Quantitative and qualitative analysis of joint stiffness in normal subjects and in patients with connective tissue diseases.

Rheology is the study of the deformation and the flow of matter. In the present communication this science is applied to the human joint, its motion, and the forces resisting joint motion. The techniques used to measure joint stiffness have long been used by physicists, physical chemists, and engineers in studying the stiffness of a wide range of materials. Such studies have been of particular use in two areas: They have provided a quantitative measure of stiffness in precise physical terms, and they have defined qualitatively the different parameters contributing to total stiffness. This latter type of analysis has been important, for example, in studies of the cross-linking in elastic polymers. Both types of study have been employed in this investigation. Rheological techniques have been used to measure alterations in joint stiffness produced by physiological changes, pathological disorders, and therapeutic methods, and it is expected that a detailed study of the parameters contributing to total stiffness will give information on alterations in the materials of which joints are made.