Little is known about how calcium pyrophosphate dihydrate (CaPPD) crystals form in vivo and give rise to chondrocalcinosis or pseudogout (pyrophosphate arthropathy or calcium pyrophosphate crystal deposition disease). In this study a simple method has been devised to define the conditions necessary for the deposition of crystals in vitro. Crystal formation is monitored by 45Ca in the presence of 1·5 mmol/l Ca and increasing concentrations of inorganic pyrophosphate (PPi) under simulated physiological conditions of pH and ionic strength. Concentrations of PPi required to initiate crystal formation were about 40 mmol/l in the absence and 175 mmol/l in the presence of 0·5 mmol/l Mg2+ at pH 7·4. Less PPi was required at higher pH values. The naturally occurring monoclinic and triclinic forms of CaPPD were produced after prolonged incubation in vitro, but the initial deposits were amorphous or orthorhombic. The physiological significance of these observations is discussed. Since much higher concentrations of PPi are required to form crystals in vitro than are found to occur naturally in synovial fluids from patients with pyrophosphate arthropathy, it is suggested that crystals are more likely to deposit initially within cartilage and that nucleating mechanisms may be important in vivo. Since other workers have observed a slow interconversion of other calcium pyrophosphate crystal forms into monoclinic and triclinic allomorphs under laboratory conditions, the reason why only these 2 forms occur under clinical conditions may reflect the long time available in vivo for the formation of crystals.
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