The macromolecular structure and mechanical properties of articular cartilage are interrelated and known to vary topographically in the human knee joint. To investigate the potential of delayed gadolinium-enhanced MRI of cartilage (dGEMRIC), T 1 , and T 2 mapping to elucidate these differences, full-thickness cartilage disks were prepared from six anatomical locations in nonarthritic human knee joints (N ؍ 13). Young's modulus and the dynamic modulus at 1 Hz were determined with the use of unconfined compression tests, followed by quantitative MRI measurements at 9.4 Tesla. Mechanical tests revealed reproducible, statistically significant differences in moduli between the patella and the medial/ lateral femoral condyles. Typically, femoral cartilage showed higher Young's (>1.0 MPa) and dynamic (>8 MPa) moduli than tibial or patellar cartilage (Young's modulus <0.9 MPa, dynamic modulus <8 MPa). dGEMRIC moderately reproduced the topographical variation in moduli. Additionally, T 1 , T 2 , and dGEMRIC revealed topographical differences that were not registered me-
These preliminary results suggest that, according to T2 measurements, ACT repair tissue at 10-15 months differs from normal cartilage and probably lacks the preferential collagen arrangement of normal cartilage, while according to dGEMRIC a varying degree of proteoglycan replenishment takes place. Combining these two quantitative magnetic resonance imaging techniques enables a more comprehensive characterization of cartilage repair than before.
Chondroitin sulfate is the major constituent of cartilage. Inadequate sulfate availability results in the production of undersulfated proteoglycans. In osteoarthritis, there is a net loss of articular cartilage proteoglycans. Theoretically, it is possible that during the progress of disease undersulfated glycosaminoglycans are synthesized producing proteoglycans with poorer biological properties. In this study, we tested whether in early human osteoarthritic articular cartilage (Mankin's score of 2 and 3) or more advanced disease (Mankin's score over 3), there are proteoglycans that contain a higher relative amount of nonsulfated chondroitin disaccharide isomer in their chondroitin sulfate chains by analyzing the molar ratios of chondroitin sulfate disaccharide isoforms with fluorophore-assisted carbohydrate electrophoresis. Our results indicated that the nonsulfated disaccharide of chondroitin sulfate formed in average only 1-2% of the total chondroitin sulfate. More important, the molar ratio of nonsulfated disaccharide did not appear to be increased in the osteoarthritic articular cartilage. We conclude that undersulfation of articular cartilage proteoglycans is not present in the human osteoarthritic joint.
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