Synthesis and characterization of porous beta-tricalcium phosphate blocks.

Porous beta-tricalcium phosphate (beta-TCP) blocks with four different macropore sizes (pore larger than 50 microm were synthesized using "calcium phosphate emulsions", and characterized by optical, geometrical, gravimetric, and radiological methods. The reproducibility of the synthesis method was excellent. Moreover, the macropore size could be easily controlled without modifying the microporosity (pore smaller than 50 microm) or the total porosity (microporosity+macroporosity). Based on the initial composition of the blocks and their final apparent density, the microporosity, macroporosity, and the total block porosity were calculated to be close to 21%, 54%, and 75%, respectively. These values were confirmed by microcomputed tomography (microCT). The mean macropore diameters were close to 150, 260, 510 and 1220 microm, as measured optically. Consistently lower values (25% lower) were obtained by microCT, but the linear correlation between microCT and optical method was high (r(2)>0.97). The macropore size distribution calculated from microCT scans appears to be narrow and normally distributed. The very good correlation between the results of the various methods and the possibility to determine the pore size distribution suggest that microCT is an ideal tool to non-destructively characterize macroporous calcium phosphate bone substitutes.

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.

Compositional changes of a dicalcium phosphate dihydrate cement after implantation in sheep.

A hydraulic calcium phosphate cement having dicalcium phosphate dihydrate (DCPD) as end-product of the setting reaction was implanted in a cylindrical defect in the diaphysis of sheep for up to 6 months. The composition of the cement was investigated as a function of time. After setting, the cement composition consisted essentially of a mixture of DCPD and beta-tricalcium phosphate (beta-TCP). In the first few weeks of implantation, the edges of the cement samples became depleted in DCPD, suggesting a selective dissolution of DCPD, possibly due to low pH conditions. The cement resorption at this stage was high. After 8 weeks, the resorption rate slowed down. Simultaneously, a change of the color and density of the cement center was observed. These changes were due to the conversion of DCPD into a poorly crystalline apatite. Precipitation started after 6-8 weeks and progressed rapidly. At 9 weeks, the colored central zone reached its maximal size. The fraction of beta-TCP in the cement was constant at all time. Therefore, this study demonstrates that the resorption rate of DCPD cement is more pronounced as long as DCPD is not transformed in vivo.