The Extensor Expansion of the Hand

Written by Oliver Jones

Last updated March 9, 2024 • 17 Revisions

The extensor expansion of the hand is a specialised connective tissue structure by which the extensor tendons insert onto the phalanges.

It is a complex structure which acts to balance forces from a large number of muscles that act on the digits.

In this article, we shall look at the anatomy of the extensor expansion – its structure, insertions and clinical correlations.

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Structure

The extensor expansion is triangular in shape and begins at the level of the metacarpophalangeal joint. It is formed from the extensor digitorum tendon (fingers) and extensor pollicis longus tendon (thumb).

As the long extensor tendons approach the metacarpophalangeal joint, they begin to flatten and form the extensor hood. Fibres from the extensor hood fan out and wrap around the distal metacarpal and proximal phalanx. The most well-defined component of the extensor hood at this level is the sagittal band.

The tendon then continues along the dorsal surface of the proximal phalanx, dividing into three parts:

  • Central slip – inserts onto the base of the middle phalanx.
  • Lateral bands (x2) – insert onto the base of the distal phalanx.

Fig 1
The deep structures of the extensor expansion of the digits of the hand.

Fig 2
Lateral view of the extensor apparatus. Note the insertion of the lumbrical and interossei muscles.

Tendinous Attachments

Whilst the extensor expansion of the hand is formed by the long extensor tendons, a number of other muscles insert onto it.

  • Fingers:
    • Extensor indicis – additional extensor of the index finger. It lies medially to the tendon of the extensor digitorum, and inserts onto the extensor hood centrally.
    • Extensor digiti minimi – additional extensor of the little finger. It lies medially to the tendon of the extensor digitorum, and inserts onto the extensor hood centrally.
    • Palmar and dorsal interossei – inserts onto the lateral bands of the extensor expansion.
    • Lumbricals – inserts onto the radial aspect of the extensor expansion (distal to the interossei insertion).
  • Thumb:
    • Adductor pollicis 
    • Abductor pollicis brevis 
    • Extensor pollicis brevis 

Fig 3
The interossei insert onto the lateral bands of the extensor apparatus

Oblique Retinacular Ligament

The oblique retinacular ligament originates from the proximal phalanx and flexor tendon sheath, and inserts onto the lateral bands of the extensor apparatus.

It acts to link the motion of the proximal interphalangeal joint (PIPJ) with that of the distal interphalangeal joint (DIPJ):

  • Flexion at the PIPJ relaxes the oblique retinacular ligament, permitting flexion of the DIPJ.
  • Extension at the PIPJ tightens the oblique retinacular ligament, facilitating extension of the DIPJ.
Clinical Relevance

Central Slip Injury

The central slip is the component of the extensor expansion that inserts onto the base of the middle phalanx. It can be injured by trauma to the dorsal surface of the proximal phalanx (e.g. laceration).

In cases of complete rupture, there is a weakening of extension at the proximal interphalangeal joint.

If left untreated, the extensor apparatus becomes unbalanced, and the lateral bands are pulled towards the volar (front) surface of the digit. This results in hyperextension of the distal interphalangeal joint.

The combination of flexion at the PIPJ and hyperextension at the distal interphalangeal joint is known as the Boutonniere Deformity.

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