Calcaneal tendon, or the Achilles tendon is the strongest and thickest tendon of the human musculoskeletal system. It is the common tendon of the two constituting muscles of the triceps surae; gastrocnemius and soleus, attaching them to the posterior surface of calcaneus bone. The calcaneal tendon has a couple of important functions; it enables the superficial posterior leg muscles to plantarflex the foot and stabilizes the ankle joint during the gait cycle.
There is a fun fact regarding how the calcaneal tendon got its eponym; the Achilles tendon. The story is related to the Greek mythology hero, Achilles. According to the myth, the entire Achilles’ body was bulletproof, except for the calcaneal tendon.
He eventually died in the Trojan war as an arrow pierced his calcaneal tendon. Based on this myth, the Achilles tendon got its name and is used in common language to describe a person’s weakest spot.
|Definition||Common tendon of superficial posterior leg muscles; gastrocnemius, soleus and plantaris|
|Function||Plantar flexion of the foot, ankle joint stabilizer|
|Clinical relations||Thompson’s test, Achilles tendon rupture|
This article will discuss the anatomy and function of the Achilles tendon.
The calcaneal tendon originates as a wide aponeurotic sheath from the distal end of the gastrocnemius muscle. It then courses downwards, gradually rounding up in shape. The tendon is joined by the soleus muscle fibers about 4 centimeters above the ankle joint. Finally, the tendon passes over the ankle joint and inserts onto the posterior surface of calcaneus.
The tendon fibers do not run strictly vertically as they descend, but rather they spiral laterally up to 90 degrees once the soleus fibers join the tendon. This results in the soleus fibers inserting medially and the gastrocnemius inserting laterally on the attachment surface of the calcaneus. Sometimes, the tendon of the plantaris muscle fuses with the calcaneal tendon, while in other cases, it inserts separately onto the plantar aponeurosis.
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The Achilles tendon is innervated by the sural nerve. Note that the sural nerve crosses the upper half of the tendon’s lateral border, which is a common spot of the nerve’s injury during surgical procedures. Vascular supply to the tendon comes from two sources; proximal and distal thirds are supplied by the posterior tibial artery, while the fibular (peroneal) artery supplies the middle third of the tendon.
The main Achilles tendon function is to enable the triceps surae muscles to plantarflex the foot.
Besides this, the tendon is crucial for normal biomechanics of the ankle joint. The spiralling of the fibers just before their insertion creates an area of concentrated stress conveyed to the calcaneus. This design of the tendon’s insertion enables adequate force transmission to the foot during walking, running and jumping. In young individuals, the tendon can bear up to 12 times its body weight during running, while in the elderly the level of endurance drops. This is one of the reasons why Achilles tendon tears are more common in the elderly population.
Learn more about the Achilles tendon anatomy and function with our 3D video tutorials.
Achilles tendon rupture
Although the strongest tendon in the body, the Achilles tendon is the one that ruptures the most in the lower extremity. Most common reasons for the tear are the excessive and sudden plantar flexion, trauma and other tendon pathologies (e.g. tendinopathy).
The symptoms onset immediately; they start with an audible snap at the site of tearing, being followed by the pain which patients describe as being kicked in the leg. Depending on the severity, the injury is classified into four degrees or types; type I refers to partial tear, while the types II-IV are complete tears that differ in degree of retraction. Type of injury defines the treatment;
- Type I is treated conservatively with immobilization and non-weight-bearing activities for at least four weeks after the injury;
- Types II-IV require surgical treatment, such as open repair, percutaneous repair, and augmentative repair. After surgery, the patient is prescribed with conservative treatment described above.
After healing, the scar tissue decreases the tensile strength of the tendon.
Thompsons (Simmonds’) test is a clinical examination procedure for discovering the signs of an Achilles tendon rupture. The steps for this Achilles tendon rupture test are the following;
- Patient lies in the prone position or kneels on the chair with their leg muscles relaxed. In both cases, their feet should be over the edge of the table or chair.
- The examiner then squeezes the calf muscles.
Normal response to the test is the plantar flexion of the foot. The absence of plantar flexion is a positive Thompson’s sign and indicates an Achilles tendon rupture.