Fourier transform infrared imaging and MR microscopy studies detect compositional and structural changes in cartilage in a rabbit model of osteoarthritis

Assessment of subtle changes in proteoglycan (PG) and collagen, the primary macromolecular components of cartilage, which is critical for diagnosis of the early stages of osteoarthritis (OA), has so far remained a challenge. In this study we induced osteoarthritic cartilage changes in a rabbit model by ligament transection and medial meniscectomy and monitored disease progression by infrared fiber optic probe (IFOP) spectroscopy, Fourier transform infrared imaging spectroscopy (FT-IRIS), and magnetic resonance imaging (MRI) microscopy. IFOP studies combined with chemometric partial least-squares analysis enabled us to monitor progressive cartilage surface changes from two to twelve weeks post-surgery. FT-IRIS studies of histological sections of femoral condyle cartilage revealed that compared with control cartilage the OA cartilage had significantly reduced PG content 2 and 4 weeks post-surgery, collagen fibril orientation changes 2 and 4 weeks post-surgery, and changes in collagen integrity 2 and 10 weeks post-surgery, but no significant changes in collagen content at any time. MR microscopy studies revealed reduced fixed charge density (FCD), indicative of reduced PG content, in the OA cartilage, compared with controls, 4 weeks post-surgery. A non-significant trend toward higher apparent MT exchange rate, k(m), was also found in the OA cartilage at this time point, suggesting changes in collagen structural features. These two MR findings for FCD and k(m) parallel the FT-IRIS findings of reduced PG content and altered collagen integrity, respectively. MR microscopy studies of the cartilage at the 12-week time point also found a trend toward longer T-2 values and reduced anisotropy in the deep zone of the OA cartilage, consistent with increased hydration and less ordered collagen. These studies reveal that FT-IRIS and MR microscopy provide complementary data on compositional changes in articular cartilage in the early stages of osteoarthritic degradation.