Serum-hydroxyapatite interaction in vitro.
In a project to develop hydroxyapatites for bone replacement, biological and synthetic types were prepared at 600 degrees C, ground to 300-600 microns and immersed in a pooled human serum for periods up to 1 month at 4 degrees C to assess material interaction. It was found that the levels of calcium in the serum were reduced at 6 h immersion, followed by an increase to reach maximum at 48 h and then stability up to 1 month. Phosphorus levels showed the opposite behaviour. Both apatites showed similar trends, although higher values were recorded for the synthetic type, suggesting higher activity. Infrared spectral analysis complemented the biochemical values, where the optical densities (O.D.) of phosphate groups were reduced, reflecting the increased phosphorus in serum and denoting leaching. Also, O.D. values of both CO3(2-) and OH- groups were reduced at 10 h, then returned to original levels. Scanning electron microscopy revealed a spongy appearance parallel with reduced O.D. and higher levels of serum Ca2+. At longer periods (48 h) the concentric needles of hydroxyapatite are clearly shown to be deposited on biological apatite. Differences in responses were attributed to their original crystalline structure assessed by X-ray diffraction analysis, as well as pore analysis using a mercury porosimeter.