Microscale frictional response of bovine articular cartilage from atomic force microscopy

The objective of this study was to compare micro- and macroscale friction coefficients of bovine articular cartilage. Microscale measurements were performed using standard atomic force microscopy (AFM) techniques, using a 5 μm spherical probe tip. Twenty-four cylindrical osteochondral plugs were harvested in pairs from adjacent positions in six fresh bovine humeral heads (4–6 months old), and divided into two groups for AFM and macroscopic friction measurements. AFM measurements of friction were observed to be time-independent, whereas macroscale measurements demonstrated the well-documented time-dependent increase from a minimum to an equilibrium value. The microscale AFM friction coefficient (μAFM, 0.152±0.079) and macroscale equilibrium friction coefficient (μeq, 0.138±0.036) exhibited no statistical differences (p=0.50), while the macroscale minimum friction coefficient (μmin, 0.004±0.001) was significantly smaller than μeq and μAFM (p<0.0001). Variations in articular surface roughness (Rq=462±216 nm) did not correlate significantly with μAFM, μeq or μmin. The effective compressive modulus determined from AFM indentation tests using a Hertz contact analysis was E*=45.8±18.8 kPa. The main finding of this study is that μAFM is more representative of the macroscale equilibrium friction coefficient, which represents the frictional response in the absence of cartilage interstitial fluid pressurization. These results suggest that AFM measurements may be highly suited for exploring the role of boundary lubricants in diarthrodial joint lubrication independently of the confounding effect of fluid pressurization to provide greater insight into articular cartilage lubrication.