The effect of crystallinity on strength development of alpha-TCP bone substitutes.

Alpha phase tricalcium phosphates (alpha-TCP) were produced using a solid-state reaction method and milled for various periods of time. The resulting four materials were alpha-TCPs, ranging in crystalline content. Powders were exposed to X-ray diffraction for material identification as well as for use in crystallinity and purity calculations. Powder particle size was investigated using laser diffraction. Materials were mixed with 2.5% Na(2)HPO(4) solution to initiate the hydration of alpha-TCP to calcium-deficient hydroxyapatite (CDHA). Isothermal calorimetry was performed to observe thermal response of the powders over a period of time. During the reaction process, at various time points up to 216 h, the material was compression tested to observe strength development. Materials proved to be predominantly alpha phase, while amorphous content determined by XRD varied. Reactivity, as measured by isothermal calorimetry, varied with crystallinity of the alpha-TCP powder. Speed of strength development did not change except for the most finely ground powder. In addition, crystal size of the CDHA was changed only in the product formed from the most highly ground material. It is proposed that increasing reactivity of alpha-TCP cements does not result in a corresponding increase in rate of strength development until there is sufficient supersaturation to produce significant crystal nucleation.

Study of the reactivity and in vitro bioactivity of Sr-substituted alpha-TCP cements.

In this study the effect of strontium substitution on the hydrolysis of alpha -tricalcium phosphate (alpha-TCP) toward the formation of calcium deficient hydroxyapatite (CDHA) was investigated. For that purpose substituted alpha-TCP powders with 1, 5 and 10 mol% Sr substitution for Ca were synthesized by reacting at 1500 degrees C stoichiometric amounts of CaCO(3), SrCO(3), and Ca(2)P(2)O(7), followed by rapid quenching in air. XRD analysis of the powders revealed the presence of alpha-TCP (traces of beta-TCP) with enlarged unit cell volume at increased Sr contents, indicating the incorporation of Sr in the crystal structure. Strontium was also incorporated in the apatite phase as revealed by XRD analysis of the set cements. The hydrolysis of milled alpha-SrTCP powders and a pure alpha-TCP (control) was monitored by isothermal calorimetry and the compressive strength of set cements was tested. The results showed a decrease in the reactivity with increasing Sr content and similar final mechanical strength within the Sr series, though lower than the control. The in vitro bioactivity of the set cements after soaking in simulated body fluid for 4 weeks was also tested. The formation of a bone-like apatite layer on the surface of the set cements indicated a potential in vivo bioactivity.

Hydration characteristics of alpha-tricalcium phosphates: Comparison of preparation routes.

Alpha tricalcium phosphate ( á -TCP) cement powders were obtained by solid state reaction and milling (M1) and by precipitation from aqueous solution followed by heating (M2). The materials were hydrated to form calcium-deficient hydroxyapatite (CDHA) with a 2.5 wt% solution of Na2 HPO4 (liquid to powder ratio = 0.34 ml/g, temperature = 37.5 degrees C) and subjected to isothermal calorimetry, mechanical compression tests, X-ray powder diffraction, at various times during hydration, as well as scanning electron microscopy (SEM), laser diffraction and gas adsorption. The particle characteristics of the two powders were similar, but M2 exhibited two reaction events in the thermal power curve, while M1 showed a single event. Both reaction events were attributed to á -TCP dissolution and CDHA recipitation. The minimum in the reaction rate response of M2 was probably due to the formation of a passivating product layer. No such layer was formed on the milled M1 due to its rougher surfaces. Both preparations reached a compressive strength of 30-40 MPa after 24 hr.

Correlating crystallinity and reactivity in an alpha-tricalcium phosphate.

In this study, the effect of how variant milling time affects material characteristics of alpha phase tricalcium phosphate powder (alpha-TCP) was studied. Two alpha-TCP batches were separated in small lots and milled for various times for up to 4 h. The resulting milled lots were characterized by measuring their crystallinity, particle size, specific surface area, thermal stability, and heat released during hydration. Mechanical treatment was seen to greatly increase the alpha-TCP X-ray amorphous fraction and heat release during hydration, almost independently of alpha-TCP particle size and specific surface area. Therefore, the results suggest that the formation and presence of an X-ray amorphous phase in the alpha-TCP powder greatly contribute to its reactivity. The exotherm of the powders increases from 103 to 238 kJ/mol after milling.