Anatomical Relations
Each extensor tendon is held in place by a fibrous extensor expansion, which runs from the metacarpophalangeal joint into each digit. The expansions form movable hoods over the dorsal aspect of the digits.
The margins of the lateral and medial aspects of the extensor expansions are thickened by the insertion of the lumbricals and the interossei muscles.
Structure
The extensor expansion is a complex aponeurotic extension of the extensor tendons that form as the tendons pass over the dorsal aspect of the digits. Each extensor expansion is triangular shaped with its base wrapped around the sides of the metacarpophalangeal joint.
The extensor expansions divide into three slips, one central and two lateral bands, near the proximal interphalangeal joint. Fibers from the central slip insert into the proximal portion of the middle phalanx and fibers from the lateral slips continue distally, unite and insert into the distal phalanx.
Function
The extensor expansions keep the extensor digits in place and ensure their insertion into the phalanges. They are also the site of insertion for other intrinsic muscles of the hand.
