Study on morphology and lubrication of articular cartilage surface with atomic force microscopy

Morphological and structural characteristics of natural articular cartilage surface were examined in a physiological solution by using an atomic force microscopy (AFM), and the lubricating ability of a cartilage surface was investigated in a reciprocating sliding tests. Freshly excised cartilage specimens were immediately immersed in phosphate-buffered saline (PBS), and observed in the liquid cell of the AFM filled with PBS by the fluid tapping method. AFM images revealed that the surface of articular cartilage was very smooth, with gentle mounds of 1-2 mu m height, and covered with a gel layer under the physiological wet condition. Chondroitinase ABC could digest the gel layer and expose the fibrous networks. Therefore, it could be considered that the main constituent of this surface layer was proteoglycans. Results of sliding tests showed that the friction between articular cartilage and glass plate was initially very low, and increased gradually with repetition of sliding. AFM images of rubbed cartilage surface showed that the surface gel layer was preserved for the Low friction stage, on the other hand, fibrous structure appeared for the higher friction stage. These results indicated that the surface gel layer play an important role on the natural synovial joint lubrication under the severe operating condition.