Chronic softening of cartilage without thickening of underlying bone in a joint trauma model

We have recently developed a trauma model to study degradation of the rabbi t patello-femoral joint. Our current working hypothesis is that alterations in retropatellar cartilage and underlying bone in our model are initiated independently by acute overstresses developed in each tissue during blunt i nsult to the joint, and that the processes of chronic degradation in each t issue are not related in a mechanical sense. The current study was conducte d in an attempt to help validate our hypothesis by impacting the patello-fe moral joint with a padded interface. Based upon earlier human cadaver exper iments, we believe this would reduce the acute overstresses in patellar bon e while the stresses developed in the overlying retropatellar cartilage wou ld be sufficient enough to initiate a chronic softening of the tissue. Twen ty-four animals received an impact to the patello-femoral joint and were sa crificed at either 0, 4.5, or 12 months post-insult. Three acute animals we re impacted to develop a simplified computational model to estimate the str esses in joint tissues. The study showed there was a significant softening of the retropatellar cartilage at 4.5 and 12 months post-trauma, compared t o unimpacted controls. However, no thickening of the underlying subchondral bone was documented at any timepoint. This was consistent with a reduction of stress in the bone compared to earlier studies, which document thickene d subchondral bone post-insult at the same applied impact load. In conclusi on, this study helped validate our hypothesis by documenting chronic soften ing of cartilage without remodeling of the underlying subchondral bone. Fur thermore, this study, along with our earlier studies, suggest that impact l oad alone, which is currently used by the automobile industry to certify ne w automobiles, is not a good predictor of chronic injuries to a diarthrodia l joint, and that simply the addition of padding to impact interfaces may n ot be adequate to protect occupants from chronic injuries. (C) 2000 Elsevie r Science Ltd. All rights reserved.