Deformation of articular cartilage collagen structure under static and cyclic loading

Relatively little is known about the morphology of articular cartilage unde r conditions of normal use, yet a more profound knowledge is both critical to the understanding of cartilage function and helpful for the validation o f tissue-engineered cartilage. In this study, the deformation of the articu lar cartilage of the tibial plateau under compressive static and cyclic loa ding is characterized. Whole knee joints of rabbits were loaded ex vivo whi le the knee was held statically or allowed to move against resistance. Load magnitudes of quadriceps were maintained at either three (high) or one (lo w) times body weight for 30 minutes. For cyclic loading, the tibia was flex ed between 70 and 150 degrees relative to the femur at 1 Hz with either a c yclic or constant force. The recovery of cartilage after unloading was exam ined for each loading condition. At the end of the loading, specimens were cryofixed while under load, freeze-substituted, and prepared for scanning e lectron microscopy. Morphological examination demonstrated significantly hi gher deformation of the collagen structure throughout all cartilage zones u nder static loading conditions compared with cyclic loading conditions in w hich deformation was limited to the superficial regions. The minimum thickn ess of the cartilage that remained after loading was dependent on the magni tude of load and was significantly smaller with static loads (54% of the th ickness of the unloaded controls) than after cyclic loading or constant-for ce cyclic loading (78 or 66% of the thickness of the unloaded controls, p < 0.05). Acute bending of the collagen fibers was observed under both loadin g conditions: in the superficial half of the articular cartilage after stat ic loading and in the superficial quarter after cyclic loading. Complete re covery of all deformation occurred within 30 minutes but was significantly faster after cyclic loading. These data suggest that the structure of the c ollagen of articular cartilage exhibits a zone-specific deformation that is dependent on the magnitude and type of load.