Abstract:

The wrist is a complex structural joint with demanding motion requirements. This study aimed to develop a mathematical model of carpal motion based on computer derived isometric constraints. This extends the previously reported concept of rules based animation which proposes that resultant motion is a net interplay of bone shape, isometric constraints, bone interaction, and applied load.

The extreme positional relationships between bones of the proximal row and the radius, and identified isometric constraints, based on a computer derived analysis were reviewed using a 3-dimensional surface rendering. These 3d models were create from the CT scan data of 10 normal wrists taken in extremes of radial and ulna deviation as well as flexion and extension.

Connecting lines were identified between specific points of the lunate and radius, which corresponded to an isometric constraint through range. Similar pairs of points were found on the trapezium and scaphoid, and in the scapho-lunate, luno-triquetral and radio-triquetral joints. There was a clear discrepancy (p <.05) between those areas (typically either volar or dorsal depending on the bones) which remained isometric and those which did not and this corresponded to previous documented anatomical structures.

Using 3D software modelling software, the centroids of the proximal and distal rows were identified and the displacement and motion of these centroid with respect to each other were assessed. On the basis of the isometric constraints and motion patterns, and in particular the relationships between the various centroids, identified in this study, the carpus appears to function as a stable central column (Lunate-Capitate-Hamate-Trapezoid-Trapezium) with a supporting lateral column (Scaphoid), which is more of a "crossed four bar linkage" than the traditionally described "slider crank" pattern. On the medial side of the central column, the triquetrum acts principally as an ulna translation restraint.

The "trapezoid" shaped trapezoid places the trapezium anterior to the transverse plane of the radius and ulna, and thus rotates the principal axis of the central column to correspond to that used in the "Dart Thrower Motion". A unifying kinetic theory of wrist motion based on isometric constraints and rules based motion is proposed, which is characterised by a stable central carpal column, a lateral column stabiliser, and medial column translation restraint.

This paper won the Best Paper prize at the conference