Abstract
Hyaluronan, lubricin and phospholipids, molecules ubiquitous in synovial joints, such as hips and knees, have separately been invoked as the lubricants responsible for the remarkable lubrication of articular cartilage; but alone, these molecules cannot explain the extremely low friction at the high pressures of such joints. We find that surface-anchored hyaluronan molecules complex synergistically with phosphatidylcholine lipids present in joints to form a boundary lubricating layer, which, with coefficient of friction μ≈0.001 at pressures to over 100 atm, has a frictional behaviour resembling that of articular cartilage in the major joints. Our findings point to a scenario where each of the molecules has a different role but must act together with the others: hyaluronan, anchored at the outer surface of articular cartilage by lubricin molecules, complexes with joint phosphatidylcholines to provide the extreme lubrication of synovial joints via the hydration–lubrication mechanism.
Highlights
Hyaluronan, lubricin and phospholipids, molecules ubiquitous in synovial joints, such as hips and knees, have separately been invoked as the lubricants responsible for the remarkable lubrication of articular cartilage; but alone, these molecules cannot explain the extremely low friction at the high pressures of such joints
Hydrodynamic effects have been considered as a lubrication mechanism[7,8,10,11], but it is likely that a mixed regime including both fluid-film and boundary lubrication operates[12], and the crucial issue concerns the nature of the boundary layer at the cartilage surface[12,13,14]
Cleaved mica sheets were mounted and calibrated in a surface force balance (SFB, see Methods), following which HA was attached via avidin– biotin chemistry, and the surfaces incubated with dipalmitoylphosphatidylcholine (DPPC) introduced into solution in the form of small unilamellar vesicles (SUVs, designated DPPC-single-unilamellar vesicles (SUVs)) (Methods)
Summary
Hyaluronan, lubricin and phospholipids, molecules ubiquitous in synovial joints, such as hips and knees, have separately been invoked as the lubricants responsible for the remarkable lubrication of articular cartilage; but alone, these molecules cannot explain the extremely low friction at the high pressures of such joints. The articular cartilage layers coating the major synovial joints such as hips or knees are remarkable constructs They support a wide range of stresses and impacts[1,2] but, in particular, cartilage surfaces sliding past each other exhibit extremely low levels of friction under physiologically high pressure values (some researchers reporting friction coefficients (m) down to 0.001 In this study we attach HA to a surface—to resemble its configuration at the outer cartilage surface10,21,30,32—and find that it complexes with phosphatidylcholines (PCs), ubiquitous in synovial joints[28], to form robust boundary layers These layers act synergistically to provide the low friction (mE0.001) characteristic of cartilage[3,33], at the highest physiological pressures, and contrast with surface-attached HA on its own, which leads to considerably higher friction. Our results point to a scenario where hyaluronan, phosphatidylcholines and lubricin, each with a very different role, act together to provide the extreme boundary lubrication in articulating joints
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