In vivo morphometry and functional analysis of human articular cartilage with quantitative magnetic resonance imaging--from image to data, from data to theory.

Analyses of form-function relationships and disease processes in human articular cartilage necessitate in vivo assessment of cartilage morphology and deformational behavior. MR imaging and advanced digital post-processing techniques have opened novel possibilities for quantitative analysis of cartilage morphology, structure, and function in health and disease. This article reviews work on three-dimensional post-processing of MR image data of articular cartilage, summarizing studies on the accuracy and precision of quantitative analyses in human joints. It presents normative values on cartilage volume, thickness, and joint surface areas in the human knee, and describes the correlation between different joints and joint surfaces as well as their association with gender, body dimensions, and age. The article summarizes ongoing work on functional adaptation of articular cartilage to mechanical loading, analyses of in situ cartilage deformation in intact joints in vivo and in vitro, and the quantitative evaluation of cartilage tissue loss in osteoarthritis. We describe evolving techniques for assessment of the structural/biochemical composition of articular cartilage, and discuss future perspectives of quantitative cartilage imaging in the context of joint mechanics, mechano-adaptation, epidemiology, and osteoarthritis research. Specifically, we show that fat-suppressed gradient echo sequences permit valid analysis of cartilage morphology, both in healthy and severely osteoarthritic joints, as well as highly reproducible measurements (CV%=1 to 3% in the knee, and 2 to 10% in the ankle). Relatively small differences in cartilage morphology exist between both limbs of the same person (approximately 5%), but large differences between individuals (CV% approximately 20%). Men display only slightly thicker cartilage then women (approximately 10%), but significantly larger joint surface areas (approximately 25%), even when accounting for differences in body weight and height. Weight and height represent relatively poor predictors of cartilage thickness (r2 <15%), but muscle cross section areas display more promising correlations (r2 >40%). The level of physical exercise (sportive activity) does not account for interindividual differences in cartilage thickness. The thickness appears to decrease slightly in the elderly--in particular in women, even in the absence of osteoarthritic cartilage lesions. Strenuous physical exercises (e.g., knee bends) cause a 6% patellar cartilage deformation in young individuals, but significantly less deformation in elderly men and women (<3%). The time required for full recovery after exercise (fluid flow back into the matrix) is relatively long (approximately 90 min). Static in situ compression of femoropatellar cartilage with 150% body weight produces large deformations after 4 h (approximately 30% volume change), but only very little deformation during the first minutes of loading. Quantitative analyses of magnetization transfer and proton density hold promise for biochemical evaluation of articular cartilage, and are shown to be related to the deformational behavior of the cartilage. Application of these techniques to larger cohorts of patients in epidemiological and clinical studies will establish the role of quantitative cartilage imaging not only in basic research on form-function relationships of articular cartilage, but also in clinical research and management of osteoarthritis.