Kinematic analysis of the instant centers of rotation of the equine metacarpophalangeal joint.

: A kinematic analysis of the instant centers of rotation analysis was performed on 21 metacarpophalangeal joints from 11 horses. Manual and computerized methods were used to locate the instant center of rotation on photocopies of transparent composite tracings of a series of radiographs of each joint. The instant centers of rotation of the proximal phalanx about the distal portion of the third metacarpal bone were located consistently on or near the eminence for attachment of the collateral ligaments. The instant centers of rotation of the sesamoids about the distal portion of the third metacarpal bone were consistently located near the dorsal articular margin of the distal portion of the third metacarpal bone. Rotation of the joint as it extended caused minor variation in radiographic projection. This variation in radiographic projection limited the precision of the analysis of the instant center of rotation and prevented the identification of a single instant center of rotation or an instant center of rotation pathway for the articulation of the proximal phalanx or the proximal sesamoids with the distal portion of the third metacarpal bone. The articular surface velocity vectors determined from the instant centers of rotation indicated that the joint surfaces slide on each other. The motion of the joint caused compression at the dorsal articular margins at maximal extension and thereby limited further extension. At this degree of extension, the proximal sesamoids articulated only with the proximal sesamoid-metacarpal articular surface of the distal portion of the third metacarpal bone.