Hyaluronic acid–collagen network interactions during the dynamic compression and recovery of cartilage

Abstract

A compression cell designed to fit inside an NMR spectrometer was used to investigate (i) the in situ dynamic strain response and structural changes of the internal pore network, and (ii) the diffusion and flow of interstitial water, in full thickness cartilage samples as they were mechanically deformed under a constant compressive load (pressure) and then allowed to recover (swell again) when the load was removed. Selective enzymatic digestion of the collagen fibril network and the glycopolysaccharide hyaluronic acid (HA) was performed to mimic some of the structural and compositional changes associated with osteoarthritis. Digestion of collagen gave rise to mechanical 'dynamic softening' and-perhaps more importantly-nearly complete loss in the ability to recover through swelling, both effects due to the disruption of the hierarchical structure and fibril interconnectivity in the collagen network which adversely affects its ability to deform reversibly and to properly regulate the pressurization and resulting rate and direction of interstitial fluid flow. In contrast, digestion of HA inside the collagen pore network caused the cartilage to 'dynamically stiffen' which is attributed to the decrease in the osmotic (entropic) pressure of the digested HA molecules confined in the cartilage pores that causes the network to contract and thereby become less permeable to flow. These digestioninduced changes in cartilage's properties reveal a complex relationship between the molecular weight and concentration of the HA in the interstitial fluid, and the structure and properties of the collagen fibril pore network, and provide new insights into how changes in either could influence the onset and progression of osteoarthritis.

Stick-slip friction and wear of articular joints

“…The disruption of the fibril network and softening of cartilage have been observed at the onset and progression of OA (20,21), which could also be induced by applying type II collagenase (41,42). Recent studies (10) indicate that enzymatic digestion of collagen causes drastic disruption of the complex microstructure and stability of the pore matrix, jeopardizing the cartilage's ability to control the transport of interstitial fluid through diffusion and/or flow, and inhibits efficient hydrodynamic lubrication. Fig.…”

“…Recent studies showed that physisorbed HA layers do not have a beneficial effect on lubrication, whereas chemically grafted and/or crosslinked HA layers exhibit excellent wear protection against surfaces, even when sheared at high pressures (∼20 MPa), regardless of the high friction coefficient of μ = 0.15-0.3 (26,27). Experiments involving selective digestion of HA from articular cartilage have shown that HA is mechanically trapped at the cartilage interface by the constricted collagen pore network (9) and that digestion of HA induces a decrease in the porosity and an increase in the stiffness of articular cartilage (10). The molecular mass of the HA is known to decrease (from ∼3-4 down to 0.5 MDa) with age (18) and/or the progression of arthritis (16,17).…”

“…3). We and others (10,41) have recently found that HA digestion significantly stiffens cartilage, which could have an impact on stick-slip behavior.…”

“…To act as a well-functioning protective layer, HA should have a high enough molecular mass to both be trapped in the porous structure of the cartilage and entangle lubricin (LUB) for efficient boundary lubrication (9,10). After digestion, the length of the HA chains is greatly shortened, and they lose the ability to become trapped in the porous structure of the cartilage.…”

“…Many different experimental setups have been proposed for measuring frictional forces between cartilage surfaces (9)(10)(11)(12). Most of these setups can be schematically represented by a mechanical equivalent depicted in Fig.…”