Dorsal interossei muscles of the hand
The dorsal interossei muscles are four short muscles of the metacarpus.
Each of them is attached to the adjacent sides of two metacarpal bones, e.g. the first one between thumb and index fingers, the second between index and middle fingers, etc.
Distally, their tendons insert at the dorsal aponeurosis and the base of the proximal phalanx of the second to fourth fingers. Thereby, they all run towards the middle finger which leads to the following insertion surfaces:
- dorsal interosseous muscle I: radial side of the index finger
- dorsal interosseous muscle II: radial side of the middle finger
- dorsal interosseous muscle III: ulnar side of the middle finger
- dorsal interosseous muscle IV: ulnar side of the ring finger
The first dorsal interosseous, the largest and strongest among the four, can be easily felt in the web between thumb and index finger. Nevertheless, it is also possible to palpate the remaining three between the metacarpal bones and the tendon of the extensor digitorum muscle.
The innervation of all dorsal interossei is carried by the deep branch of the ulnar nerve (C8-Th1).
The dorsal interossei partially perform opposite actions in the fingers: While the first and second pull the index and middle finger laterally (radial abduction), the third and fourth move the middle and ring finger medially (ulnar abduction).
Together they complement the abductor pollicis (abductor of the thumb) and the abductor digiti minimi (abductor of the little finger) in the spread of the fingers and, thus, act antagonistically to the palmar interossei muscles.
(mnemonic: DAB = Dorsals ABduct, PAD = Palmars ADduct).
Furthermore, their contraction leads to a flexion in the metacarpophalangeal joints (MCP) and an extension in the proximal and distal interphalangeal joints (PIP and DIP).
An analogous muscle group can be found in the foot which is responsible, at the bottom, for identical movements at the toes (dorsal interossei of the foot).
Lesions of the deep branch of the ulnar nerve can cause paralysis of the dorsal and palmar interossei whereby the fingers are extended in the MCP and flexed in the PIP and DIP. In that case, spreading the fingers also becomes a more difficult task. This characteristic clinical picture is referred to as the ulnar claw (or claw hand).
An atrophy of the metacarpal and hypothenar muscles occurs in later stages. Interestingly the index and middle fingers are usually less affected in the ulnar claw because the first and second lumbricals can compensate this loss up to a certain degree. The reason behind this is that those two muscles are not supplied by the ulnar nerve, but instead the median nerve.