Bones and muscles are connected by incredibly strong soft tissue known as tendons. These fibrous bands of collagen are subjected to tension so that skeletal muscles can move bone structures. Some tendons are covered by a membrane called a tendon sheath. This sheath helps lubricate and protect the tendon. There are three types of tendon sheath: palmar carpal, dorsal and digital.
A palmar carpal tendon sheath lubricates the hand flexor tendons that pass through the flexor retinuaculum segment of the carpal tunnel. The trapezium bone is grooved to accommodate the flexor carpi radialis's synovial sheath. Nonetheless, the failure of the body to produce adequate amounts of lubricating synovial fluid can result in inflammation and swelling in the carpal tunnel. This swelling can cramp the movement of these tendons, increasing friction, pain and inflammation in a condition known as carpal tunnel syndrome.
Tendons receive a poor blood supply, so synovial fluid also is an important source of cellular nutrition for tendons. The dorsal tendon sheath is a synovial sheath that helps nourish the tendons of the flexor pollicis longus and flexor carpi radialis. These tendon sheaths attach to bony ridges of the ulna and radius to the bottom of the retinaculum. They are found in the compartments created by the septa and extensor reticulatum. A common sheath encloses the tendons for both the flexor digitorum profundus and flexor digitorum superficialis muscles.
Synovial sheaths are secured along the length of the tendon by a series of pulleys. The peritendineum of the sheaths form the inner lining of the pulleys, which are in turn made up of a mesh of collagen fiber bundles, fibroblasts and chondrocyte-like cells. The pulleys are not attached to the free ends of the pulleys. Instead, the membrane of the sheath overlaps the pulleys to prevent it from becoming pinched when the attachment is flexed. The transverse fascicles of the palmar aponeurosis might also play a role in the securing of synovial sheaths.
Unlike dorsal and palmar carpal sheaths, digital tendon sheaths are surrounded by strong fibrous sheaths. These sheaths are comprised of both cruciate and annular fibers comprised of collagen bundles. They assist in the protection of tendons that have both nervous and vascular tissue within them, also known as mesotendons. Mesotendons derive a smaller percentage of their nutrient supply from synovial fluid, because of the presence of a strong blood supply.
