The objective of this study was to find a correlation between magnetic resonance (MR) appearance and biochemical composition of the normal articular cartilage by comparing the laminar aspects with the distribution of the two principal matrix components: proteoglycans and collagen.
T2-weighted MR microimages of porcine cartilage-bone plugs, excised from both the habitually loaded and habitually unloaded regions of the proximal end of the humerus, were obtained using a spin-echo sequence. Proteoglycans (PGs) were monitored by histology and by measuring the uronate and the sulfur content of the tissue; a histologic method and the chemical determination of hydroxyproline were used for the evaluation of the collagen content.
The 'loaded' cartilage exhibited the expected MR laminar appearance whereas the 'unloaded' tissue appeared to be more homogeneous. The PG content in the 'loaded' cartilage, was found to be 2.4 times higher than in the habitually unloaded tissue, exhibiting an increasing trend from the articular surface to the bone. In the 'unloaded' cartilage the uronate distribution was more uniform with a higher concentration in the intermediate zone. The mean collagen content of both cartilage regions was found to be about 39% of the tissue dry weight. Histology and hydroxyproline distribution pattern showed that collagen was particularly concentrated at the surface and in a central zone of the 'loaded' cartilage whereas in the 'unloaded' tissue collagen was evident only at the surface. In accordance with the collagen distribution, transverse relaxation (T2) times in 'loaded' cartilage showed a minimum value at the articular surface and another minimum in a central region. On the contrary, the average T2 value of the 'unloaded' tissue was high at the surface and decreased rapidly in the deeper zones.
These results demonstrate that the MR appearance of articular cartilage correlates with the collagen content, but not with that of PGs, of the different zones. Other matrix components might, however, influence the MR appearance by contributing to the macromolecular organization of the tissue.