The Characterization and Distribution of Magnesium Whitlockite Crystals in Human Articular Cartilage

The occurrence of crystals (previously termed 'cuboid crystals': 50-500nmsize range) not apparent by light microscopy, in human articular cartilage has been confirmed by transmission electron microscopy of tissue prepared by various techniques, including anhydrous and cryo processing. Earlier reports of such crystal deposition had been limited to osteoarthritic and elderly femoral head articular cartilage. In this study crystals have been reported in articular cartilage across an age range from five to ninety two years in normal and osteoarthritic tissue from a variety of joint sites. The distribution of crystal deposition within articular cartilage was described both qualitatively and quantitatively; normal femoral head tissue was investigated in most detail. Over 90 % of crystals were commonly deposited within the first 50μm below the articular surface; crystals appeared either in a band parallel to the surface or in a pericellular distribution. In deeper zones crystal deposition was restricted to pericellular distribution, and areas of chondrocyte necrosis. Quantitative analysis of crystal deposition distribution in articular cartilage at sites around the femoral head revealed a significantly greater deposition in the superior (zenith) region than the inferior (infrafoveal) region. Elemental analysis of crystals confirmed a calcium, phosphorus and magnesium content. It also demonstrated no variation in the mean calcium to phosphorus ratio with crystal size, specimen age, or between normal and osteoarthritic specimens. A crystal isolation technique involving collagenase digestion, centrifugation and sodium hypochlorite treatment was developed, enabling crystal characterization by electron and x-ray diffraction. Crystals were identified as magnesium whitlockite; the first report of this mineral in articular cartilage. The mode of formation and role of these crystals remain unknown, although histological and histochemical investigations revealed a consistent association with intramatrical lipid, containing a phospholipid component. The results of this study are most tenable with a concept of opportunistic crystal deposition