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

Analyses of form-function relationships and disease processes in human arti cular cartilage necessitate in vivo assessment of cartilage morphology and deformational behavior. MR imaging and advanced digital post-processing tec hniques have opened novel possibilities for quantitative analysis of cartil age morphology, structure, and function in health and disease. This article reviews work on three-dimensional post-processing of MR image data of arti cular cartilage, summarizing studies on the accuracy and precision of quant itative analyses in human joints. It presents normative values on cartilage volume, thickness, and joint surface areas in the human knee, and describe s the correlation between different joints and joint surfaces as well as th eir association with gender, body dimensions, and age. The article summariz es ongoing work on functional adaptation of articular cartilage to mechanic al loading, analyses of in situ cartilage deformation in intact joints in v ivo and in vitro, and the quantitative evaluation of cartilage tissue loss in osteoarthritis. We describe evolving techniques for assessment of the st ructural/biochemical composition of articular cartilage, and discuss future perspectives of quantitative cartilage imaging in the context of joint mec hanics, mechano-adaptation, epidemiology, and osteoarthritis research. Spec ifically, we show that fat-suppressed gradient echo sequences permit valid analysis of cartilage morphology, both in healthy and severely osteoarthrit ic 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 (similar to5%), but large differences between individuals (CV% similar to 20%). Men display on ly slightly thicker cartilage then women (similar to 10%), but significantl y larger joint surface areas (similar to 25%), even when accounting for dif ferences in body weight and height. Weight and height represent relatively poor predictors of cartilage thickness (r(2) <15%), but muscle cross sectio n areas display more promising correlations (r(2) >40%). The level of physi cal exercise (sportive activity) does not account for interindividual diffe rences in cartilage thickness. The thickness appears to decrease slightly i n the elderly - in particular in women, even in the absence of osteoarthrit ic cartilage lesions. Strenuous physical exercises (e.g., knee bends) cause a 6% patellar cartilage deformation in young individuals, but significantl y less deformation in elderly men and women (<3%). The time required for fu ll recovery after exercise (fluid flow back into the matrix) is relatively long (<similar to>90 min). Static in situ compression of femoropatellar car tilage with 150% body weight produces large deformations after 4 h (similar to 30% volume change), but only very little deformation during the first m inutes of loading. Quantitative analyses of magnetization transfer and prot on 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 epidemiol ogical and clinical studies will establish the role of quantitative cartila ge imaging not only in basic research on form-function relationships of art icular cartilage, but also in clinical research and management of osteoarth ritis.