Finger Extensor Apparatus – Simplified

Synonyms: Extensor expansion, Extensor hood, Dorsal expansion, Dorsal hood, Dorsal aponeurosis

Components:

1. Extrinsic tendons
2. Intrinsic tendons
3. Soft-tissue stabilizing structures

Extrinsic tendon

Extensor Digitorum Communis (EDC; may be absent in little fingers) ± Extensor Indicis Proprius (EIP; usually ulnar to EDC) ± Extensor Digiti Minimi (EDM; ulnar to EDC) passes dorsally over MCP joint and trifurcates:

1. Central slip: Inserts into dorsal middle phalanx base (rupture leads to boutonniere deformity)
2. 2 Lateral slips: Join with the lateral bands formed by intrinsic muscles (Interosseous ± Lumbrical) to form conjoined lateral band.

Intrinsic tendon

1. Interosseous: Travel dorsal to transverse intermetacarpal ligament on both sides
2. Lumbrical: Travels volar to transverse intermetacarpal ligament only on radial side

Both the intrinsic muscles travel volar to the metacarpophalangeal joint’s axis of rotation – hence, they aid in MCP flexion.

Interosseous slip ± Lumbrical (radial) slip = Lateral band

Lateral band + Lateral slip of EDC = Conjoined lateral band

2 Conjoined lateral bands converge at dorsal distal phalanx base = Terminal extensor tendon

Soft-tissue stabilizing structures

1. Sagittal bands: Connect the extrinsic tendon to MCP joint volar plate and deep transverse metacarpal ligament
   • Primary stabilizer of extensor tendon at MCP joint
2. Transverse retinacular ligament: Connect the lateral bands to PIP joint volar plate
   • Attenuation = Dorsal translation of lateral bands = PIPJ hyperextension = swan-neck deformity
   • Contracture = Volar translation of lateral bands = PIPJ flexion = boutonniere deformity
3. Triangular ligament (Triangle of stack): Counteracts transverse and oblique retinacular ligament to prevent volar subluxation of lateral bands
4. Oblique retinacular ligament (ORL): variably present and assist with linking IP joint motion
   • Arises from volar plate of PIP joint and flexor tendon sheath and insert onto dorsal base of distal phalanx
   • PIP flexion relaxes ORL allowing DIP flexion and PIP extension tenses ORL extending the DIP
   • Retraction leads to boutonnière deformity of finger

Proximal phalanx is controlled by: Extensor, Flexors and Interossei

Middle and distal phalanx is controlled by: Extensor, Flexors and Retinacular ligaments

Extensor apparatus of thumb: It is less complex and comprises of –

a. Extrinsic tendons: EPL (to dorsal base of distal phalanx) and EPB (radial to EPL and inserts on dorsal base of proximal phalanx)

b. Intrinsic tendons: APB and FPB

c. Soft-tissue stabilizing structures: 2 saggital bands (connect EPL to MCP volar plate)

Few mnemonics that may be handy:

1. eXtensor eXpansion is in proXimal phalanX
2. Lumbrical is a part of Lateral band only Laterally (radial side)
3. Central slip of Common extensor tendon (Communis) inserts into Central (middle) phalanx
4. Lateral band inserts into Last (distal) phalanx

References:

1. ASSH Manual of Hand Surgery edited by Warren C. Hammert, David J. Bozentka, Martin I. Boyer
2. Musculoskeletal key – Injuries of the Extensor Apparatus

Try yourself and Answer (in comments)

1. We can extend the PIP and DIP joint without extending the MCP joints. But we can’t extend the PIP joint without extending the DIP joint at the same time. Why?
2. Flexing only the DIP joint without concurrently flexing PIP joint is difficult. Why?
3. Full flexion of PIP joint prevents active extension of DIP joint. Why?