Adaptive mechanically controlled lubrication mechanism found in articular joints

TLDR

Articular cartilage functions as a highly efficient water‑based tribological system that maintains low friction and wear over a lifetime, relying on a synergistic combination of multiple lubrication mechanisms, including the abundant hyaluronic acid (HA) and its cross‑linked complex with lubricin (LUB) that is essential for wear prevention. The study investigates the role of hyaluronic acid in cartilage lubrication, emphasizing that optimal function requires both HA and lubricin. Friction experiments on porcine cartilage show that, under compression, the HA‑LUB complex becomes mechanically trapped by the collagen pore network, acting as an effective boundary lubricant that slightly reduces friction and, more importantly, eliminates wear damage.

Abstract

Articular cartilage is a highly efficacious water-based tribological system that is optimized to provide low friction and wear protection at both low and high loads (pressures) and sliding velocities that must last over a lifetime. Although many different lubrication mechanisms have been proposed, it is becoming increasingly apparent that the tribological performance of cartilage cannot be attributed to a single mechanism acting alone but on the synergistic action of multiple "modes" of lubrication that are adapted to provide optimum lubrication as the normal loads, shear stresses, and rates change. Hyaluronic acid (HA) is abundant in cartilage and synovial fluid and widely thought to play a principal role in joint lubrication although this role remains unclear. HA is also known to complex readily with the glycoprotein lubricin (LUB) to form a cross-linked network that has also been shown to be critical to the wear prevention mechanism of joints. Friction experiments on porcine cartilage using the surface forces apparatus, and enzymatic digestion, reveal an "adaptive" role for an HA-LUB complex whereby, under compression, nominally free HA diffusing out of the cartilage becomes mechanically, i.e., physically, trapped at the interface by the increasingly constricted collagen pore network. The mechanically trapped HA-LUB complex now acts as an effective (chemically bound) "boundary lubricant"--reducing the friction force slightly but, more importantly, eliminating wear damage to the rubbing/shearing surfaces. This paper focuses on the contribution of HA in cartilage lubrication; however, the system as a whole requires both HA and LUB to function optimally under all conditions.