Common flexor tendon sheath
Vagina communis tendinum flexorum
- Latin synonym: Vagina communis tendinum musculorum flexorum
- Synonym: Ulnar bursa
- Related terms: Common flexor sheath
Definition
The common flexor tendon sheath, also known as the ulnar bursa, is a synovial sheath on the palmar side of the hand that encloses the long tendons of flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP), as they pass deep to the flexor retinaculum within the carpal tunnel.
Extent
Proximally: Begins about 2.5 cm above the flexor retinaculum, extending into the distal forearm.
Distally: Continues into the palm, usually ending at the level of the mid-palm. In many cases, the common flexor tendon sheath extends into the digital synovial sheath of the little finger, forming a continuous synovial channel.
Note: The digital sheaths of the index, middle, and ring fingers are separate and do not communicate with the common flexor tendon sheath. In some cases, this common flexor sheath (radial bursa) may communicate with the ulnar bursa, forming a continuous synovial channel across the palm.
Parietal and visceral layers of the common flexor tendon sheath
The common flexor tendon sheath (ulnar bursa) is a double-layered synovial tube that surrounds the tendons of FDS and FDP as they pass through the carpal tunnel.
The outer (parietal) layer lines the inside of the carpal tunnel, attached to the flexor retinaculum and carpal bones. It stays fixed in place.
The inner (visceral) layer closely covers the FDS and FDP tendons, moving with them as they glide.
At the upper and lower ends of the sheath, the parietal layer reflects around the tendons to become the visceral layer, forming a closed space between them that contains synovial fluid. This arrangement lets the tendons slide smoothly and without friction during finger movements — similar to how layers of pleura move over each other in the lungs.
Flexor tenosynovitis
Inflammation or infection of the common flexor tendon sheath (often due to penetrating injury or spread from the little finger) can present with pain, swelling, and impaired finger flexion. Effusion or thickening within the sheath may contribute to carpal tunnel compression of the median nerve.
References
Gray, H. (2016) Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 41st edn. Edited by S. Standring. New York: Elsevier. Chapter 50: Wrist and hand, pp. 879.
Mehta, P., Thoppil, J., Koyfman, A. and Long, B., 2024. High risk and low prevalence diseases: Flexor tenosynovitis. The American Journal of Emergency Medicine, 77, pp.132-138.
Drake, R.L., Vogl, A.W. and Mitchell, A.W.M. (2009). ‘Chapter 7: Hand’ in Gray’s anatomy for Students. (2nd ed.) Philadelphia PA 19103-2899: Elsevier, pp. 757; Fig. 7.95.
