Articular cartilage surface changes following immobilization of the rat knee joint. A semiquantitative scanning...

Normal articular cartilage surfaces are not flat and smooth, but are contoured with various degrees of roughness. We applied the articular surface classification system developed by Jurvelin to evaluate contour and surface quality changes in rat patellae after varying periods of knee joint immobilization. In our study, one knee joint was immobilized in each of 20 rats, while the contralateral joint served as a free-moving control. Scanning electron-microscopic montages were constructed and the entire cartilage surface was subdivided by contour and surface quality areas. The total articular surface area and all contour and surface quality subclass areas were then calculated for both joints in each rat. Numerous studies have demonstrated that joint immobilization induces degenerative changes in articular cartilage. We found a correlation between the duration of immobilization and changes in the measured area of contour and surface quality subclasses. There was a statistically significant decrease of the even surface area in the weight-bearing, central region of the patellae. This corresponded to a statistically significant increase in the slightly uneven articular surface area usually found just peripheral to this weight-bearing region. There was also a less pronounced (but not statistically significant) decrease in the smooth surface area with a corresponding increase in the rough surface area of the patellae following immobilization. Closer examination of these surfaces revealed that the superficial cartilage layer in the central, weight-bearing areas had less ground substance than other areas. In addition, there appeared to be two different fiber diameters in the superficial cartilage layer of the rat patellae. These were predominantly collagen fibers (142.3 +/- 31.5 nm in diameter) generally running parallel to the long axis of the patella, occasionally random in orientation, and sometimes presenting on the cartilage surface as collagen tips and much more scarce fibrils (20.0 +/- 4.8 nm in diameter) randomly crossing the larger collagen fibers.