Extensor Tendons of the Hand

Overview

The Six Dorsal Compartments (Radial → Ulnar)

• Miller's Review of Orthopaedics, 9th Ed.; Roberts and Hedges' Clinical Procedures in Emergency Medicine; Sabiston Textbook of Surgery

Key Tendons and Their Insertions

Extensor Digitorum Communis (EDC)

• Extends fingers II-V via a single muscle belly splitting into four tendons
• Inserts loosely at the base of the proximal phalanx (via sagittal bands), then trifurcates over the proximal phalanx into:
  • A central slip → inserts at the base of the middle phalanx
  • Two lateral branches → join the interosseous/lumbrical lateral bands → fuse to form the terminal extensor mechanism (TEM) → inserts at the base of the distal phalanx
• Tendons are interlinked on the dorsum by oblique bands called juncturae tendinum (intertendinous connections) - this is why a complete EDC laceration may not abolish extension entirely

Extensor Indicis Proprius (EIP) and Extensor Digiti Minimi (EDM)

• The index and little fingers have their own dedicated extensor muscles, allowing independent extension
• Their tendons always lie on the ulnar side of the corresponding EDC tendon
• No junctura tendinum is present on the EIP tendon

Thumb Extensors

• EPL: extends IP joint; forms ulnar border of anatomic snuffbox
• EPB: extends MCP joint; forms radial border of anatomic snuffbox
• APL: abducts thumb; inserts on base of 1st metacarpal

The Dorsal Digital Expansion (Extensor Hood)

• Sagittal bands (proximal hood at MCP level): loop around the joint and blend into the volar plate; keep the EDC tendon centered over the metacarpal head; extend the MCP joint
• Central slip: EDC central continuation to the base of the middle phalanx; extends the PIP joint
• Lateral bands: contributions from lumbricals and interossei; lie dorsal to the axis of the PIP joint so they act as PIP extensors and MCP flexors
• Triangular ligament: connects the two lateral bands over the middle phalanx, preventing volar subluxation of the lateral bands (injury → boutonniere deformity)
• Terminal tendon (terminal slip): fusion of the conjoined lateral bands; inserts at the base of the distal phalanx; extends the DIP joint
• Oblique retinacular ligament (Landsmeer's ligament): links PIP and DIP extension; relaxed in PIP flexion (allows DIP flexion), taut in PIP extension (assists DIP extension)
• General Anatomy and Musculoskeletal System, THIEME Atlas of Anatomy; Sabiston Textbook of Surgery

Zones of Extensor Tendon Injury

• Campbell's Operative Orthopaedics, 15th Ed.; Sabiston Textbook of Surgery

Dorsal Hand Overview

Key Clinical Points

Extensor Zones and Compartments of the Hand

Part 1 - The Six Dorsal Compartments (at the Wrist)

• Campbell's Operative Orthopaedics, 15th Ed.; Sabiston Textbook of Surgery

Part 2 - Extensor Zones of Injury

• Odd-numbered zones = over joints
• Even-numbered zones = over bones
• Zone VIII and IX = proximal to the wrist

Zone-by-Zone: Anatomy, Injury, and Management

Zone I - DIP Joint

• Structure at risk: Terminal extensor tendon (insertion at base of distal phalanx)
• Injury: Mallet finger - forced DIP flexion avulses the tendon (sometimes with a bony fragment); common in ball sports ("jammed finger")
• Signs: DIP extensor lag, pain/tenderness over DIP; check X-ray for avulsion fracture
• Treatment:
  • Closed: DIP extension splinting x 6-8 weeks continuously, then night-only splint for 2-4 more weeks
  • Surgical: K-wire DIP fixation; suture anchor if complete tendon avulsion; operative fixation if >50% joint surface in fragment or volar subluxation present
  • Splinting within 2 weeks is as effective as delayed splinting; can work up to ~12 weeks post injury

Zone II - Middle Phalanx

• Structure at risk: Tendon over middle phalanx, triangular ligament, lateral bands
• Injury: Usually sharp lacerations; rarely closed
• Treatment: Repair with figure-of-eight or running sutures + static splinting

Zone III - PIP Joint ⚠️ Most Clinically Important

• Structure at risk: Central slip (± lateral bands, triangular ligament)
• Injury: Boutonniere deformity - central slip rupture → lateral bands migrate volar to PIP axis → PIP flexion + DIP hyperextension
• Mechanisms: Forced PIP flexion, blunt dorsal PIP trauma, volar PIP dislocation
• Key test: Elson's test - PIP held in 90° passive flexion; ask patient to extend against resistance. If central slip intact: DIP cannot extend (lateral bands go lax). If central slip ruptured: DIP extends/hyperextends + PIP extends (positive test)
• Note: Deformity may not be apparent acutely if the triangular ligament is intact
• Treatment: 6 weeks PIP extension splinting (DIP left free); surgical repair if open injury

Zone IV - Proximal Phalanx

• Structure at risk: Flat, broad portion of EDC where it begins to trifurcate
• Injury: Usually partial lacerations (tendon is wide here)
• Treatment: Partial injuries may be managed conservatively; complete lacerations require repair

Zone V - MCP Joint

• Structure at risk: Sagittal bands (hold EDC tendon centered over metacarpal head)
• Injury: Sagittal band rupture → EDC subluxation/dislocation (usually ulnar); "snapping" with extension
  • Boxer's knuckle: Closed sagittal band rupture from punching
  • Most common at the radial sagittal band of the middle finger
  • Human fight bite: MCP puncture from opponent's tooth; high infection risk, may penetrate joint
• Classification: Type I (no instability), Type II (subluxation), Type III (complete dislocation)
• Treatment: Type I/II: 4-6 weeks MCP extension splinting; Type III/open: surgical repair

Zone VI - Dorsum of Hand (Metacarpals)

• Structure at risk: EDC tendons + juncturae tendinum (intertendinous connections)
• Injury: Usually lacerations
• Critical pitfall: Juncturae tendinum transmit extension forces from adjacent EDC tendons → patient can STILL extend the digit despite complete tendon laceration
• Rule: Always test strength of extension, not just presence of active motion

Zone VII - Wrist (Extensor Retinaculum)

• Structure at risk: Tendons within their synovial compartments beneath the retinaculum
• Injury: Lacerations over the carpus; closed injuries with distal radius fractures
• Treatment: Surgical repair; extensor retinaculum usually repaired (to prevent bowstringing) but not too tightly (avoid compartment syndrome)

Zone VIII - Distal Forearm

• Structure at risk: Muscle-tendon junction
• Injury: Lacerations, crush; junctions are less amenable to primary repair

Zone IX - Forearm

• Structure at risk: Extensor muscle bellies
• Injury: Lacerations and crush of muscle body

Thumb Zones (TI - TV)

Part 3 - Classic Deformities at a Glance

• Rosen's Emergency Medicine; Sabiston Textbook of Surgery; Campbell's Operative Orthopaedics, 15th Ed.