Kinematics of the wrist using 2D and 3D analysis: biomechanical and clinical deductions.

Surgery of the wrist relies on the known notions of biomechanics of the wrist. But these notions are incomplete. For a better understanding of the movements of the wrist, we studied five wrists of healthy volunteers with CT scanning. Each wrist was studied in neutral position, and in the four extreme positions: flexion, extension, radial and ulnar deviations. Using oblique reformatted CT sections, we measured the angular displacements in frontal and sagittal views of every carpal bone in the different positions of the wrist. This allowed us to construct a table of intracarpal mobility. By comparing the angle values and the three-dimensional pictures of these wrists, we illustrate some fundamental points regarding intracarpal movement. The dynamics of the wrist are like those of two super-imposed mobile cups with different movements. The proximal row is malleable with flattening and torsion according to the transverse axis and its behavior is like that of an articular meniscus. The distal row, more rigid but deformable, behaves like a T-handle giving attachment to the hand and articulating under the proximal row around the head of the capitate and the proximal pole of the hamate. During radial and ulnar deviations of the wrist, the movement between the two rows is like an inverse pronation-supination shearing. During flexion-extension, the distortion of the two rows allows maximal congruence to be maintained between the different carpal bones.