Impact loading and functional tissue engineering of articular cartilage
Natoli, Roman M.
Doctor of Philosophy
This thesis presents two advances for alleviating the problem of articular cartilage degeneration: mitigating degradative changes that follow mechanically induced injuries and growing functional neo-cartilage for diseased tissue replacement. Experiments demonstrate that cartilage subjected to a single, non-surface disrupting 1.1 J (Low) impact experiences sufficient degeneration over 4 weeks to become functionally equivalent to tissue subjected to a single, surface disrupting 2.8 J (High) impact. By 24 hrs post High impact, cell death and sulfated glycosaminoglycan (sGAG) release increased, changes in gene expression distinguished injured from adjacent tissue, and compressive stiffness decreased. In contrast, Low impacted tissue did not show decreased compressive stiffness until 4 weeks, revealing that Low impacted tissue experiences a delayed biological response. Post-injury treatment with the polymer P188, growth factor IGF-I, or matrix metalloproteinase inhibitor doxycycline partially ameliorated cell death and sGAG loss, two detrimental changes that occurred following either Low or High impact. With 1 week of treatment after Low impact, P188 reduced cell death 75% and IGF-I decreased sGAG release 49%. Following High impact, doxycycline treatment reduced 1 and 2 week sGAG release by 30% and 38%, respectively. As a novel method for engineering functional replacement tissue to use in cases of established disease, the GAG degrading enzyme chondroitinase ABC (C-ABC) improved the tensile integrity of articular cartilage constructs grown with a scaffold-less approach. C-ABC application increased ultimate tensile strength and tensile stiffness, reaching values of 1.4 and 3.4 MPa, respectively. Moreover, construct collagen concentration was ∼22% by wet weight. Though C-ABC temporarily depleted sGAG, by 6 weeks no significant differences in compressive stiffness remained. Furthermore, chondrocyte phenotype was maintained, as constructs contained collagen type II, but not collagen type I. Decorin decreased following C-ABC treatment, but recovered during subsequent culture. The known ability of decorin to control collagen fibrillogenesis suggests a putative mechanism for C-ABC's effects. Diseased articular cartilage heals poorly. For patients, the last resort is total joint replacement, though its associated morbidity and the limited lifespan of its results drive the need for alternate treatment strategies. Decreasing degradative changes post-injury and increasing functional properties of engineered cartilage are two significant improvements.