Cell death after cartilage impact occurs around matrix cracks

The damage from rapid high energy impacts to cartilage may contribute to the development of osteoarthritis (OA). Understanding how and when cells are damaged during and after the impact may provide insight into how these lesions progress. Mature bovine articular cartilage on the intact patella was impacted with a flat impacter to 53 MPa in 250 ms. Cell viability was determined by culturing the cartilage with nitroblue tetrazolium for 18 h or for 4 days in medium containing 5% serum before labeling (5-day sample) and compared to adjacent, non-impacted tissue as viable cells per area. There was a decrease in viable cell density only in specimens with macroscopic cracks and the loss was localized primarily near matrix cracks, which were in the upper 25% of the tissue. This was confirmed using confocal microscopy with a fluorescent live/dead assay, using 5'-chloromethylfluorescein diacetate and propidium iodide. Cell viability in the impacted regions distant from visible cracks was no different than the non-impacted control. At 5 days, viable cell density decreased in the surface layer in both the control and impacted tissue, but there was no additional impact-related change. In summary, cell death after the impaction of cartilage on bone occurred around impact induced cracks, but not in impacted areas without cracks. If true in vivo, early stabilization of the damaged area may prevent late sequelae that lead to OA.