The twisted structure of the Achilles tendon unraveled: A detailed quantitative and qualitative anatomical investigation

Sports participation has undergone an increase in recent decades. Injury due to sporting activity has also recently risen. The Achilles tendon has been one of the most common sports-related injuries. A 2 in 100,000 individual Achilles tendon injury rate increased to a 12 in 100,000 individual injury rate in less than 10 years. The injury is typically observed in men in the fourth to fifth decades of life. Male to female injury ratios range from 2:1 to 12:1. Running, jumping, and agility activities involving eccentric loading and explosive plyometric contractions are usual mechanisms. Natural aging allows predisposing chronic degeneration of the tendon. Blood flow decreases and stiffness increases with aging to decrease the ability to withstand stress. Noninflammatory tendinosis and chronic tendinopathy are 2 separate processes proposed for tendon degeneration and subsequent rupture. Rupture typically occurs 2 to 6 cm proximal to the calcaneal insertion. Predisposing factors are grouped into 2 categories: intrinsic and extrinsic risk factors. Avoidance of degenerative changes within the tendon is the primary method to prevent rupture. Regular physical activity as athletes age also promotes tendon hypertrophy, increases nutrient delivery, and reduces collagen fiber fatigue.

We determined the incidence of a total Achilles tendon rupture in the city of Oulu and changes over the 16-year period 1979-1994. During this time, 110 ruptures occurred. The incidence increased from 2 ruptures/10(5) inhabitants in 1979-1986 to 12 in 1987-1994, with a mean of 7. The peak annual incidence, 18, was recorded in 1994. The incidence was highest in the age group 30-39 years. Male dominance was 5.5:1, and 81% of the ruptures were related to sports, with 88% occurring in ball games. The mean age was significantly lower for the sports injuries.

The Achilles tendon is the strongest and thickest tendon in the human body. It is also the commonest tendon to rupture. It begins near the middle of the calf and is the conjoint tendon of the gastrocnemius and soleus muscles. The relative contribution of the two muscles to the tendon varies. Spiralisation of the fibres of the tendon produces an area of concentrated stress and confers a mechanical advantage. The calcaneal insertion is specialised and designed to aid the dissipation of stress from the tendon to the calcaneum. The insertion is crescent shaped and has significant medial and lateral projections. The blood supply of the tendon is from the musculotendinous junction, vessels in surrounding connective tissue and the osteotendinous junction. The vascular territories can be classified simply in three, with the midsection supplied by the peroneal artery, and the proximal and distal sections supplied by the posterior tibial artery. This leaves a relatively hypovascular area in the mid-portion of the tendon where most problems occur. The Achilles tendon derives its innervation from the sural nerve with a smaller supply from the tibial nerve. Tenocytes produce type I collagen and form 90% of the cellular component of the normal tendon. Evidence suggests ruptured or pathological tendon produce more type III collagen, which may affect the tensile strength of the tendon. Direct measurements of forces reveal loading in the Achilles tendon as high as 9 KN during running, which is up to 12.5 times body weight.