Effects of neutral sodium hydrogen phosphate on setting reaction and mechanical strength of hydroxyapatite putty

The setting reaction and mechanical strength in terms of diametral tensile strength (DTS) of hydroxyapatite (HAP) putty made of tetracalcium phosphate, dicalcium phosphate anhydrous, and neutral sodium hydrogen phosphate (Na_1.8H_1.2PO₄) solution containing 8 wt % sodium alginate were evaluated as a function of the Na_1.8H_1.2PO₄ concentration. In one condition, HAP putty was placed in an incubator kept at 37°C and 100% relative humidity. In the other condition, immediately after mixing HAP putty was immersed in serum kept at 37°C. Longer setting times and lower DTS values were observed when HAP putty was immersed in serum regardless of the Na_1.8H_1.2PO₄ concentration. The setting times of the HAP putty in both conditions became shorter with an increase in the Na_1.8H_1.2PO₄ concentration, reaching approximately 7-13 min when the Na_1.8H_1.2PO₄ concentration was 0.6 mol/L or higher. The DTS value of HAP putty was relatively constant (10 MPa) regardless of the Na_1.8H_1.2PO₄ concentration (0.2-1.0 mol/L) when HAP putty was kept in an incubator. In contrast, when HAP putty was immersed in serum, the DTS value was dependent on the Na_1.8H_1.2PO₄ concentration. It increased with the Na_1.8H_1.2PO₄ concentration and reached approximately 5 MPa when the Na_1.8H_1.2PO₄ concentration was 0.6 mol/L, after which it showed a relatively constant DTS value. We therefore would recommend a HAP putty that uses 0.6 mol/L Na_1.8H_1.2PO₄ since at that concentration the putty's setting time (approximately 10 min) is proper for clinical use and it shows good DTS value (approximately 5 MPa) even when it is immersed in serum immediately after mixing.