In vivo kinematic behavior of the radio-capitate joint during wrist flexion-extension and radio-ulnar deviation

The capitate is often considered the "keystone" of the carpus, not simply b ecause of its central and prominent position in the wrist, but also because of its mechanical interactions with neighboring bones. The purpose of this study was to determine in vivo three-dimensional capitate kinematics. Twen ty uninjured wrists were investigated using a recently developed, non-invas ive markerless bone registration (MBR) technique. Surface contours of the c apitate, third metacarpal and radius were extracted front computed tomograp hy images of seven wrist positions and the three-dimensional motions of the capitate and third metacarpal were calculated with respect to the radius i n wrist flexion-extension and radio-ulnar deviation. We found that in vivo capitate motion does not simply occur about a single pivot point like a uni versal joint, as demonstrated by non-intersecting rotation axes for differe nt capitate motions. The distance between flexion and ulnar deviation axes was 3.9+/-2.0 mm, and the distance between extension and ulnar deviation ax es was 3.9+/-1.4 mm. Furthermore, capitate axes for males tended to be loca ted more distally than axes for females. However, we believe that this resu lt is related to subject size and not to gender. We also found that there i s minimal relative motion between the capitate and third metacarpal during these in vivo wrist motions. These findings demonstrate the complexity of c apitate kinematics, as well as the different mechanisms through which wrist flexion, extension, radial deviation and ulnar deviation occur. (C) 2001 E lsevier Science Ltd. All rights reserved.