Tendon sheath of flexor pollicis longus

The tendon sheath of the flexor pollicis longus (a.k.a. the radial bursa) is a synovial sheath that surrounds the tendon of flexor pollicis longus (FPL) as it passes from the forearm, through the carpal tunnel, into the thumb.

Extent

• Proximally: Begins about 2–3 cm above the flexor retinaculum in the distal forearm.

• Distally: Extends along the entire length of the FPL tendon, reaching the distal phalanx of the thumb.

The sheath therefore runs uninterrupted from the wrist into the thumb. In some cases, the radial and ulnar bursae communicate with each other, forming a continuous synovial channel across the palm.

Parietal and visceral layers of flexor pollicis longus tendon sheath

Like all tendon sheaths, the tendon sheath of the flexor pollicis longus also has two layers:

• The parietal layer, which lines the inside of the fibrous tunnel beneath the flexor retinaculum.

• The visceral layer, which closely invests the FPL tendon, allowing it to glide freely.

Between these layers lies a thin film of synovial fluid, reducing friction during thumb flexion.

Function

• The tendon sheath allows smooth, friction-free movement of the FPL tendon as it flexes the thumb joints.

• It also protects the tendon from mechanical wear as it passes under the flexor retinaculum and through the narrow osseofibrous tunnel of the thumb.

Tenosynovitis

• Tenosynovitis (inflammation of the sheath) may occur from overuse or infection, presenting with pain along the thumb and wrist.