A extensor expansion is an anatomical term that refers to the special connective attachments by which the extensor tendons insert into the phalanges.

These flattened tendons (aponeurosis

Aponeurosis

Aponeuroses are layers of flat broad tendons. They have a shiny, whitish-silvery color, are histologically similar to tendons, and are very sparingly supplied with blood vessels and nerves. When dissected, aponeuroses are papery, and peel off by sections...

) of extensor muscles span the proximal and middle phalanges.

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

Phalanx bones

In anatomy, phalanx bones are those that form the fingers and toes. In primates such as humans and monkeys, the thumb and big toe have two phalanges, while the other fingers and toes consist of three. Phalanges are classified as long bones.The phalanges do not have individual names...

.

Bands

The expansion soon divides into three bands:

• lateral bands pass on either side of the proximal phalanx and stretch all the way to the distal phalanx. The lumbricals of the hand
  Lumbricals of the hand
  The lumbricals are intrinsic muscles of the hand that flex the metacarpophalangeal joints and extend the interphalangeal joints.-Structure:There are four of these small, worm-like muscles on each hand. These muscles are unusual in that they do not attach to bone...
  
  , extensor indicis muscle, dorsal interossei of the hand
  Dorsal interossei of the hand
  The dorsal interossei of the hand are muscles that occupy the space between the metacarpals.-Structure:There are four dorsal interossei in each hand...
  
  , and 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.