Enhanced osteoconductivity of sodium‐substituted hydroxyapatite by system instability

Abstract

The effect of substituting sodium for calcium on enhanced osteoconductivity of hydroxyapatite was newly investigated. Sodium-substituted hydroxyapatite was synthesized by reacting calcium hydroxide and phosphoric acid with sodium nitrate followed by sintering. As a control, pure hydroxyapatite was prepared under identical conditions, but without the addition of sodium nitrate. Substitution of calcium with sodium in hydroxyapatite produced the structural vacancies for carbonate ion from phosphate site and hydrogen ion from hydroxide site of hydroxyapatite after sintering. The total system energy of sodium-substituted hydroxyapatite with structural defects calculated by ab initio methods based on quantum mechanics was much higher than that of hydroxyapatite, suggesting that the sodium-substituted hydroxyapatite was energetically less stable compared with hydroxyapatite. Indeed, sodium-substituted hydroxyapatite exhibited higher dissolution behavior of constituent elements of hydroxyapatite in simulated body fluid (SBF) and Tris-buffered deionized water compared with hydroxyapatite, which directly affected low-crystalline hydroxyl-carbonate apatite forming capacity by increasing the degree of apatite supersaturation in SBF. Actually, sodium-substituted hydroxyapatite exhibited markedly improved low-crystalline hydroxyl-carbonate apatite forming capacity in SBF and noticeably higher osteoconductivity 4 weeks after implantation in calvarial defects of New Zealand white rabbits compared with hydroxyapatite. In addition, there were no statistically significant differences between hydroxyapatite and sodium-substituted hydroxyapatite on cytotoxicity as determined by BCA assay. Taken together, these results indicate that sodium-substituted hydroxyapatite with structural defects has promising potential for use as a bone grafting material due to its enhanced osteoconductivity compared with hydroxyapatite.

Strontium substituted biomimetic calcium phosphate system derived from cuttlefish bone

“…When the Na concentration in the treatment solutions increased, the formation of HA as a residual material was inhibited. Additionally, as reported by Sang Cho et al () Na‐substituted HA exhibited higher osteoconductivity compared to nonsubstituted HA in vivo.…”

In vitro cytotoxicity and ion release of multi‐ion doped hydroxyapatite

“…Similarly, anionic dopants such as by F − , Cl − , CO 3 − , PO 4 3− , and CO 3 2− replace OH − . Sang Cho et al reported that Na‐HA improves osteoconductivity in rabbits due to enhanced dissolution of HA at the bone interface leading to better bioactivity compared to undoped HA. Bose et al reported that Mg doped HA (Mg‐HA) affects osteoblasts’ activity and accelerates bone ingrowth in to these doped HA implants.…”

Is there a relationship between solubility and resorbability of different calcium phosphate phases in vitro ?

“…Hydroxyapatite (HAP), the chemically pure version of the mineral component of bone, however, is typified by impractically low resorption rates [1–3]. Reduction of the particle size [4], an increase in porosity [5] and introduction of ionic substitutions, such as magnesium [6], sodium [7], fluorine [8] and/or carbonate [9], so as to mimic the composition and microstructure of biological apatite, have presented approaches to resolving this issue, though with limited success. Namely, even the least sparingly soluble calcium-deficient apatites usually resorb slower than the new bone tissue formation rate [10], whereas HAP with high porosity is weak even to compression and not suitable for load-bearing applications [11].…”