The function of articular cartilage depends on the interaction between the tissue matrix and the interstitial fluid bound to the proteoglycan molecules. Mechanical loading has been shown to be involved in both the metabolic regulation of chondrocytes and in matrix degeneration. The purpose of the present study was therefore to analyze the deformation, recovery, and fluid flow in human articular cartilage after dynamic loading in vivo. The patellae of 7 volunteers were imaged at physical rest and after performing knee bends, with a specifically optimized fat-suppressed FLASH-3D magnetic resonance (MR) sequence. To measure cartilage deformation, the total volume of the patellar cartilage was determined, employing 3D digital image analysis. Patellar cartilage deformation ranged from 2.4 to 8.6% after 50 knee bends, and from 2.4% to 8.5% after 100 knee bends. Repeated sets of dynamic exercise at intervals of 15 min did not cause further deformation. After 100 knee bends, the cartilage required more than 90 min to recover from loading. The rate of fluid flow during relaxation ranged from 1.1 to 3.5 mm(3)/min (0.08 to 0.22 mm(3)/min per square centimeter of the articular surface) and was highly correlated with the individual degree of deformation after knee bends. The data provide the first quantification of articular cartilage recovery and of the rate of fluid flow between the cartilage matrix and surrounding tissue in intact joints in vivo. Measurement in the living opens the possibility of relating interindividual variations of mechanical cartilage properties to the susceptibility of developing joint failure, to assess the load-partitioning between the fluid phase and solid cartilage matrix during load transfer, and to determine the role of mechanically induced fluid flow in the regulation of the metabolic activity of chondrocytes.

Women displayed smaller cartilage volumes than men, the percentage difference ranging from 19.9% in the patella, to 46.6% in the medial tibia. The gender differences of the cartilage thickness were smaller, ranging from 2.0% in the femoral trochlea to 13.3% in the medial tibia for the mean thickness, and from 4.3% in the medial femoral condyle to 18.3% in the medial tibia for the maximal cartilage thickness. The differences between the cartilage surface areas were similar to those of the volumes, with values ranging from 21.0% in the femur to 33.4% in the lateral tibia. Gender differences could be reduced for cartilage volume and surface area when normalized to body weight and body weight x body height. The study demonstrates significant gender differences in cartilage volume and surface area of men and women, which need to be taken into account when retrospectively estimating articular cartilage loss in patients with symptoms of degenerative joint disease. Differences in cartilage volume are primarily due to differences in joint surface areas (epiphyseal bone size), not to differences in cartilage thickness.

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Objective. Quantitative diagnostic tools for osteoarthritis (OA) are important for evaluating the treatment response to structure-modifying drugs. This study was undertaken to test the technical validity (accuracy) of quantitative magnetic resonance imaging (qMRI) for reliable determination of the total bone interface area, percentage of cartilaginous (denuded) joint surface area, and cartilage thickness in OA.Methods. High-resolution MRIs of femorotibial and patellar cartilage were acquired in 21 patients prior to total knee arthroplasty, using a T1-weighted gradient-echo sequence with water excitation. After segmentation of original bone interface areas (before disease onset) and the actual cartilage layer, the percentages of cartilaginous joint surface area, cartilage thickness, and cartilage volume were determined using proprietary software. During surgery, the patella and the medial and lateral tibia were resected. Results obtained with qMRI were compared with those obtained by direct image analysis of surface area, cartilage thickness, and cartilage volume of the surgically removed tissue.Results. Pairwise differences between results obtained with qMRI and morphologic analysis were ؎4.6% for percentage of cartilaginous surface area, ؎8.9% for cartilage thickness, and ؎9.1% for cartilage volume. Correlation coefficients ranged from 0.92 (thickness) to 0.98 (volume).Conclusion. Quantitative MRI permits technically accurate and differential assessment of increases in eroded joint surface area and reductions in cartilage thickness in OA. The surrogate validity of these parameters requires testing in longitudinal studies. These parameters may be advantageous over determination of cartilage volume alone when diagnosing OA, exploring its progression, or testing responsiveness to new therapies.