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Author: Brian A Jacobs, MD, FACSM; Chief Editor: Sherwin SW Ho, MD more...
Achilles tendon ruptures commonly occur to otherwise healthy men between the ages of 30 and 50 years who have had no previous injury or problem reported in the affected leg. Those who suffer this injury are typically "weekend warriors" who are active intermittently.
Most Achilles tendon tears occur in the left leg in the substance of the tendoachilles, approximately 2-6 cm – the "watershed zone" – above the calcaneal insertion of the tendon. That the left Achilles tendon is torn more frequently may be related to handedness; right-handed individuals "push off" more frequently with the left foot.
The most common mechanisms of injury include sudden forced plantar flexion of the foot, unexpected dorsiflexion of the foot, and violent dorsiflexion of a plantar flexed foot. Other mechanisms include direct trauma and, less frequently, attrition of the tendon as a result of longstanding peritenonitis with or without tendinosis. [1, 2, 3]
Other populations at risk for an Achilles tendon rupture include those who are poorly conditioned, those of advanced age, those who have been using fluoroquinolone antibiotics, those who have used corticosteroids, and those who overexert themselves.
Besides Achilles tendon rupture, which the remainder of this article will focus on, there is also a spectrum of Achilles tendon injuries, including peritenonitis, tendinosis, and peritenonitis with tendinosis. [1, 2, 3]
Patients with peritenonitis will usually note a localized burning pain that accompanies or follows activity and tracks along the tendoachilles.
Peritenonitis with tendinosis will generally present with activity-related pain, swelling, and sometimes crepitation along the tendon sheath, with or without the presence of nodularity. More severe symptoms may include pain at rest.
Tendinosis is a late-stage manifestation of this problem, characterized by mucoid degeneration of the tendoachilles itself, with a lack of inflammatory response and symptoms characterized by a sense of fullness or nodularity in the posterior aspect of the tendoachilles.
Although the worldwide frequency of Achilles tendon ruptures is not known, data collected from Finland estimates that it occurs in 18 per 100,000 people yearly. The male-to-female ratio of rupture is estimated from 1.7:1 to 12:1.
The Achilles tendon is the largest and strongest tendon in the human body, and it is formed from tendinous contributions of the gastrocnemius and soleus muscles. The tendons converge approximately 15 cm proximal to the insertion at the posterior calcaneus. When viewed in cross section, the right Achilles tendon appears to spiral counterclockwise 30-150 º toward its insertion at the calcaneus; the left Achilles tendon spirals clockwise analogously.
The spiraling of the tendon as it reaches the calcaneus allows for elongation and elastic recoil within the tendon, facilitating storage and release of energy during locomotion. This phenomenon also allows higher shortening velocities and greater instantaneous muscle power than could be generated by the gastrocsoleus complex alone.
Because actin and myosin are present in tenocytes, tendons have almost ideal mechanical properties for the transmission of force from muscle to bone. Tendons are stiff but resilient, possess a high tensile strength, and have the ability to stretch up to 4% before damage occurs. [4, 5] With stretch greater than 8%, macroscopic rupture occurs.
The blood supply for the Achilles tendon is derived from the posterior tibial artery and its contributions to the musculotendinous junction, as well as the mesosternal vessels which cross the paratenon, infiltrating the tendon and the bone-tendon junction at the calcaneus.  The watershed zone is an area 2-6 cm proximal to the calcaneus, in which the blood supply is less abundant and becomes even sparser with age. It is in this region that most degeneration and therefore rupture of the Achilles tendon occurs. Because younger tendons have better blood supply, significantly higher tensile strength, and less stiffness, they tend to rupture less frequently. [4, 5]
The peak Achilles tendon force (F) and the mechanical work (W) by the calf muscles are respectively approximately 2200N and 35J in the squat jump, 1900N and 30J in the countermove jump, and 3800N and 50J when hopping.  The estimated peak load is 6-8 times the body weight during running with a tensile force of greater than 3000N. On average, Achilles tendons in women have a smaller cross-sectional area than in men. This possibly suggests that less force is generated in a woman’s Achilles tendon than the figures noted above, which may account for the lower rate of rupture in women. 
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