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
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.