Synovial Sheaths at the Wrist
Overview
As the long flexor tendons pass through the carpal tunnel, they are enclosed in synovial sheaths that reduce friction. There are two main bursae at the wrist — one for the finger flexors and one for the thumb flexor.
Ulnar Bursa (Common Flexor Synovial Sheath)
Contents: Encloses the tendons of flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) — all eight finger flexor tendons together.
Extent:
- Proximally: extends 2.5 cm above the proximal border of the flexor retinaculum (into the forearm)
- Distally: extends to the middle of the palm (level of the upper transverse crease of the palm), corresponding approximately to the shafts of the metacarpal bones
Important exception: The distal medial end of the ulnar bursa is directly continuous with the digital synovial sheath of the little finger — this continuity explains why infection in the little finger digital sheath can spread into the carpal tunnel (and vice versa).
Arrangement: The synovial sac appears to have been invaginated from its lateral side by the tendons — giving a parietal layer lining the carpal tunnel walls and a visceral layer applied to the tendons.
Radial Bursa (Synovial Sheath of Flexor Pollicis Longus)
Contents: Encloses the tendon of flexor pollicis longus (FPL) only.
Extent:
- Proximally: coextensive with the ulnar bursa — extends 2.5 cm above the flexor retinaculum
- Distally: extends the full length of the thumb — to the distal phalanx of the thumb
The radial bursa is continuous throughout from forearm to thumb tip — this means infection in the thumb digital sheath communicates directly with the carpal tunnel.
Digital Synovial Sheaths
The flexor tendons of the index, middle, and ring fingers have their own individual digital synovial sheaths within the fibrous flexor sheaths of the fingers:
Extent:
- Proximally: from the necks of the metacarpal bones (level of the lower transverse crease of the palm)
- Distally: to the base of the distal phalanx of the respective digit
Important: These sheaths do not connect with the ulnar bursa — they are isolated. This means infections in these three fingers remain confined unless the proximal extent of the sheath communicates (which can occur when the sheath extends proximally further than usual).
Summary Table
| Sheath | Tendons Enclosed | Proximal Extent | Distal Extent | Communicates With |
|---|---|---|---|---|
| Ulnar bursa | FDS + FDP (8 tendons) | 2.5 cm above retinaculum | Mid palm | Little finger digital sheath |
| Radial bursa | FPL only | 2.5 cm above retinaculum | Tip of thumb | Thumb digital sheath |
| Index digital sheath | FDS + FDP (index) | Neck of 2nd metacarpal | Base of distal phalanx | None normally |
| Middle digital sheath | FDS + FDP (middle) | Neck of 3rd metacarpal | Base of distal phalanx | None normally |
| Ring digital sheath | FDS + FDP (ring) | Neck of 4th metacarpal | Base of distal phalanx | None normally |
Clinical Significance
- Horseshoe abscess: Infection of the little finger digital sheath → spreads to ulnar bursa in carpal tunnel → spreads to radial bursa → infects thumb digital sheath. The resulting U-shaped spread of infection is called a horseshoe abscess (or compound palmar space infection)
- Tendon sheath infections (tenosynovitis): Kanavel’s four cardinal signs — flexed posture of digit, fusiform swelling, tenderness along sheath, pain on passive extension
- Safe zone for incision: When draining palmar infections, the palmar cutaneous branch of the median nerve (running superficial to the retinaculum) must be avoided