A flexor retinaculum is a ligament, or a piece of tough, fibrous tissue in the body. There are four flexor retinacula in the human body, one in each wrist, and one in each ankle. In the wrist, they are also known as transverse carpel ligaments, and in the ankle they are also known as laciniate ligaments or internal annular ligaments. The function of ligaments is to connect and support bones and structures, maintaining correct anatomical positioning.
In the wrist, the flexor retinaculum stretches across the carpal bones, creating the carpal tunnel. The median nerve passes through the carpal tunnel from the forearm into the hand. The flexor tendons of the fingers also pass through the carpal tunnel. If this ligament sustains an injury, it may swell, stretch too tightly, or tear, allowing the carpel bones to move incorrectly. These actions may compress the median nerve, which passes through the tunnel, causing pain, numbness and lack of mobility in the hand.
Carpal tunnel syndrome is a condition generally caused by swelling of the membrane that protects the tendons as they pass through the tunnel. As the membrane swells, the median nerve is compressed between the carpal bones and the flexor retinaculum. One of the surgical options for treating carpal tunnel syndrome involves making an incision in the flexor retinaculum to relieve the compression of the median nerve.
The flexor retinaculum has a similar function in the ankle as in the wrist. It stretches across a number of bones in the ankle, creating a narrow passage or tunnel known as the tarsal tunnel. The function of the flexor retinaculum in the ankle is to maintain, protect and support the tarsal tunnel and the structures that pass through it. The tibial nerve passes through the tarsal tunnel, along with veins, arteries and tendons, on the way into the foot.
If the tibial nerve is compressed inside the tarsal tunnel, a condition called tarsal tunnel syndrome can occur. Tarsal tunnel syndrome is similar to carpal tunnel syndrome in that both conditions involve symptoms arising from a nerve that is compressed in a confined space, but tarsal tunnel syndrome is less common than carpal tunnel syndrome. The flexor retinaculum is unable to stretch very far, so anything that takes up additional space within the tarsal tunnel may contribute to tarsal tunnel syndrome. Varicose veins, inflammation of surrounding tissues, tumors or cysts within the tunnel may bring on symptoms including pain, numbness and muscle weakness.
People who have fallen arches, are flat-footed, or walk on the outer edge of the foot are at greater risk of developing tarsal tunnel syndrome. This is because these conditions place greater strain on the flexor retinaculum, as well of many of the structures it protects, and this can result in irritation and inflammation that may cause the tibial nerve to become compressed.