Thumb Flexor Sheath (Pulley) Anatomy

The thumb has two annular pulleys and a thinner oblique pulley. These pulleys provide the fibrous component of the flexor tendon’s fibro-osseous tunnel.
The functions of the pulleys include providing a mechanical advantage which allows for a small shortening, i.e. excursion, to cause significant rotational motion and torque at the joints. The pulley mechanism also limits bowstringing of the flexor tendon and enhances selective motion of individual joints of the digit. Finally, the flexor sheath (pulleys) produces synovial fluid to facilitate smooth gliding of the flexor tendon (FPL).

Diagrams & Photos

Note thumb A1 , Oblique, and A2 pulleys. Also observe the radial digital artery goes dorsal to the FPL.
The thumb flexor sheath connects to the radial bursa.
A-1 pulley of the thumb flexor sheath (yellow ellipse). Also note the FPL under the pulley and tip of the scissors. Digital nerves and flexor pollicis brevis are also labeled.
Oblique pulley of the thumb flexor sheath (yellow ellipse).
A-2 pulley of the thumb flexor sheath (yellow ellipse).

Key Points

The annular pulleys have three layers: 1) the inner layer which secrets hyaluronic acid; 2) the middle structural layer which contains abundant collagen; 3) the outer vascular layer.
The thumb flexor sheath can become infected.
The thumb infection can spread to the radial bursa, the connected ulnar bursa, and finally to the little finger flexor sheath. The result can be a "horseshoe" abscess of the hand.