A primary complication often thought to be associated with blunt impact onto the knee, especially from motor vehicle accidents, is post-traumatic arthritis. This research describes the results of blunt impact trauma administered to the flexed hind limb (knee) of anesthetized rabbits. The animals were closely monitored in cages for as long as 12 months post-impact. At predetermined times the animals were euthanized. The degree of posttraumatic osteoarthritis was determined using biomechanical, biochemical and histological methods. A mathematical model, based on elastic finite element theory, was developed to help understand the mechanisms of impact-induced fissures to the cartilage and damages to the underlying bone. Corresponding experiments have also been performed on human cadavers.

In low intensity insults minimal surface fissuring developed and the articular cartilage became soft and permeable. It retumed to nearly normal levels at one year. Histologically the only change noted was disorganization of the chondrocytes, suggesting a swelling response that was measured as increased water content. In severe levels of insult the surface of the cartilage was often fissured, tissue water was lost, and the tissue again was softer and more permeable than contralateral controls. Biomechanically the tissue retumed to normal at 6 days post-impact. At 3 months the tissue may have even stiffened above controls with a subsequent degeneration thereafter. All impacted cartilage lost proteoglycans post-impact. Cellular cloning, etc. was noted at 1 year after severe insults. The finite element analysis suggested that surface fissures were induced via excessive tensile stress in areas adjacent to the contact pressure. High compressive and shear stresses were also developed at the zone of calcified cartilage and underlying subchondral bone. Interestingly, in human cadaver experiments low levels of contact pressure were recorded even in cases of patellar fracture. This appears due to the load distributing capacity of an intact layer of cartilage in the patello-femoral joint. The results of in vivo animal experiments, cadaver experiments and mathematical modeling are being used in our research to help understand and predict the pathogenesis of a posttraumatic osteoarthritis.