Load-induced Adaptation of the Site-specific Zonal Properties of Tibial Articular Cartilage by Microscopic MRI (µMRI)

Articular cartilage is a thin layer of load-bearing tissue in joint, being compressed constantly in our daily activities. Compression (loading) would cause complex changes in cartilage, due to its unique molecular composition and structural organization (1). An extensive project is ongoing in our lab, which uses external loading to force cartilage to reach a new equilibrium with its environment when the tissue is being imaged by µMRI. We hypothesize that healthy and lesioned cartilage will have very different responses to loading.

Aims: To map the site-specific zonal properties of articular cartilage over canine medial tibia as the function of external loading by µMRI.

Materials and Methods: T2 and T1 relaxation maps and GAG (glycosaminoglycan) maps from a total of 70 specimens were obtained without and with mechanical loading at 17.6 µm depth resolution. In addition, bulk mechanical modulus and water content were measured from the tissue.

Results: Complex relationships were found in the tissue properties as the function of external loading topographically. Cartilage in the meniscus-covered area had lower water content, a thinner total thickness, a higher GAG concentration, and a higher mechanical modulus, when comparing with the corresponding values on the meniscus-uncovered area. These trends are mutually consistent and also consistent with the findings in the literature (2). In addition, T2 values at the magic angle are lower in the covered area, which agrees with the trend in µMRI GAG concentration. It was found that the tissue parameters in the superficial zone changed more profoundly than the same properties in the radial zone. The tissue parameters in the meniscus-covered areas changed differently when comparing with the same parameters in the uncovered areas.

Conclusions: This project confirms that the load-induced adaptation in the molecular distribution and structure of cartilage are both depth-dependent and topographically distributed. Since a softened cartilage deforms more under a fixed loading, quantitative knowledge of strain and site dependent zonal variations of cartilage in knee joints can provide valuable insights to the evaluation of cartilage degradation and malfunction. Such detailed knowledge of the strain-modified MRI tissue parameters would be great benefit for future diagnosis and possible treatment of osteoarthritis, and ultimately, for the prevention of articular diseases.

Acknowledgements: NIH R01 grants (AR045172, AR052353), J Matyas (University of Calgary) for lesion tissue, XG Qu (Oakland University) for statistical assistance, J Spann (Michigan Resonance Imaging) for contrast agent.

References: (1) J Biomech Eng, 137, 054502 (2015). (2) Connective Tissue Research, 55, 205 (2014).