Collateral bands of extensor expansion of hand

The collateral (or lateral) bands of extensor expansion of hand are formed at the proximal phalanx, where the extensor digitorum tendon trifurcates into a central and two lateral slips. These slips then unite with the interosseous lateral bands (derived from the interossei and lumbrical tendons) to create the conjoint collateral (or lateral bands) of extensor expansion of hand in the vicinity of the proximal interphalangeal joint.