Carpal Tendinous Sheaths
The sheaths around the long flexor and extensor tendons of the wrist and hand are essential for their smooth functioning beneath the flexor retinaculum and fibrous flexor sheaths with minimum friction. There are:
- Palmar carpal tendinous sheaths
- Dorsal carpal tendinous sheaths
- Palmar digital tendinous sheaths
This article will discuss the anatomy of these tendinous sheaths along with their function and clinical relevance.
Palmar Carpal Tendinous Sheaths
Flexor Carpi Radialis Tendon Sheath
The flexor retinaculum forms the roof of the carpal tunnel, through which the median nerve and the long finger flexors tendons of finger and thumb run. The tendons run in three palmar synovial tendon sheaths. On the radial side, the flexor carpi radialis (FCR) tendon runs in the synovial tendon sheath in the groove of trapezium bone, consequently dividing the radial attachment of the flexor retinaculum into two parts. The function of the muscle tendon is flexion and abduction of the wrist. It lies superficially in the wrist along palmaris longus (a vestigial muscle that inserts into the palmar aponeurosis).
In this region of the wrist, the radial artery lies radial to the median nerve. The radial artery passes under the tendons of the long thumb muscles, and the median nerve passes into the hand through the carpal tunnel. Immediately adjacent and deep to FCR in the carpal tunnel is the flexor pollicis longus (FPL) tendon synovial sheath. The FPL tendon flexes the distal phalanx of the thumb. This tendon sheath is sometimes referred to as the radial bursa.
The flexor digitorum superficialis and flexor digitorum profundus run together within a common synovial sheath of the flexor muscles, which is incomplete on the radial side. The muscles are responsible for flexing the proximal and distal interphalangeal joints respectively, as well as the wrist. The common flexor synovial sheath contains the tendons of flexor digitorum superficialis and flexor digitorum profundus. It is also known as the ulnar bursa, as it lies on the ulnar side of the palm. With the little finger, the ulnar bursa only extends from just proximal to the first phalanx to a few centimeters proximal to the wrist. The flexor tendon sheaths of the remaining three fingers are separate. The radial bursa extends for the entire length of the flexor pollicis longus tendon and ends just proximal to the flexor retinaculum.
The radial & ulnar bursa communicate at the level of the wrist joint in almost 50% of individuals.
Dorsal Carpal Tendinous Sheaths
The dorsal synovial sheaths lie in six tendon compartments, which are formed by the extensor retinaculum and the fascial septa that arise from the inferior surface of the extensor retinaculum. The septa attach to the bony ridges on the ulna and radius.
In the first compartment (counted from radial to ulnar side) are the tendons of extensor pollicis brevis and abductor pollicis longus. These two tendons form the anterior border of the anatomical snuffbox and lie in the separate synovial sheaths. An easy way to learn the tendons of the anatomical snuffbox is to extend your thumbs fully. The most obvious tendon that runs in the same direction as your thumb is the extensor pollicis longus.
The snuffbox is bordered radially by the extensor pollicis brevis, i.e. the two extensor tendons are adjacent. The other tendon that attaches (just medial to the extensor pollicus brevis) is that of abductor pollicis longus i.e. the longus muscles lie either side of the snuffbox.
The second compartment contains the separate tendon sheaths of extensor carpi radialis longus and brevis. These muscles extend the wrist, and are known as the ‘punching muscles.’ This is because the wrist is slightly extended in preparation for throwing a punch.
The third compartment is an oblique canal that contains the extensor pollicis longus tendon. This muscle extends the distal phalanx of the thumb.
The fourth compartment lies between the groove and the ulnar border of radius. It contains the extensor indicis along with the four extensor digitorum tendons. All five tendons are within a common synovial sheath. The index finger is one of the two fingers that has its own extensor tendon in addition to its slip from the extensor digitorum.
The fifth compartment lies over the radioulnar joint and contains extensor digiti minimi tendon in its sheath. The index and little finger have their own extensor tendons in addition to a slip from the extensor digitorum.
The sixth and final compartment contains the tendon sheath of the extensor carpi ulnaris muscle in a groove near the base of the ulnar styloid. This muscle causes extension and adduction of the wrist.
Palmar Digital Tendinous Sheaths
There are five synovial sheaths of digits in the hand. They are surrounded by fibrous flexor sheaths consisting of annular and cruciate fibers, often clinically referred to as the pulleys. These include 5 annular pulleys (A1-A5, fibers run horizontally) and 3 cruciate pulleys (C1-C3, cross shaped). They alternate, apart from between A1 and A2, where there is no cruciate pulley. In the thumb you only have the flexor pollicis longus tendon, whereas in the fingers you have a deep and superficial tendon. The sheaths keep the tendon close to the bone and therefore prevent bowstringing. They also lubricate the smooth passage of the tendon on movement.
Between the parietal and visceral layers of the synovial sheath is a mesotendon which contains blood vessels and nerves. The mesotendon in the region of the digital tendon sheath is called vinculum longum and vinculum breve.