Heterogeneity of cartilage laminae in MR imaging

The purpose of this study was to investigate the discrepancy in the number of laminae observed in magnetic resonance (MR) images of articular cartilage (the magic angle effect in MRI of cartilage). Microscopic MR imaging (muMRI) experiments were carried out at 14-micrometer pixel resolution on full-depth cartilage-bone plugs from several locations (central, intermediate, and peripheral) on the humeral heads of two young healthy beagles. When the articular surface of the plug was perpendicular to the direction of the magnetic field, the cartilage appeared to have two layers in the plugs from the central locations of the humeral head, three layers in the plugs from the greater tubercle side of the humeral head, and three or five layers in the plugs from the lesser tubercle side. This heterogeneity of cartilage laminae was observed within a single humeral head and was symmetrical about the median plane of the animal. This result suggests that some structural variations related to cartilage structure in various regions of load bearing may cause some unique laminar patterns seen in MRI of cartilage. This novel and new observation may resolve the controversy about whether cartilage appears as two or three layers in MR images. A comprehensive model for the collagen structure over a curved two-dimensional surface of a joint is suggested as a replacement of the classic three-zone model of fiber orientation in collagen. This heterogeneity of cartilage laminae is speculated to be related to the load-bearing status of the tissue in the joint. The ability to visualize such structural heterogeneity is important because of the direct connection between collagen structure and the mechanical characteristics of cartilage.