Magnesium whitlockite, a calcium phosphate crystal of special interest in pathology

Whitlockite (in fact magnesium whitlockite) is a calcium orthophosphate crystal in which, in biological conditions, magnesium is partly substituted for calcium. Identified in X-ray or electron diffraction patterns, it occurs in physiological or pathological conditions at extra or intratissular sites, mainly in tissues of non-epithelial origin. In a range of pathological calcifications investigated by X-ray diffraction, we noted that whitlockite appeared to be frequently associated with apatite, particularly in "dystrophic calcifications" of tuberculous origin. These personal observations could be correlated with documented data in oral pathology (dental calculus, salivary stones, and dental caries). Whitlockite deposits have also been reported in non-infectious conditions, such as in aortic media, cartilage, and bone tissue. Whereas the formation of both apatite and magnesium whitlockite appears to be caused by the binding of their constituting ions with proteolipids, magnesium inhibits apatite originating from amorphous calcium phosphate to the benefit of whitlockite formation. Possibly, the development of magnesium whitlockite may provide an interesting marker for magnesium metabolism. Further studies linking histology to crystallography might relate the crystal to issues, such as tuberculous calcifications or diseases of bone tissue, and might be useful for potential diagnostic orientation.