Background: The human carpus is a complex joint system. Many problems that arise in the wrist are the result of an alteration of intercarpal motion. Although the midcarpal joint is a major component of the wrist joint, the global kinematics of the midcarpal joint have not been described. The purpose of this study was to provide a simplified description of the motion and function of the midcarpal joint.

Methods: We studied the in vivo three-dimensional kinematics of the midcarpal joint with use of a markerless bone-registration technique. Magnetic resonance images of the wrists of twenty-four healthy volunteers were acquired during a dart-throwing motion or flexion-extension motion of the wrist. Three-dimensional animations of the isolated midcarpal joint were created. Relative midcarpal motions were investigated qualitatively and quantitatively.

Results: The direction of the capitate motion relative to the scaphoid was always similar: it was oblique and it extended from radiodorsal to ulnopalmar in radioulnar deviation, in the dart-throwing motion, and in the flexion-extension motion. The directions of the capitate motions relative to the lunate and triquetrum inclined in a similar way, while the ranges of motion were almost unchanged. As the wrist motion changed from radioulnar deviation to flexion-extension motion, the range of the capitate rotation relative to the scaphoid decreased while the range of the lunate rotation relative to the scaphoid increased. Regardless of the type of wrist motion, the loci of the displacement of all of the joint surfaces of the midcarpal joint were located within a midcarpal ovoid space, and a line connecting the centers of the joint surfaces of the midcarpal joint could be schematized as a letter "C" entwining the midcarpal ovoid.

Conclusions: Midcarpal motion is essentially the combined motion of three types of joint systems: (1) the uniaxial joint between the scaphoid and the distal row; (2) the biaxial joint between the lunate and triquetrum and the distal row; and (3) the intercarpal joints of the proximal row, which have an adaptive mechanism that accommodates the above-mentioned two types of joint systems in the midcarpal joint.

Clinical Relevance: We advocate use of the "ovoid/C" concept to describe the function of the midcarpal joint that contributes to both the stability and the mobility of the wrist, to assist clinicians in achieving a better understanding of the kinematics of the wrist joint.