Objective. To determine the inflammatory potential of basic calcium phosphate (BCP) crystals, which have been identified in human joints.Methods. Hydroxyapatite, carbonate apatite, whitlockite, and octacalcium phosphate crystals were injected in rat air pouches. Volume and cellularity of the exudate were measured. Physicochemical properties of the injected BCP crystals were determined, and correlations with the magnitude of induced inflammatory responses were sought.Results. Significant differences were observed among the volumes and white blood cell (WBC) counts of the pouch exudates, based on the various crystal types used to induce inflammation. A strong correlation was demonstrated between the specific surface (SS) area of the injected crystals and the area under the curve for induced WBC count versus time (R2 = 0.88, P = 0.05). Conclusion. The inflammatory potential of BCP crystals appeared to vary according to crystal features. SS area and the Ca:P ratio (which correlates with crystal solubility) influenced inflammatory properties. These results could explain the variable clinical consequences of BCP deposits, and must be taken into account in the choice of apatite ceramics for use as biomaterials. This correlation was observed forVarious basic calcium phosphatc (BCP) crystals (i.c., synthcsizcd under basic pH, as opposed to acidic CP crystals such as brushite) can be identified in human joints, where tolerance of them appears highly variable. Crystal dcposits are usually asymptomatic, but have been considered responsible for acute periarticular (1-3) or articular (43) inflammation, subchondral ( 6 ) or diaphyseal (7) bone erosions, and destructive arthropathies (8).

Labile environments of carbonate and HPO4 present in synthetic poorly crystalline apatites, analogous to bone mineral crystals, can be easily and reversibly exchanged. Such reactions depend on the maturation stage of the apatites and are related to both the short range order and organization of the crystals and their specific surface area. Similar reactions may also occur in bone mineral altering its properties and function as an ion reservoir in vivo. INTRODUCHONPoorly crystalline apatites are the major constituent of mineralized tissues of vertebrates. Their reactivity governs several biological processes such as the adsorption of mineral ions, matrix and serum proteins, adhesion of cells (example osteoclasts), and the subsequent resorption of bone. The reactivity of poorly crystalline apatites and consequently their biological functions are directly related to the structure, short range order and compositional characteristics of the crystals.Poorly crystalline apatites contain significant amounts of carbonate and phosphate groups in non-apatitic environments which can be demonstrated by FTIR and 31-P MAS-NMR. Many of the CO32-and HPO42-groups are labile and in non-apatitic environments. These are present at higher concentrations in newly formed crystals and in the bone of young animals, and gradually diminish with time (maturation). The aim of this work was to study the role of non-apatitic labile environments in the reactivity of poorly crystalline apatites. MATERIALS AND METHODSPoorly crystalline apatites were prepared according to the methods published in previous papers 1. Their composition was determined by chemical analysis 1 of calcium, phosphate and carbonate ions. The exchange experiments were made by exposing the poorly crystalline apatites (100 mg) in ammonium bicarbonate or ammonium hydrogen phosphate 1 molar (10 ml) . The samples were then centrifuged, washed with distilled water and lyophilized. The environments of mineral ions were analyzed by FTIR spectroscopy

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