The mechanical axes of the wrist are oriented obliquely to the anatomical axes.

The movements of the wrist are described with use of the four orthogonal and anatomically defined directions of flexion and extension in the sagittal plane and radial and ulnar deviation in the coronal plane1. More complex movements of the wrist are required for daily living activities, and these motions are described with use of combinations of these motions2,3. For example, circumduction is the “circular” motion that traces an envelope about these four motions. The so-called dart thrower's motion, a functionally important wrist motion4-6, describes a direction of wrist motion that is oblique to these orthogonal anatomical planes. Specifically, the dart thrower's motion is used to define a direction of wrist movement that is from a position of combined extension and radial deviation to a position of combined flexion and ulnar deviation. All wrist movements are accomplished through the complex motions of the eight carpal bones7-13. With no tendon attachments, the static and dynamic stability of the carpus, particularly the proximal row of carpal bones (scaphoid, lunate, and triquetrum), is dependent solely on the shape of the carpal articulations and on the mechanical properties of the attached capsule and ligaments. Accordingly, numerous studies have focused on carpal bone shape14-17, carpal bone kinematics11-13, ligament properties18,19, and cartilage properties20. While these studies have focused on the elements of the wrist, few studies have examined the mechanical properties of the wrist itself, defined herein as the compound joint from the distal end of the radius to the proximal part of the metacarpals. To date, the determination of the mechanical properties of the wrist has focused on distractive loading21,22 with the aim of understanding the effects of traction in the reduction of intra-articular distal radial fractures23 and in arthroscopic surgery24,25. Determining the mechanical properties of the wrist along the directions of the various wrist movements can aid in understanding the function of the normal wrist and wrists with a pathological condition, improving treatments for wrist disorders26, constructing and validating mathematical models27-30, and developing design specifications for wrist implants31. Given that radial deviation has the smallest range of motion of the four primary anatomical directions26, we anticipated that radial deviation would have the largest stiffness values, but we were unable to find support for this in the literature. Moreover, recent studies of the dart thrower's motion and the carpal kinematics associated with this motion5,13,32,33 have suggested that the mechanical behavior of the wrist may be more complex than previously appreciated and led us to hypothesize that the mechanical axes of the wrist are not aligned with the anatomical axes. The purpose of this study was to determine the mechanical properties of the wrist in the four anatomical directions of wrist motion and in twenty additional directions that are combinations of these motions. We quantified the mechanical properties by measuring the moment-rotation behavior of the cadaver wrist.