Biomechanics and relaxivity for functional imaging of articular cartilage injury and degradation
Osteoarthritis (OA) is a major debilitating health concern and economic burden worldwide, affecting 27 million people in the United States alone. OA often follows tissue injury, and is marked by changes in the structure and biomechanical function of cartilage, including breakdown of extracellular matrix molecules, loss of bulk tissue stiffness, and increase in articular surface friction and wear. Unlike bone and many other tissues, cartilage lacks an intrinsic capacity for regeneration. Advanced OA is typically diagnosed by patient symptoms (e.g. joint pain) and confirmed by radiographic evaluation of joint space narrowing. However, the application of functional imaging to assess cartilage physiology may provide an early diagnosis of joint changes prior to patient symptoms. One such functional imaging modality, magnetic resonance imaging (MRI), may be used to characterize the mechanics of joint cartilage in vitro and in vivo, but it has not yet been applied to evaluate cartilage injury in defined damage models. Here, we studied the changes in MRI-assessed intratissue strain following cartilage injury, and correlate those changes with traditional assessment metrics such as relaxivity, biochemical composition, and microstructure. Osteochondral samples were harvested from the load-bearing region of juvenile bovine knees. Samples were exposed to injurious compressive loading at 100% strain/second and incubated over four weeks. Tissue strain throughout the cartilage interior was measured by displacements under applied loading by MRI (dualMRI) and coregistered to relaxivity measures of T1 and T2. Proteoglycan and collagen content, cartilage microstructure, and cell viability were also assessed by biochemical, histochemical, and microscopy assays. Injurious compressive strain magnitudes of 50% resulted in decreased chondrocyte viability. By three weeks post-injury, dualMRI strains in the compressive loading direction of injured cartilage increased compared to controls, suggesting a regional loss of tissue stiffness. T2 and sample height increased with incubation time. Changes in proteoglycan and collagen content, and microstructure, were also observed to change with incubation time. These finding indicate that dualMRI may be a promising technology to detect and diagnose the early onset of injury-related degeneration compared to conventional techniques like MR relaxivity. The results also indicate the utility and potential for functional imaging to assess disease progression and treatment.
Neu, Purdue University.
Biomedical engineering|Medical imaging|Biomechanics
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