RADIO-CARPAL JOINT FORCE AND THE VOLAR TILT ANGLE OF THE DISTAL RADIUS

Abstract

It is generally appreciated that the internal structure and external shape of living bone adapt to mechanical stimuli according to Wolff’s law. However, the precise details of bone adaptation to external forces are not fully understood and there has been no previous investigation of the association between specific loading conditions and the skeletal architecture of a particular anatomical area using case-specific observations in a group of individuals. The aim of the present study was to investigate a previously unreported correlation between the maximum wrist joint reaction force and the volar tilt angle of the distal radius using parameters radiographically obtained parameters from normal wrist joints.

Using free body analysis of the forces acting on the distal radius for the loading condition that corresponds to the lift of a weight using the supinated hand, the wrist joint reaction force F and the angle formed between the vector of F and the long axis of the radius have been expressed as a function of the lifted weight, the lever-arm of the wrist flexor tendons and that of the lifted weight. Measurements of the volar tilt angle of distal radius and the lever-arms of the flexor tendons and the lifted weight were performed from lateral wrist radiographs of 30 normal wrists. Subsequently, using the equations obtained from free body analysis, the maximum wrist joint reaction force F and the angle that the latter forms with the long axis of the radius were calculated for each the cases. Statistical analysis compared the angle of the maximum wrist force and the volar tilt of the distal radius (two-tailed paired t-test) and correlated (a) the angle of the maximum wrist force and the volar tilt angle and (b) the maximum joint reaction force and the volar tilt angle.

Results showed no significant difference (p=0.33, 95% confidence interval −0.64° to 0.22°) but a statistically significant correlation (R² = 0.74, r = 0.86, p < 0.001) between the angle of the maximum wrist force and the volar tilt angle of the distal radius. Additionally, an inverse relationship between the volar tilt angle and the magnitude of the maximum wrist force (R² = 0.71, r =−0.84, p< 0.001) was found.

These observations may explain the mechanism of the phylogenetical development of the volar tilt angle and support the ‘minimum effective strain’ theory of adaptive bone remodeling1. The importance of accurate restoration of the volar tilt during treatment of distal radius fractures, especially in wrists that are normally characterised by a low volar tilt angle, is also emphasized by the results of the present study.