Strains and forces in selected carpal ligaments during in vitro flexion and deviation movements of the hand

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Abstract
The forces induced in tiny wrist joint ligaments must be estimated in order to understand their role in the mechanism of the joint. We estimated forces in a number of selected ligaments in seven human wrist joint specimens, using a noninvasive method. The method is based on the rationale that the force generated in a ligament depends on its change of length with the joint under load. In vitro length changes of the ligaments were determined during flexion and deviation movements of the hand, using a roentgenstereophotogrammetric analysis technique. Subsequently, bone‐ligament‐bone (BLB) preparations were dissected from teh specimens. From these BLB preparations the zero‐force length and the force‐elongation relationship were determined in a material testing machine. The forces generated in the ligaments during flexion and deviation were calculated by combining results on the in vitro ligament length changes, the zero‐force length, and the force‐elongation relationship. Large interspecimen variations of the force patterns were found. Due to this variability, it is not possible to obtain quantitative models for the kinetic behavior of the ligaments. However, qualitative trends could be distilled from the strain and force patterns. It is clear that for most ligaments, the zero‐force lengths were not equal to the lengths they possessed in the neutral position of the hand. Furthermore, it could be shown which motions of the hand would most likely strain a particular ligament. It could be shown that the variations in the force patterns originate mainly from variations in the zero‐force lengths, and from variations in the force‐strain relationship between specimens. These variations might result from real differences between individuals as well as from artifacts introduced by the test method. Uncontrolled effects of frozen storage and unintentional preconditioning might negatively influence the variation. Because these artifacts are present from the initiation of the contribution of ligament forces to the kinematics of the wrist joint, which is very consistent between specimens, is of minor importance.