Objective:​ Articular chondrocytes respond to chemical and mechanical signals depending on their zone of origin with respect to distance from the tissue surface. However, little is known of the zonal variations in cellular mechanical properties in cartilage. The goal of this study was to determine the zonal variations in the elastic and viscoelastic properties of porcine chondrocytes using atomic force microscopy (AFM), and to validate this method against micropipette aspiration.

Methods:​ A theoretical solution for stress relaxation of a viscoelastic, incompressible, isotropic surface indented with a hard, spherical indenter (5 μm diameter) was derived and fit to experimental stress-relaxation data for AFM indentation of chondrocytes isolated from the superficial or middle/deep zones of cartilage.

Results:​ The instantaneous moduli of chondrocytes were 0.55 ± 0.23 kPa for superficial cells (S) and 0.29 ± 0.14 kPa for middle/deep cells (M/D) (P < 0.0001), and the relaxed moduli were 0.31 ± 0.15 kPa (S) and 0.17 ± 0.09 kPa (M/D) (P < 0.0001). The apparent viscosities were 1.15 ± 0.66 kPa s (S) and 0.61 ± 0.69 kPa-s (M/D) (P < 0.0001). Results from the micropipette aspiration test showed similar cell moduli but higher apparent viscosities, indicating that mechanical properties measured by these two techniques are similar.

Conclusion:​ Our findings suggest that chondrocyte biomechanical properties differ significantly with the zone of origin, consistent with previous studies showing zonal differences in chondrocyte biosynthetic activity and gene expression. Given the versatility and dynamic testing capabilities of AFM, the ability to conduct stress-relaxation measurements using this technique may provide further insight into the viscoelastic properties of isolated cells.