backwards and the illusion of a forward body tilt in standing subjects. This is because vibrations stimulate muscle spindles in the calf muscles. The muscle spindles alert the brain that the body is moving forward, so the central nervous system compensates by moving the body backwards.

Clinical significance

Inflammation

Inflammation of the Achilles tendon is called Achilles tendinitis. Achilles tendinosis is the soreness or stiffness of the tendon, particularly worse when exercising, and generally due to overuse. The most common symptoms are sharp achy pain and swelling around the affected tendon. The pain is typically worse at the start of exercise and decreases thereafter. Other risk factors include trauma, a lifestyle that includes little exercise, high-heel shoes, rheumatoid arthritis, and medications of the fluoroquinolone or steroid class. Treatment typically involves rest, ice, non-steroidal antiinflammatory agents (NSAIDs), and physical therapy.

Rupture

Achilles tendon rupture is when the Achilles tendon breaks. A snapping sound may be heard as the tendon breaks and walking becomes difficult.

Rupture typically occurs as a result of a sudden bending up of the foot when the calf muscle is engaged, direct trauma, or long-standing tendonitis. Males are more commonly affected than females. The risk of re-rupture is about 25% with casting.

Xanthomas

Tendon xanthomas are cholesterol deposits that commonly develop in the Achilles tendon of people with lipid metabolism disorders such as familial hypercholesterolemia.

Neurological exam

The Achilles tendon is often tested as part of a neurological examination. In this examination, the tendon is hit with a tendon hammer. This tests the S1 and S2 spinal nerves: a normal response is plantar flexion (downward movement) of the foot.

Level or portion of tendon affected:

  • Paratendinopathy: The inflammation of a connective tissue sleeve which surrounds the tendon and protects it from friction, irritation, and repeated trauma
  • Insertional: Eminently overuse-injury which frequently occurs in running and jumping athletes. Patients affected by insertional Achilles tendinopathy complain of pain on the posterior aspect of the heel and may have morning stiffness, swelling with activity and tenderness at the tendon insertion level. If this condition becomes chronic, calcific deposits at the Achilles insertional level may be developed (due to microfractures and healing of the osteotendinous union) which can degenerate, if it persists over time, in the abnormal bony prominence on the posterior aspect of heel, condition known as Haglund deformity, which can be painful and difficult close-shoes fitting due to friction and irritation.
  • Mid-portion: Occurs approximately 2–7 cm proximal from the Achilles insertion into the calcaneus. Characterized by a combination of pain and swelling at this level. It has associated a remarkable impaired performance.

Other animals

The Achilles tendon is short or absent in great apes, but long in arboreal gibbons and humans. It provides elastic energy storage in hopping, walking, and running.