MECHANICAL-PROPERTIES OF HYDROXYAPATITE AND OH-CARBONATED HYDROXYAPATITE SINGLE-CRYSTALS

Single-crystal hydroxyapatite and OH-carbonated hydroxyapatite have be nding strength much higher than that of dense hydroxyapatite ceramic, indicating potential applicability to a load-bearing biomaterial. Howe ver, the effects of carbonate on the strength are less clear. The obje ctive of the present study was to determine the bending strength and Y oung's modulus of these single crystals with carbonate contents from 0 to 0.62 in CO2 wt%. Three-point bending tests were performed by means of a modified ultramicrohardness tester with a span of 380 mu m and a bending direction <210> of the single crystals. The crystals were bro ken in air, water, and air after immersion in a cell culture medium fo r 3 wks. The average Young's modulus of the single crystals was from 5 4 to 79 GPa. The average bending strength of the single crystals in ai r was 500 +/- 184, 468 +/- 205, 513 +/- 151, and 450 +/- 162 MPa for t hose with 0, 0.09, 0.37, and 0.62 wt% carbonate, respectively. No sign ificant decrease in strength was found for hydroxyapatite single cryst als both in water and after the immersion in the medium. However, the strength of OH-carbonated hydroxyapatite single crystals decreased sig nificantly by 23 to 43% in water in proportion to the carbonate conten t. The strength of single crystals with 0.37 and 0.62 wt% carbonate de creased significantly, even after the immersion in the medium. Therefo re, hydroxyapatite single crystals are superior to OH-carbonated hydro xyapatite single crystals as a biomaterial for a load-bearing purpose.