Imaging the physical and morphological properties of a multi‐zone young articular cartilage at microscopic resolution

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Abstract
Purpose: To characterize a number of physical and morphologic properties of young articular cartilage. These properties include the anisotropy of T2relaxation, optical retardation, orientation of the collagen fibrils, total thickness of the tissue, number of histologic sub‐zones in the tissue, width of individual sub‐zones, and correlation between the depths of the local T2maxima and the local retardation minima.Materials and Methods: Microscopic magnetic resonance imaging (μMRI) and polarized light microscopy (PLM) were used to examine three side‐by‐side specimens from a humeral head of a three‐month‐old beagle that exhibited a unique topographic heterogeneity from three‐zones centrally to multi‐zone peripherally.Results: The centrally located specimen showed that the collagen fibrils across the tissue depth have a pattern of the classic three histologic sub‐zones (tangential, transitional, and radial). A much more complicated multi‐zone structure was found in the specimen located peripherally, with a second transitional zone and a second tangential zone located at the deep part of the tissue. We also showed that the orientation of the collagen fibrils that form the cocoon‐shaped territorial matrix surrounding the clusters of chondrocytes can be imaged by our PLM technique.Conclusion: The results from the young animal in this report, together with our observations from older animals, demonstrate that MRI and PLM can be used to study the epiphyseal expansion of cartilage in young animals during its growth and subsequent loss in older animals. An illustrative model for the structure of collagen fibrils in a humeral head is suggested as an extension to the classic three‐zone model for young articular cartilage. J. Magn. Reson. Imaging 2003;17:365–374.