Heterogeneous nanomechanical properties of superficial and zonal regions of articular cartilage of the rabbit proximal radius condyle by atomic force microscopy

Articular chondrocytes have been thought to reside in a homogenous matrix. The physical characteristics of the intercellular matrix of articular cartilage are not well characterized, especially at a nanoscopic scale. The present work tested the hypothesis that the nanomechanical properties of the intercellular matrices of articular cartilage in both the articulating surface and various cellular zones are non-homogeneous. Nanoindentation by atomic force microscopy was applied to the geometric center of the medial, lateral and groove regions of the superficial zone of the rabbit proximal radius cartilage, and then the intercellular matrices of chondrocytes from the superficial to calcifying zones in 40 microm increments. The elastic modulus of the articular surface of the medial condyle (1.46+/-0.11 MPa) was significantly higher than the lateral condyle (1.18+/-0.10 MPa), and the groove (0.96+/-0.07 MPa). There is a significant gradient increase in Young's moduli from the superficial zone (0.52+/-0.05 MPa) to calcifying zone (1.69+/-0.12 MPa). Thus, the nanomechanical properties of the intercellular matrices of the articulating surface are region-specific and likely related to articular function. Heterogeneous biophysical properties of intercellular matrices along the depth from the superficial to calcifying zones suggest that chondrocytes likely reside in a heterogeneous matrix.