Imaging structural and mechanical properties of articular cartilage using optical polarization tractography

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Osteoarthritis (OA) is an extremely common joint disease, which affects more than one-third of all adults in the USA. Although the entire joint compartments are involved, OA is considered as a cartilage disease. Articular cartilage is a thin tissue covering the end of bones in the diarthrodial joints and plays a crucial role in providing a frictionless articulation. In spite of the harsh mechanical environment, cartilage has an amazingly long life due to its unique structure and composition. Cartilage is composed of ~80% water and ~20% solid matrix that mainly consists of collagen fibers and proteoglycans. Collagen degeneration is often an early symptom in OA. Since the fiber structure governs normal functionality in cartilage, the disease progression leads to impaired mechanical functions. Hence, an effective imaging technology that can visualize the collagen organization and its effects on cartilage mechanical properties will help to understand the sophisticated structure-function relationship in cartilage. Polarized light macroscopy (PLM) has been broadly utilized for collagen assessment; however, it requires thin, sectioned samples and thus remains a destructive technology. We introduced a nondestructive alternative to PLM for cartilage imaging using optical polarization tractography (OPT). OPT improved visualization and characterization of the zonal structure in cartilage by calculating the depth-resolved local birefringence and fiber orientation. We demonstrated that parametric imaging can be implemented using multiple complementary tissue contrasts obtained in OPT including surface roughness, birefringence, and fiber dispersion. We showed that parametric OPT imaging provided a morphometric evaluation of collagen damage in human OA cartilage samples. Because OPT can accurately quantify tissue optical birefringence, it can reveal the higher level of complexity in collagen architecture of cartilage. Our multi-incident OPT based biaxial birefringence measurement provided strong evidence of the existence of a leaf-like structure in cartilage. Furthermore, we expanded the capability of OPT technology by developing a method that can simultaneously image the fiber organization and mechanical responses in cartilage. This new method enabled a precise characterization of the zonal structural and mechanical responses to unconfined compressive and directional shear loading. We discovered that the upper part of the radial zone plays a critical role in absorbing compression-induced deformation in cartilage. Young's modulus in cartilage was strongly correlated with the optical birefringence. In the shear test, we found a remarkably higher shear modulus in the radial zone when the sample was sheared along the fibers. In summary, this dissertation research developed new OPT based imaging methods that can fully characterize the collagen organization and its responses during mechanical loading. This new technology has a great potential for nondestructive structural and functional imaging in articular cartilage.