The Achilles tendon is located in the posterior ankle and is the strongest as well as the thickest tendon in the body. It is formed by the Soleus and Gastrocnemuis muscles. The Achilles tendon is inserted into the calcaneus bone and the subcutaneous calcaneal bursa is found superficial to the Achilles tendon. The bursa lies between the skin and the distal aspect of the Achilles tendon. There is also the subtendinous calcaneal bursa located deep to the Achilles tendon, between the achilles tendon and the calcaneous. Inflammation of one or both of these bursa can cause pain in the posterior heel or ankle.
The Achilles tendon is prone to tears and ruptures. Most Achilles tendon ruptures occur above the calcaneal insertion of the tendon. Like all tendons, the Achilles tendon does not have great vascularity. The Watershed zone, is the area of the Achilles tendon with the worst blood supply. This is a very narrow area in width; between 2-6 cm proximal to the calcaneus. The Achilles tendon is also susceptible to tendonitis or tendon ruptures within this watershed zone due to limited blood supply. When the Achilles tendon ruptures within this narrow area, the result may be similar to pulling apart a rope. This injury is sometimes referred to as “The Weekend Warrior’s Injury”. This injury typically occurs due to over performing or overdoing a physical activity. This injury also more commonly occurs in men, between the ages of 30-40 year.
The mechanism of injury for a rupture of the Achilles is usually from eccentric loading on a dorsiflexed ankle with knee extension. If the strain placed on the tendon is less than 4%, it is considered to be within the normal limits of the physiological load and the fibers return to the original configuration after the load is removed. Strains between 4-8% may lead to microscopic failure. Strain beyond 8% will result in a macroscopic failure and a rupture of the Achilles tendon. The Thompson test is performed in order to confirm an Achilles tendon tear diagnosis.
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