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

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