Achilles tendon

Did You Know?

• The Achilles tendon is short or absent in great apes but long in arboreal gibbons and humans.
• Computer models suggest the tendon's energy storage increases top running speed by over 80% and reduces running costs by more than 75%.
• The absence of a well-developed Achilles tendon in nonhuman African apes would preclude them from effective running at high speeds or over extended distances.

History

The earliest known record of the tendon being called after Achilles appears in a 1693 work by Flemish anatomist Philip Verheyen. In his text Corporis Humani Anatomia, he noted the tendon's location and referred to it as "the cord of Achilles." Earlier anatomists, including Hippocrates, described it as the "tendo magnus" (great tendon) or "chorda Hippocratis" (Hippocrates' string). Verheyen linked the name to the myth of Achilles, whose mother Thetis held him by the heel while dipping him in the River Styx, leaving that spot vulnerable. This vulnerability led to his death by a poison dart to the heel, and the name also reflects the disabling effect of injuring this tendon. The first closed rupture was documented by Ambroise Pare in the sixteenth century. The tendon is also called the "tendo calcaneus," a descriptive term from the Latin calcaneum, meaning heel.

Structure

The Achilles tendon connects the gastrocnemius and soleus muscles to the calcaneal tuberosity on the heel bone. It begins near the middle of the calf, receiving muscle fibers from the soleus almost to its lower end, and gradually thins before inserting into the middle part of the back of the calcaneus. Its narrowest point is about 4 cm above the insertion. The tendon is covered by fascia and skin, with areolar and adipose tissue filling the gap behind the bone. A bursa, called the Achilles bursa, lies between the tendon and the upper calcaneus. The tendon is about 15 cm long. Along its side runs the small saphenous vein, and the sural nerve travels inferolateral to it as it crosses the lateral border. Blood supply is poor, coming mainly from a recurrent branch of the posterior tibial artery and some arterial branches through surrounding muscles.

Function

The gastrocnemius and soleus muscles work through the Achilles tendon to point the foot downward, pulling the sole toward the back of the leg. The gastrocnemius also bends the knee. Both muscles receive signals from the tibial nerve. The tendon’s fibers twist about 90 degrees, so those from the gastrocnemius connect to the bone’s outer side, while soleus fibers attach nearer the midline. Vibrating the tendon without sight makes a standing person sway backward and feel as if they are leaning forward. This happens because the vibration triggers muscle spindles in the calf, which tell the brain the body is moving forward, prompting a backward correction.

Clinical significance

Inflammation of the Achilles tendon is called Achilles tendinitis, while tendinosis refers to soreness or stiffness, typically worsening with exercise and due to overuse. Common symptoms include sharp achy pain and swelling around the tendon, often worse at the start of exercise and decreasing afterward, with possible ankle stiffness. Onset is generally gradual. Risk factors include overuse from running, trauma, a sedentary lifestyle, high-heel shoes, rheumatoid arthritis, and medications such as fluoroquinolones or steroids. Diagnosis is based on symptoms and examination. Prevention through stretching and strengthening exercises has poor evidence. Treatment involves rest, ice, NSAIDs, and physical therapy; heel lifts or orthotics may help. Surgery is considered if symptoms persist over six months. Tendinosis is investigated with MRI or ultrasound, showing a thickened tendon, possible inflammation, paratenonitis, bursitis, increased blood flow, fibril disorganization, and partial tears. It often affects the mid portion but may involve the insertion (enthesopathy), associated with aging or arthritis like gout. Tendinosis is a risk factor for calf muscle tears. Achilles tendon rupture causes sudden sharp heel pain, a snapping sound, and difficulty walking.