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Summary: The current criteria used by the automotive industry for predicting joint injury are based on fracture of bone, but clinical studies suggest that chronic diseases such as osteoarthrosis can occur from a single blunt insult without bone fracture. In the current study, blunt insults were delivered to the patellofemoral joints of rabbits without producing bone fractures. Biomechanical and histological studies were performed on joint tissues at various times after insult. The functional integrity of the retropatellar cartilage on the lateral facet was measured with mechanical indentation experiments, and the thickness of the subchondral bone was measured from histological sections. Impacts produced surface lesions on the retropatellar cartilage. The thickness of the subchondral bone in representative animals tended to increase with time after insult, and the bone exhibited significant thickening at 12 months. The overlying cartilage showed signs of degeneration. However, the mechanical stiffness of the cartilage did not change until 12 months after the insult.
Although the number of automotive fatalities is decreasing due to mandated use of seat belts and airbags (11,20), more occupants are reporting musculoskeletal injuries (3,5). In the automotive setting, joint injury is largely based on observable fracture of bone, yet clinical studies reveal cases of early post-traumatic pain and even development of osteoarthrosis without documentation of bone fracture (4,16,18).
Animal studies have been conducted in recent years to better elucidate the relationship between a single blunt insult and the pathogenesis of osteoarthrosis. Thompson et al. (19) observed surface lesions on the retropatellar cartilage and clefts in the zone of calcified cartilage and the subchondral bone plate after a single blunt insult that did not produce a bone fracture. Six months later, changes resembling those of early osteoarthrosis occurred in the cartilage and subchondral bone. Radin et al. (14) documented that changes in bone occur prior to changes in the overlying cartilage following low-level, repetitive loading of the rabbit femorotibial joint. In contrast, a study by Donohue et al. (6) indicated early histological changes in the articular and calcified cartilage after a blunt insult. They hypothesized that the changes indicated softening of the cartilage and increased stress in the underlying bone, producing microcracks and sclerosis that would lead to osteoarthrosis. Armstrong and Mow (1), however, suggested that the...