Influence of joint laxity on scaphoid kinematics.

Excessively lax wrists more frequently become symptomatic if overloaded or injured than normal joints. Whether this is the consequence of biological or mechanical factors or both remains unknown. This study evaluates the relationship between the degree of joint laxity and scaphoid kinematic behaviour during radio-ulnar deviation of the wrist in 60 normal volunteers. There is a significant linear relationship between the direction of scaphoid rotation and the amount of wrist joint laxity. During lateral deviation of the wrist, joints that are more lax have a scaphoid rotating mainly along the sagittal plane of flexion and extension, with little lateral deviation. In contrast, the scaphoid of volunteers with decreased laxity rotate mostly along the frontal plane of radioulnar deviation with minimal flexion extension. These results support the concept of increased out-of-plane scaphoid rotation as a factor of increased vulnerability during over-work or injury.