The ulnar bursa of the hand (common flexor sheath) is a synovial-lined sac enclosing the flexor digitorum superficialis and profundus tendons as they pass through the carpal tunnel and into the palm. It extends from approximately 2 cm proximal to the flexor retinaculum to the midpalm, enclosing the tendons of all four fingers. It communicates with the digital tendon sheath of the small finger in approximately 80% of individuals, but does not normally communicate with the digital sheaths of the other fingers.
The ulnar bursa is one of the primary bursae involved in suppurative flexor tenosynovitis of the hand. Kanavel four cardinal signs (fusiform swelling, flexed posture, tenderness along the tendon sheath, and pain with passive extension) must be sought in any patient with finger infection, as untreated tendon sheath infection can rapidly destroy the tendons and spread proximally through the ulnar bursa into the forearm space of Parona. Proximal spread from the small finger sheath through the ulnar bursa to Parona space and distally through the radial bursa to the thumb creates the horseshoe abscess pattern.
Bacterial infection of the ulnar bursa from a penetrating hand wound produces a rapidly progressive closed-space infection with Kanavel signs requiring urgent surgical irrigation, debridement, and intravenous antibiotics to prevent irreversible tendon damage.
Communicating infection between the radial bursa (FPL sheath) and ulnar bursa via the space of Parona produces a characteristic horseshoe-pattern abscess involving both the thumb and small finger tendon sheaths, a severe deep space hand infection requiring extensive surgical drainage.
