Causes of prolapse and collapse of the proximal interphalangeal joint.

INTRODUCTION An understanding of abnormal finger motion depends on a basic knowledge of normal functional anatomy, which can be found in the contributions made by Eyler and Markee (! 954), Kaplan (! 953), Milford (! 968), Stack (! 962), Tubiana and Valeillin (1963). It is then possible to appreciate that the finger is a combination of two biarticular, bimuscular ~ystems: a proximal system consisting of the metacarpophalangeal joint and the proximal interphalangeal joint, and a distal system consisting of the proximal interphalangeal and the distal interphalangeal joints. In such a biarticular bimuscular system four combinations of joint positions are found: Extension-extension, extension-flexion. flexion-flexion, and flexionextension (Stack, 1962). Movement being a synonym for a change of position, twelve different motions are possible: eight single actions involving only one joint, and four double motions involving two joints.