An indirect method to assess wrist ligament forces with particular regard to the effect of preconditioning

A method has been developed to calculate the forces that are developed in the ligaments of a joint specimen during motions. This indirect method is needed since direct measurements fail in the case of small ligaments. As an example the small ligaments of the carpal joint are considered. The rationale of the method is that the force generated in a ligament depends on the amount of strain to which it is subjected and on its material characteristics. In the method presented the lengths of the ligaments are determined in vitro at several joint positions by means of röntgenstereophotogrammetry. The zero-force length and the force-elongation relationship are determined on the same ligaments isolated in a materials testing machine. Over a considerable part of the strain range the measurement errors are relatively small compared to the forces determined, less than 10%. The method is applicable to joints in situations where other measuring methods cannot be used. The present analysis shows, however, that the force values determined are susceptible to preconditioning of the ligaments. In preconditioned ligaments the forces could be up to 50% lower than in the non-preconditioned situation. This suggests that ligament forces may vary considerably in vivo, depending on the extent of preconditioning provoked by a particular function.