Muscles extending the fingers

Extensor digitorum

Extensor digiti minimi

Extensor indicis

Interossei

Lumbricals

Palmar interossei

There are four palmar interossei situated between the metacarpal bones, with one running to the thumb, index, ring and little fingers. Each muscle arises from the shaft of the metacarpal of the digit on which it acts, and inserts into the dorsal digital expansion and the base of the proximal phalanx of the same digit.

The first palmar interosseus lies on the medial side of the thumb and passes between the first dorsal interosseus and the oblique head of adductor pollicis to insert into the medial side of the base of its proximal phalanx with adductor pollicis. Of the remaining palmar interossei, the second lies on the medial side of the index finger, while the third and fourth lie on the lateral side of the ring finger and little finger respectively. The tendons of all but the first palmar interosseus pass posterior to the deep transverse metacarpal ligament on their way to the extensor expansion.

Nerve supply

All four palmar interossei are supplied by the deep branch of the ulnar nerve, root value T1.

Action

The palmar interossei adduct the thumb, index, ring and little finger towards the middle finger(the midline of the hand). The first palmar interosseus also assists flexor pollicis brevis in flexing the thumb at the metacarpophalangeal joint. The three remaining interossei, by their attachment to the dorsal digital expansion, assist the lumbricals in flexing the metacarpophalangeal joint and extending both interphalangeal joints.

Dorsal interossei

The four dorsal interossei lie superficially in the spaces between the metacarpals on the dorsum of the hand. Two attach to the middle finger, and one each to the index and ring fingers. Each is a bipennate muscle arising from the sides of adjacent metacarpals. The first and second dorsal interossei lie on the lateral side of the index and middle finger respectively, while the third and fourth lie on the medial side of the middle and ring fingers respectively. All of the dorsal interossei insert into the proximal phalanx and dorsal digital expansion of the appropriate finger, having passed posterior to the deep transverse metacarpal ligament.

Nerve supply

All four dorsal interossei are supplied by the deep branch of the ulnar nerve, root value T1. The skin overlying the muscles is supplied by roots C6, 7 and 8.

Action

The dorsal interossei are abductors of the index, middle and ring fingers. In addition, the first dorsal interosseus can rotate the index finger at the metacarpophalangeal joint, and may assist adductor pollicis in adduction of the thumb. Their attachment to the dorsal digital expansion means that, like the palmar interossei, they assist the lumbricals in producing flexion and extension of the metacarpophalangeal and interphalangeal joints respectively.

Palpation

During resisted abduction of the index, middle or ring fingers the dorsal interossei can be palpated on the dorsum of the hand between the metacarpals. The first dorsal interosseus may be seen contracting against resistance in the thumb web. The palmar interossei and lumbricals are too deep to palpate but their actions can be demonstrated by accurate electrical stimulation.