The extensor expansions (also known as the extensor hood or dorsal digital expansion) are triangular aponeuroses by which the extensor tendons insert onto the phalanges.
On the dorsal aspect of the fingers, the tendons of the long extensors of the posterior forearm (extensor digitorum, extensor indicis, extensor digiti minimi) have a characteristic configuration. As they cross the metacarpophalangeal joint, the deepest fibers are partly adherent to the posterior joint capsule with the remaining tendon bulk passing the joint. They flatten and fan out as they traverse the dorsal proximal phalanx and separate into 3 slips:
- one central slip: inserts on to the base of the middle phalanx
- two lateral slips: diverge laterally around the central slip to insert on the base of the distal phalanx
These slips are reinforced by the inserting tendons of the palmar/dorsal interossei and lumbricals forming the extensor expansion. The extensor expansion surrounds the distal metacarpal head and proximal phalanx and serves to hold the extensor tendons in place and allow the extensors, lumbricals, and interossei to effect extension at the proximal and distal interphalangeal joints.
On the extensor surface of the thumb, there is no extensor expansion proper: the tendons of the extensor pollicis brevis and longus are inserted separately in the proximal and distal phalanx respectively. However, the tendon of the extensor pollicis longus does receive a fibrous expansion from the abductor pollicis brevis and adductor pollicis which serves a similar purpose as the digital extensor expansion: to hold the extensor tendon in place on the dorsal phalangeal surface.
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