Abstract

The major mammalian joints such as hips or knees are uniquely-efficient tribological systems. In health, the articular cartilage coating them presents the most remarkably lubricated surfaces known in nature. They can slide past each other with friction coefficients down to 0.001 or lower under pressures of 100 atmospheres or more at sliding velocities from rest to several cm/second (shear rates ca. 1 – 106 sec-1), and they do this over a lifetime. No man-made surfaces can approach this. But when this lubrication breaks down, the result can be degradation of the articular cartilage, and onset of osteoarthritis (OA), a debilitating joint disease affecting millions (some 6M in the UK alone). An understanding of the molecular origins and the physics of the very efficient lubrication at the cartilage surface is thus essential for better treatments of OA.

The talk will describe recent breakthoughs in our understanding of cartilage friction based on the emergent hydration lubrication paradigm. In this mechanism, water molecules in hydration shells just a few Å thick can massively reduce frictional dissipation between sliding surfaces. The roles of lubricin, hyaluronic acid and phospholipids – crucial lubricating components of the joints – in enabling such lubrication is considered. In particular, the effect of the low friction not only on the wear and tear of the cartilage but especially on shear of the chondrocyte cells embedded within it, and the ensuing mechanotransductive effects on gene regulation, is emphasized as part of the underlying picture for the well-being of the joints arising from their tribological properties