Palmar Bursae and Tendon Sheaths

Palmar Bursae and Tendon Sheaths

The term ‘tendon sheath’ is broadly used to describe histologically distinct tissues. The classic two-layered tendon sheath is only found in certain tendons in locations of mechanical stress. The two layers consist of an outer fibrous sheath and an inner synovial sheath. The synovial sheath is composed of two sheets of synovial cells-a visceral sheet applied to the surface of the tendon and a parietal sheet applied to the inner aspect of the surrounding fibrous sheath-separated by a capillary layer of synovial fluid.

Synovial tendon sheaths and bursae are fluid-filled sacs lined by a synovial membrane, acting to decrease friction between adjacent structures, allowing smooth, gliding movement. They are located throughout the body in locations where tendons are closely apposed to bones and rigid fibrous structures. The flexor tendons of the carpal tunnel are enveloped in two palmar bursae: the ulnar bursa and the radial bursa. The ulnar and radial bursae begin proximal to the carpal tunnel, usually near the distal margin of the pronator quadratus muscle, and they extend distally to the level of the mid-palm, allowing for longitudinal excursion of the tendons during normal wrist movement. These bursae extend over a longer distance than the extensor tendon sheaths, a difference likely related to the greater range of wrist motion that takes place in flexion compared with extension.

The nine flexor tendons traversing the carpal tunnel consist of the flexor pollicis longus (FPL) tendon to the thumb along with four pairs of flexor digitorum tendons serving the index (FD2), long (FD3), ring (FD4), and little fingers (FD5), each pair consisting of a superficialis tendon and a profundus tendon. (Although the superficialis and profundus tendons originate from separate superficialis and profundus muscles in the forearm, the abbreviation ‘FD’ will be used for brevity to refer to both tendons as a pair, unless otherwise specified). The flexor digitorum superficialis and profundus tendons-inserting respectively on the middle and distal phalanges-are located at the ulnar aspect of the canal and arranged in two rows, the profundus tendons dorsal to the superficialis tendons. On axial images through the carpal tunnel, the four superficialis tendons often show a U-shaped configuration with the superficialis tendons of FD3 and FD4 volar to the superficialis tendons of FD2 and FD5 (5a). The median nerve occupies a superficial position within the carpal tunnel, located just deep to the flexor retinaculum.

Illustration depicting the relationship of the palmar bursae to the flexor tendons of the carpal tunnel.

The smaller radial bursa surrounds the flexor pollicis longus tendon. The larger ulnar bursa surrounds the eight flexor digitorum tendons, doing so by means of three invaginations: a deep invagination between the carpal bones and the profundus tendons, a middle invagination between the profundus and superficialis tendons, and a superficial invagination between the superficialis tendons and the flexor retinaculum5. These invaginations arise from a common space at the ulnar side of the tendons adjacent to the FD5 tendon (7a). Although there are several anatomic variations in the communication patterns between the ulnar bursa, the radial bursa, and the digital flexor tendon sheaths6, a typical anatomic arrangement found in the majority of persons has been established through anatomic dissection as well as tenography in cadavers.

A fat-suppressed T1-weighted axial image from an MR arthrogram of the wrist in a healthy 24-year-old man. The transverse carpal ligament (short arrows) extends from the hook of the hamate (long arrow) to the tubercle of the trapezium (arrowhead), forming the floor of the carpal tunnel. At the radial aspect of the carpal tunnel, the flexor digitorum tendons are arranged in two rows (separated by dotted line), the profundus tendons deep to the superficialis tendons. The flexor pollicis longus tendon (star) is positioned at the radial aspect of the tunnel, separated from the flexor carpi radialis tendon (curved arrow) by a ligamentous reflection of the transverse carpal ligament. The median nerve is indicated (asterisk).

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