Regional quantitative analysis of knee cartilage in a population study using MRI and model based correspondences

Degeneration and loss of articular cartilage in osteoarthritis (OA) is difficult to measure, because changes are small and localised. We present a method that uses statistical shape models of the knee bones to define an anatomically consistent frame of reference across a population, providing sensitive measures of cartilage morphology in anatomically equivalent regions of interest. Bone and cartilage were manually segmented from magnetic resonance images (MRI) of volunteers´ knees. Dense correspondences were defined across all subjects by constructing minimum description length (MDL) statistical shape models of the bones. Regions of interest were manually delineated on the mean bone shapes provided by the models, and propagated to each individual in an anatomically consistent manner, using the model-based correspondences. We show that this approach results in precise measurements that can be used to detect small localised changes in cartilage thickness. Results are reported for an OA study, in which significant focal loss of cartilage was detected over 6 months in a cohort of just 31 patients