Flexor Tendon

Finger Zones

Zone 1

  • Contains only FDP

Zone 2

  • From origin of sheath (MCPJ – distal palmar crease) to middle phalanx
  • Contains both FDS and FDP within a tight non-distensible sheath
  • Often both tendons are lacerated
  • Worst prognosis

Zone 3

  • Between carpal tunnel and origin of synovial sheath (distal palm crease)
  • Contain the origin of the lumbricals from FDP
  • Lumbrical muscle tends to be injured

Zone 4

  • Carpal tunnel – contains 9 tendons and median nerve

Zone 5

  • Proximal to carpal tunnel
  • Best prognosis for repair

Thumb Zones

  • Only one tendon > FPL - zones are similar in pattern
Zone Location
Zone 1 Distal insertion of FPL
Zone 2 Over proximal phalanx and MCPJ
Zone 3 Over thenar muscles
Zone 4 & 5 Same as for fingers

Flexor Tendons

  • Flexors are all long, extrinsic tendons – 6 in total
    • No contribution from intrinsics
  • Wrist Flexors: FCR, PL, FCU
  • Digital Flexors: FDP, FDS, FPL
    • Little finger FDS absent in 20% of people
    • Long to little fingers FDP have common muscle belly
    • Index finger has its own FDP belly
  • Testing FDS
    • To test FDS to fingers, flex PIPJ with other fingers extended (neutralizes FDP)
    • To test FDS in the index separately, ask for a pincer grip with DIPJ hyperextended
    • If FDS is absent, only an “OK” sign is possible

Tendon Nutrition

  • Flexors are mainly intrasynovial tendons
  • Lie within a fibro-osseous sheath with a double-layer synovium
  • No paratenon (unlike extensors)
  • Sheath Entry Points:
    • Index, middle, ring fingers enter at MC neck level
    • Thumb and little finger have synovial sheath in whole palm
  • 2 Modes of Nutrition:
    1. Longitudinal vessels from phalanges enter via vinculae system
      • Vincula longus and brevis per tendon
      • Tendon relatively avascular on palmar surface
    2. Passive diffusion from synovial fluid (imbibition)
      • Relies on motion

Structure

  • FDS initially volar to FDP in forearm and palm
  • FDS splits and encircles FDP over P1 (Camper’s chiasm)
  • FDS reforms dorsal to FDP and attaches to P2
  • In Zone 2, FDS and FDP glide over each other → high adhesion risk
  • Blood Supply via Vinculae:
    • VBP (Vinculum Brevis Profundus)
    • VBS (Vinculum Brevis Superficialis)
    • VLP (Vinculum Longus Profundus)
    • VLS (Vinculum Longus Superficialis)

Lumbricals

  • 4 Lumbricals in total
  • Arise from FDP proximal to MCPJ in Zone 3
  • Radial lumbricals: single-headed
  • Ulnar lumbricals: two-headed
  • Insertion:
    • Radial aspect of extensor mechanism
    • Pass dorsally and insert distal to PIPJ
    • Contribute to extensor hood & lateral bands
    • Function: Extend PIPJ & Flex MCPJ

Pulley System

  • Fibro-osseous tendon sheath anchored dorsally to bone
  • Pulley Types:
    • Annular (A1-5): Thick and fibrous (prevent bowstringing)
    • Cruciate (C1-3): Flimsy, allow flexibility
  • Odd-numbered pulleys arise from volar plate
  • Even-numbered pulleys arise from periosteum
    • A2 and A4 are most important for function
  • Thumb Pulleys:
    • 2 Annular (A1 & A2) and 1 Oblique
    • Oblique Pulley: Runs ulnar-proximal to radial-distal on P1
    • Oblique pulley is critical for stability

Prognosis for Flexor Tendon Repair

  • Order from best to worst: Zone 5 > Zone 4 > Zone 3 > Zone 1 > Zone 2
  • Prognosis Factors:
    • Ease of access for repair
    • Room for tendons to glide
    • Less room = higher adhesion risk → Worse prognosis
  • Zone 2 has worst prognosis:
    • Tight sheath with FDS & FDP close together
    • High adhesion risk
  • Zone 3 Complications:
    • Lumbrical repair or scarring → Intrinsic plus hand deformity
    • Leads to MCP flexion contracture and restricted IPJ flexion
  • Zone 5 has best prognosis:
    • No enclosing sheath → free-moving tendons
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