Deformation of articular cartilage collagen structure under static and cyclic loading

Relatively little is known about the morphology of articular cartilage under conditions of normal use, yet a more profound knowledge is both critical to the understanding of cartilage function and helpful for the validation of tissue-engineered cartilage. In this study, the deformation of the articular cartilage of the tibial plateau under compressive static and cyclic loading is characterized. Whole knee joints of rabbits were loaded ex vivo while the knee was held statically or allowed to move against resistance. Load magnitudes of quadriceps were maintained at either three (high) or one (low) times body weight for 30 minutes. For cyclic loading, the tibia was flexed between 70 and 150 degrees relative to the femur at 1 Hz with either a cyclic or constant force. The recovery of cartilage after unloading was examined for each loading condition. At the end of the loading, specimens were cryofixed while under load, freeze-substituted, and prepared for scanning electron microscopy. Morphological examination demonstrated significantly higher deformation of the collagen structure throughout all cartilage zones under static loading conditions compared with cyclic loading conditions in which deformation was limited to the superficial regions. The minimum thickness of the cartilage that remained after loading was dependent on the magnitude of load and was significantly smaller with static loads (54% of the thickness of the unloaded controls) than after cyclic loading or constant-force 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 loading conditions: in the superficial half of the articular cartilage after static loading and in the superficial quarter after cyclic loading. Complete recovery of all deformation occurred within 30 minutes but was significantly faster after cyclic loading. These data suggest that the structure of the collagen of articular cartilage exhibits a zone-specific deformation that is dependent on the magnitude and type of load.