In Situ Studies of Cartilage Microtribology: Roles of Speed and Contact Area

Abstract

The progression of local cartilage surface damage toward early stage osteoarthritis (OA) likely depends on the severity of the damage and its impact on the local lubrication and stress distribution in the surrounding tissue. It is difficult to study the local responses using traditional methods; in-situ microtribological methods are being pursued here as a means to elucidate the mechanical aspects of OA progression. While decades of research have been dedicated to the macrotribological properties of articular cartilage, the microscale response is unclear. An experimental study of healthy cartilage microtribology was undertaken to assess the physiological relevance of a microscale friction probe. Normal forces were on the orderof50 mN. Sliding speed varied from 0 to 5 mm/s, and two probes radii, 0.8 mm and 3.2 mm, were used in the study. In-situ measurements of the indentation depth into the cartilage enabled calculations of contact area, effective elastic modulus, elastic and fluid normal force contributions, and the interfacial friction coefficient. This work resulted in the following findings: 1) at high sliding speed (V=1–5 mm/s), the friction coefficient was low (μ = 0.025) and insensitive to probe radius (0.8 mm 3.2 mm) despite the 4-folddifference in the resulting contact areas; 2) The contact area was a strong function of the probe radius and sliding speed; 3) the friction coefficient was proportional to contact area when sliding speed varied from 0.05mm/s-5mm/s; 4) the fluid load support was greater than 85% for all sliding conditions (0% fluid support when V=0) and was insensitive to both probe radius and sliding speed. The findings were consistent with the adhesive theory of friction; as speed increased, increased effective hardness reduced the area of solid-solid contact which subsequently reduced the friction force. Where the severity of the sliding conditions dominates the wear and degradation of typical engineering tribomaterials, the results suggest that joint motion is actually beneficial for maintaining low matrix stresses, low contact areas, and effective lubrication for the fluid-saturated porous cartilage tissue. Further, the results demonstrated effective pressurization and lubrication beneath single asperity microscale contacts. With carefully designed experimental conditions, local friction probes can facilitate more fundamental studies of cartilage lubrication, friction and wear, and potentially add important insights into the mechanical mechanisms of OA.

Experimental characterization of biphasic materials using rate-controlled Hertzian indentation

“…The effective contact modulus, E c , includes contributions from fluid and solid stresses, while E c0 , a material property, is the equilibrium contact modulus of the tissue [15]. Inserting (1) into (2) and rearranging gives the following equation for the effective contact modulus of a biphasic half-space entirely in terms of controllable mechanical conditions and measurable material properties: $$E_c=\frac{E_{c0}}{1-\left(\frac{E_t}{E_t+E_{c0}}·\frac{\stackrel{.}{\delta }·R}{\stackrel{.}{\delta }·R+E_{c0}·k}\right)}$$…”

Mesh Size Control of Polymer Fluctuation Lubrication in Gemini Hydrogels

“…A central theme in cartilage lubrication is that the cartilage permeability and the shear thinning viscosity of natural synovial fluid work together to provide a thin film of fluid lubrication that separates the surfaces and fully supports the applied load [3,8,11,[19][20][21][22][23][24]. The microstructure of cartilage is nearly universally described as a complex, porous, elastic-solid, and it is often treated as a biphasic material permeated with water [7].…”

An investigation into the contact between soft elastic and poroelastic bodies rotating under load

“…However fluid flow is not permitted into or out of the material in this model, under flooded conditions this could be a possible means for even further improvements to the tribology of the system. Experiments have identified the benefit of such flows in cartilage which boost the lubrication performance, recently this has been referred to as tribological rehydration [9][10][11]. This model indicates the potential means by which this is facilitated, in which flow across the contacting interface would be permitted and lubrication theory used to govern fluid flow outside the contact [18].…”