Achilles tendon

The Achilles tendon (or occasionally Achilles’ tendon), also known as the calcaneal tendon or the tendo calcaneus, is a tendon of the posterior leg. It serves to attach the plantaris, gastrocnemius (calf) and soleus muscles to the calcaneus (heel) bone.

Anatomy
The Achilles is the tendonous extension of 2 muscles in the lower leg: gastrocnemius and soleus. In humans, the tendon passes posterior to the ankle. It is the thickest and strongest tendon in the body. It is about 15 cm long, and begins near the middle of the calf, but receives fleshy fibers on its anterior surface, almost to its lower end. Gradually becoming contracted below, it is inserted into the middle part of the posterior surface of the calcaneus, a bursa being interposed between the tendon and the upper part of this surface. The tendon spreads out somewhat at its lower end, so that its narrowest part is about 4 cm above its insertion. It is covered by the fascia and the integument, and stands out prominently behind the bone; the gap is filled up with areolar and adipose tissue. Along its lateral side, but superficial to it, is the small saphenous vein. The Achilles' muscle reflex tests the integrity of the S1 spinal root. The tendon can receive a load stress 3.9 times body weight during walking and 7.7 times body weight when running.

Nomenclature
The oldest-known written record of the tendon being named for Achilles is in 1693 by the Flemish/Dutch anatomist Philip Verheyen. In his widely used text Corporis Humani Anatomia, Chapter XV, page 328, he described the tendon's location and said that it was commonly called "the cord of Achilles", now also called "tendo achillies" by anatomists. ("quae vulgo dicitur chorda Achillis")

The name Achilles' heel comes from Greek mythology. Achilles' mother, the goddess Thetis, received a prophecy of her son's death. Hearing this, she dipped him into the River Styx to protect his body from harm. However, she kept hold of his heel, meaning that the water did not touch this part of his body and it was therefore vulnerable. During the Trojan War, Achilles was struck on his unprotected heel by a poisoned arrow shot by Paris, that killed him. In the same war, Achilles is also said to have cut behind Hector's Achilles tendons,having killed him,and threaded leather thongs through the incisions in order to drag him behind a chariot.

The Achilles tendon is also known as the calcaneal tendon. Because eponyms, names relating to people, have no relationship to the subject matter, most anatomical eponyms also have scientifically descriptive terms. The term calcaneal comes from the Latin calcaneum, meaning heel.

Role in disease
The most common Achilles tendon injuries are Achilles tendinosis and Achilles tendon rupture. Achilles tendinosis is the soreness or stiffness of the tendon, generally due to overuse. Achilles tendinitis (inflammation of the tendon) was thought to be the cause of most tendon pain, until the late 90s when scientists discovered no evidence of inflammation. Partial and full Achilles tendon ruptures are most likely to occur in sports requiring sudden eccentric stretching, such as sprinting. The area approximately two inches above the calcaneal attachment is most susceptible to these ruptures due to a zone of avascularity. Maffulli et al. suggested that the clinical label of tendinopathy should be given to the combination of tendon pain, swelling and impaired performance. Achilles tendon rupture is a partial or complete break in the tendon; it requires immobilization or surgery. Xanthoma can develop in the Achilles tendon in patients with familial hypercholesterolemia.

Treatment of damage
Initial treatment of damage to the tendon is generally nonoperative. Orthotics can produce early relief to the tendon by the correction of malalignments, non-steroidal anti-inflammatory drugs (NSAIDs) are generally to be avoided as they make the more-common tendinopathy (degenerative) injuries worse; though they may very occasionally be indicated for the rarer tendinitis (inflammatory) injuries. Physiotherapy by eccentric calf stretching under resistance is commonly recommended, usually in conjunction with podiatric insoles or heel cushioning. According to reports by Hakan Alfredson, M.D., and associates of clinical trials in Sweden, the pain in Achilles tendinopathy arises from the nerves associated with neovascularization and can be effectively treated with 1–4 small injections of a sclerosant. In a cross-over trial, 19 of 20 of his patients were successfully treated with this sclerotherapy.

In a case where Achilles tendon rupture is concerned, there are three main types of treatment: the open and the percutaneous operative methods, and nonoperative approaches.

Depending on the severity of the injury, recovery from an Achilles injury can take up to 12–16 months.

Imaging findings
Achilles tendon degeneration (tendinosis) is typically investigated with either MRI or ultrasound. In both cases, the tendon is thickened, may demonstrate surrounding inflammation by virtue of the presence of paratenonitis, retrocalcaneal or retroAchilles bursitis. Within the tendon, increased blood flow, tendon fibril disorganisation and partial thickness tears may be identified. Achilles tendinosis frequently involves the mid portion of the tendon, however may involve the insertion where this is known as enthesopathy. Though enthesopathy may be seen in the context of advancing age, it is also associated with arthritis such as gout and the seronegative spondyloarthitides. Achilles tendinosis is a known risk factor for calf muscle tears.

Evolution and function
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. Computer models suggest this energy storage Achilles tendon increases top running speed by >80% and reduces running costs by more than three-quarters. It has been suggested that the "absence of a well-developed Achilles tendon in the nonhuman African apes would preclude them from effective running, both at high speeds and over extended distances."

Role in postural orientation
Bilateral Achilles tendon vibration in the absence of vision has a major impact on postural orientation. Vibration applied to the Achilles tendon is well known to induce in freely standing subjects a backward body displacement and in restrained subjects an illusory forward body tilt. The 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.