Effect of added NaHCO₃ on the basic properties of apatite cement

Alternation of a composition of set apatite cement (AC) from carbonate-free apatite (CO₃-free AP) to carbonate apatite (CO₃-AP) may accelerate replacement of the set AC with bone because CO₃-AP can be dissolved much faster than CO₃-free AP in the weak acidic solution produced by osteoclasts. In this investigation, NaHCO₃ was added to the AC component and the effects of added NaHCO₃ on cement-setting behavior and on set mass were studied as an initial step for the fabrication of AC, which can be replaced with bone faster than current AC. When NaHCO₃ was added to AC, the resultant set mass contained CO₃. Although not all CO₃ was incorporated in the set AC, the amount of CO₃ incorporated into the set AC increased with the amount of added NaHCO₃. Powder X-ray diffraction analysis and Fourier transform infrared spectrometer measurements revealed the formation of B-type CO₃-AP. The setting time measured in an incubator at 37°C and 100% relative humidity slowed from 30 to 45 min, indicating that NaHCO₃ has an inhibitory effect on AP formation. The diametral tensile strength of the set AC decreased significantly with the addition of NaHCO₃. Scanning electron microscopic observation revealed that fine crystals were formed in the set AC when NaHCO₃ was added to AC. As a result, the crystallinity indices of the set AC measured using X-ray diffraction and Fourier transform infrared spectroscopy decreased with an increase in the amount of added NaHCO₃. The dissolution rate of set AC in weak acid, pH 5.5, increased with the amount of added NaHCO₃. We concluded that the formation of B-type CO₃-AP and the resulting faster dissolution of set AC in weak acidic solution is preferable for the faster replacement of set AC with bone even though the decreased diametral tensile strength value is a shortcoming.