Can magnetization transfer magnetic resonance imaging follow proteoglycan depletion in articular cartilage?

In this study we determined the efficiency of magnetization transfer magnetic resonance imaging (MT-MRI) to differentiate native and enzymatically degraded cartilage, using bovine sesamoid bones from the metacarpophalangeal joint as a model system. Gradual proteoglycan (PG) depletion was achieved by increasing incubation periods with testicular hyaluronidase. For native cartilage a Ms/Mo ratio of 0.303 +/- 0.09 (mean +/- SEM) was measured. Biochemically determined PG diminution up to 50% correlated strongly (r = 0.953) with changes in the Ms/Mo ratio. Further PG loss is not reflected in an equally drastic Ms/Mo increase, whereas subsequent treatment of PG-depleted cartilage samples with collagenase led to an additional rise in the Ms/Mo ratio. Proteoglycan depletion and the beginning destruction of the collagen structure were also assessed histochemically. Our study confirms that collagen contributes to the baseline MT effect observed in articular cartilage. However, the changes in the MT ratio in gradually PG-depleted cartilage with a largely intact collagen network indicate that PG contributes to the MT effect as well. Therefore MT-MRI might become a sensitive technique for the monitoring of subtle degradational changes in articular cartilage, the still inaccessible process in osteoarthritis.