[Changes in the form of the interosseous hood during extension and flexion of the metacarpophalangeal joint].

Finger flexion initiated at the distal and proximal interphalangeal joint level forces the extensor tendon to move distally. Therefore, the interosseous hood fixed to the extensor apparatus moves distally, too. The proximal part of the hood which is located at the level of the metacarpal head during extension of the finger slides distally along the basis of the proximal phalanx. Due to the much smaller cross-section of the phalanx compared to the metacarpal head, the palmar border of the interosseous hood is shifted palmarly. This produces a two-fold effect concerning the metacarpophalangeal joint. First, it leads to an increasing flexion moment of the intrinsic muscles during flexion of the metacarpophalangeal joint. Second, the same mechanism improves the ability of abduction and adduction in the metacarpophalangeal joint during extension of the finger. In this position, the proximal part of the interosseous hood covers the metacarpal head and the strong palmar bundles of the hood are pushed to the ulnar and radial sides resulting in a greater distance to the abduction/adduction axis of the MP-joint. The other effect concerns the proximal interphalangeal joint. The described transformation of the interosseous hood during flexion especially of its proximal part causes a curved deformation of the strong palmar border of the interosseous hood. The power of the intrinsic muscles inserting at the interosseous hood is passed along this smooth curve on its way to the dorsal side of the proximal interphalangeal joint thus allowing a continuous extension of the proximal interphalangeal joint in all flexion phases of the MP-joint. The typical transformation of the interosseous hood is regulated by the form of the underlying bone and ligament apparatus and can be understood as a passive mechanism effecting in a senseful change of muscle function as the active element during finger flexion and extension. These not yet described morphological data concerning the transformation of the interosseous hood during finger flexion and the functional interpretation complete the former described mechanism of flexion by Landsmeer (1955) and Landsmeer and Long (1965).