Interosseous muscles in claw finger.

The third and fourth fingers of the normal hands of 75 adult subjects were tested for active finger extension with the wrists in neutral position. Seventy-two of the subjects could not actively extend the interphalangeal (IP) joints when the metacarpophalangeal (MCP) joints were passively and maximally extended. However, the IP joints could be passively extended without pain in 67 subjects and with pain in 5 subjects. This indicated that the inability to actively extend the IP joints when the MCP joints were maximally extended is seldom due to tightness of the flexor tendons. Electromyographic recordings of the interossei muscles in one subject showed voluntary activity of these muscles with the MCP joints maximally extended while the IP joints remained flexed. A macrospoic study was made of the dissected third and fourth fingers of five embalmed cadaver hands and, in two of these specimens polygraphic recordings were made with the use of the strain gauge. In each of these specimens, when traction was applied in the appropriate interosseous muscle, movement of the lateral band of the extensor aponeurosis was noted when the MCP joint was at neutral or slight flexion, while no motion could be detected when the MCP joint was in maximal extension. Thus, maximal extension of the MCP joint blocks not only the action of the extensor digitorum communis tendon, as shown by Mulder and Landsmeer, but also blocks the action of the interossei in extension of the fingers. This explains the mechanism of development of claw finger in those cases without intrinsic hand muscle paralysis.