Synovial sheaths of digits of hand

The synovial sheaths of digits of hand are thin, double-layered tubular membranes that surround and lubricate the long flexor tendons as they pass through the fibrous flexor sheaths on the palmar side of the fingers. These include the tendons of flexor digitorum superficialis (FDS), flexor digitorum profundus (FDP), and flexor pollicis longus (FPL) muscles.

Structure

Each synovial sheath consists of two layers:

• A visceral layer, which is closely applied to the tendon surface.

• A parietal layer, which lines the inner surface of the fibrous sheath. A thin film of synovial fluid between these layers allows the tendon to glide smoothly with minimal friction.

Distribution

• The second, third, and fourth digits each have independent digital synovial sheaths, extending from just proximal to the metacarpophalangeal (MCP) joints to the distal phalanges.

• The little finger and thumb synovial sheaths are continuous with the palmar bursae:

  • Thumb: Continuous with the radial bursa, enclosing the flexor pollicis longus (FPL) tendon.

  • Little finger: Continuous with the ulnar bursa, enclosing the FDS and FDP tendons of that digit.

Function

The synovial sheaths of digits of hand reduce friction, protect the flexor tendons, and enable smooth, efficient gliding during flexion and extension movements of fingers of the hands.