HS4 “STABLE CENTRAL CARPAL COLUMN”: CARPAL MECHANICS BASED ON COMPUTER DERIVED ISOMETRIC CONSTRAINTS

Abstract

This study has developed a unifying theory of carpal motion based on computer derived isometric constraints which guides the movement of particular bones. 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 positional relationship between bones of the proximal row and the radius at extremes of motion was assessed to identify isometric constraints, based on a computer derived analysis rather than by observation of carpal bone motion or ligamentous anatomy. Using 3-D surface rendering software, models were created from the CT scan data of 10 normal wrists taken in extremes of radial and ulna deviation as well as flexion and extension.

Virtual 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 at the trapezium and scaphoid and dorsally at the scapho-lunate joint. There was a clear discrepancy (p < .05) between those areas (typically either volar or dorsal depending on the bones) which remain isometric and those which did not and this corresponded to previous documented anatomical structures. Variability in the pattern of isometric lines correlated with variation in scaphoid motion, thus providing a correlation with previous carpal motion observations. The Carpus can be seen to function as a stable central column (lunate/capitate/hamate/trapezoid/trapezium), with a supporting lateral column (scaphoid). This functions more as a “crossed four bar linkage” than the traditionally described “slider crank”. 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”.

This model provides a unifying theory for understanding normal and abnormal wrist motion based on isometric constraints and more broadly rules based motion. The characterisation of isometric constraints within the proximal carpal row has allowed the quantitative analysis of carpal dynamics, which has as its core, a stable central carpal column – with a lateral column stabiliser, and medial column translation restraint key to safe administration of anaesthetic in the upright position.