Biochemistry of Joint Hypermobility

In pathology there has always been an urge to find a “unifying concept” and a “basic defect” in a group of conditions with clinical similarities. So it has been with joint hypermobility. However this is not always a legitimate goal, and, except at a very superficial level, does not help us to understand the molecular basis of abnormal joint laxity. The reason for this lies in the diverse aetiologies of joint hypermobility, which span the inherited and the acquired, with both qualitative and quantitative differences in joint tissues. “But surely it’s a collagen defect?” is a common and understandable response — indeed one which has influenced the direction of research into the molecular basis of hypermobility. Why should this be so? Firstly, common sense tells us that the collagenous components of the joint connective tissues dictate the mechanical limitations of movements; after all, we are all aware of the physical attributes of tendons, ligaments and joint capsules — notably their tensile strength and high elastic modulus (resistance to deformation). Secondly, joint laxity can undoubtedly occur when these tissues are physically weakened — either surgically or by the erosion accompanying, for instance, rheumatoid arthritis. Thirdly, some genetic defects of collagen biosynthesis have been identified and associated with joint hypermobility — notably variants of the Ehlers—Danlos syndrome (EDS) and osteogenesis imperfecta (see Chap. 10). Fourthly, pharmacological interference with collagen biosynthesis, for instance with d-penicillamine, can increase joint laxity.