The object of this study was to isolate the effect of loading rate alone on fracture tolerance of the human knee joint. Paired experiments were conducted on eight pairs of isolated cadaver knees impacted with a rigid interface to approximately 5 kN at a high (5 ms to peak) or low (50 ms to peak) rate of loading. The study also examined some chronic effects of loading rate on "subfracture" injuries in an animal. Thirty-four rabbits were subjected to a "subfracture" knee load at the same rates as used in the human studies. The study documented an opposite effect than that expected based on 70's experiments with seated cadavers. There was an increase in the number of gross fractures and occult microfractures in high versus low rate of loading experiments. A similar effect was also seen in the "subfracture" chronic animal experiments, which showed relatively more degradative change in the mechanical properties of cartilage following high versus low rate of loading experiments. There was also a significant increase in subchondral bone thickening underlying cartilage and increased fissuring of cartilage in high versus low rate of loading experiments. The study suggests a relative decrease in tolerance of the knee at high versus low rates of loading in acute experiments with human cadavers and in the chronic setting with animals.
Abstract