Extensor expansion

An extensor expansion (dorsal expansion, dorsal hood) is an anatomical term that refers to the flattened tendons (aponeurosis) of extensor muscles that run into the back of the hand. At the distal end of the metacarpal, the extensor tendon will expand to form a hood, which covers the back and sides of the head of the metacarpal and the proximal phalanx.

The expansion soon divides into three bands:
 * Two lateral bands pass on either side of the proximal phalanx and stretch all the way to the distal phalanx. The lumbricals of the hand, the dorsal interossei of the hand, and the palmar interossei insert on these bands.
 * A single median band passes down the middle of the finger along the back of the proximal phalanx, ending at the base of the middle phalanx.

A band known as the retinacular ligament runs obliquely along the middle phalanx, and connects the fibrous digital sheath on the anterior side of the phalanges to the extensor expansion.