Peri-implantitis is a chronic inflammatory process with advanced bone loss and impaired healing potential. For peri-implantitis treatment, tissue engineering can be applied to enhance bone regeneration of peri-implant defects. This study aimed to evaluate ex vivo bone morphogenetic protein 2 (BMP2) gene delivery using canine periodontal ligament stem cells (PDLSCs) for regeneration of peri-implantitis defects. Canine PDLSCs were transduced with adenoviral vectors containing BMP2 (BMP2/PDLSCs). After peri-implantitis was induced by ligature placement in six beagle dogs, regenerative procedures were performed; hydroxyapatite (HA) particles and collagen gel with autologous canine PDLSCs (PDLSC group) or BMP2/PDLSCs (BMP/PDLSC group) or without cells (control group) were grafted into the defects and covered by an absorbable membrane. Three months later, the animals were sacrificed. In vitro, BMP2/PDLSCs showed similar levels of stem cell properties to PDLSCs, such as colony-forming efficiency and expression of MSC markers STRO-1 and CD 146. BMP2/PDLSCs produced BMP-2 until day 21 at a concentration of 4-8 ng/mL. In vivo, the BMP2/PDLSC group showed significantly more new bone formation and re-osseointegration in peri-implantitis defects compared to the other groups. In conclusion, ex vivo BMP2 gene delivery using PDLSCs enhanced new bone formation and re-osseointegration in peri-implantitis defects.

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.

In the synthesis of hydroxyapatite powders by spray pyrolysis, control of the particle size was investigated by varying the initial concentration of the precursor solution and the pyrolysis temperature. Calcium phosphate solutions (Ca/P ratio of 1.67) with a range of concentrations from 0.1 to 2.0 mol/L were prepared by dissolving calcium nitrate tetrahydrate and diammonium hydrogen phosphate in deionized water and subsequently adding nitric acid. Hydroxyapatite powders were then synthesized by spray pyrolysis at 900°C and at 1500°C, using these calcium phosphate precursor solutions, under the fixed carrier gas flow rate of 10 L/min. The particle size decreased as the precursor concentration decreased and the spray pyrolysis temperature increased. Sinterability tests conducted at 1100°C for 1 h showed that the smaller and denser the particles were, the higher the relative densities were of sintered hydroxyapatite disks formed from these particles. The practical implication of these results is that highly sinterable small and dense hydroxyapatite particles can be synthesized by means of spray pyrolysis using a low-concentration precursor solution and a high pyrolysis temperature under a fixed carrier gas flow rate.

Although the immediate peri-implantitis model is challenging for regeneration, it may be able to replace the conventional model to study regenerative peri-implantitis treatment due to its short experimental time and similar defect configuration.

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