To examine the relative susceptibility of cartilage and meniscus tissues to mechanical injury by applying a single, controlled overload and observing cellular, biochemical, and mechanical changes.
Cartilage and meniscus tissue explants in radial confinement were subjected to a range of injury by indenting to 40% strain at three different strain rates: 0.5%/s (slow), 5%/s (medium), or 50%/s (fast). Following injury, samples were cultured for either 1 or 9 days. Explants were assayed for cell metabolic activity, water content, and sulfated glycosaminoglycan (sGAG) content. Mechanical properties of explants were determined in torsional shear and unconfined compression. Conditioned medium was assayed for sGAG and lactate dehydrogenase (LDH) release.
Peak injury force increased with strain rate but both tissues displayed little to no macroscopic damage. Cell metabolism was lowest in medium and fast groups on day 1. Cell lysis increased with peak injury force and loading rate in both tissues. In contrast, sGAG content and release did not significantly vary with loading rate. Additionally, mechanical properties did not significantly vary with loading rate in either tissue.
By use of a custom confinement chamber, large peak forces were obtained without macroscopic destruction of the explants. At the loads achieved in this studied, cell damage was induced without detectable physical or compositional changes. These results indicate that sub-failure injury can induce biologic damage that may not be readily detected and could be an early event in osteoarthritis genesis.