tendon sheath

STRUCTURE: A tendon sheath has two layers. The outer fibrous sheath (including annular A1–A5 and cruciate pulleys in the fingers) anchors the tendon and prevents bowstringing. The inner synovial sheath has a parietal layer lining the fibrous sheath and a visceral layer covering the tendon. Between them is a potential space with a small amount of synovial fluid that lubricates and nourishes the tendon. The mesotendon (vincula) carries the blood supply. Synovial sheaths exist only where friction is high (distal limbs: palm, fingers, wrist, ankle). Tendons elsewhere are surrounded by loose connective tissue (paratenon) instead.

KEY ANATOMY: In the hand, the flexor digitorum superficialis and profundus tendons share the common flexor sheath (ulnar bursa), which continues to the 5th digit's distal phalanx. The flexor pollicis longus has its own sheath (radial bursa). The thumb and 5th digit sheaths usually communicate through the carpal tunnel (the "horseshoe abscess" pathway). The index, middle, and ring finger sheaths are separate and end at the palm (the proximal ends are blind pouches). The dorsal wrist has six extensor compartments, each with its own sheath.

CLINICAL CORRELATES: - Infectious (suppurative) flexor tenosynovitis: Usually from penetrating trauma, most often Staphylococcus aureus. Kanavel's four signs: (1) finger held in slight flexion, (2) fusiform swelling, (3) tenderness along the flexor sheath, and (4) pain on passive extension (the most sensitive sign). It is a surgical emergency (irrigation/debridement plus IV antibiotics). The infection can spread to the deep space of Parona and form a horseshoe abscess. - De Quervain tenosynovitis: Affects the 1st dorsal compartment (abductor pollicis longus and extensor pollicis brevis). It is seen in new mothers and repetitive thumb use. Findings: radial wrist pain and a positive Finkelstein test. Treatment is a thumb spica splint, NSAIDs, and corticosteroid injection. - Stenosing tenosynovitis (trigger finger): The flexor tendon nodule catches at the A1 pulley, causing locking or snapping. Risk factors include diabetes and rheumatoid arthritis. - Ganglion cyst: A mucin-filled cyst arising from a joint capsule or tendon sheath. It is most common on the dorsal wrist. It is benign and transilluminates. - Giant cell tumor of the tendon sheath: A benign, painless mass (the extra-articular form of pigmented villonodular synovitis). It is the second most common hand mass after ganglion cysts. Histology: multinucleated giant cells, hemosiderin-laden macrophages, and foam cells. - Rheumatoid arthritis: Causes tenosynovitis and can lead to tendon rupture (e.g., extensor tendons at the wrist). - Gonococcal infection (disseminated gonococcal infection): Classic triad of tenosynovitis, migratory polyarthralgia, and dermatitis (pustular lesions). - Carpal tunnel syndrome: Tenosynovial thickening can compress the median nerve.

HIGH-YIELD PEARL: Infection of the tendon sheath in the hand is a clinical diagnosis. Do not delay treatment for imaging. Passive extension pain is the earliest and most sensitive finding.