In Vitro Evaluation of Biocompatibility of Orthodontic Miniscrews

OBJECTIVES: Removable miniscrews are successfully used as compliance-independent anchorage devices in orthodontics. Although the clinical use of miniscrews has been investigate on a large scale, little is known about their biocompatibility. Since oral environment characteristics, including pH, can affect corrosion resistance and influence the release of metal ions, the aim of this study was to evaluate the cytotoxic effect of orthodontic miniscrews in different pH conditions.

METHODS: The cytotoxicity of four miniscrews for orthodontic anchorage was studied. The samples, including miniscrews of stainless steel, titanium grade IV and titanium grade V, were immersed in saline solution (0,9% NaCl) at pH 4 and 7 for 1, 3, 7, 14, 21, 28 and 84 days. The ratio between the sample weight and the volume of extraction solution was 0,1 mg/ml. Human osteogenic sarcoma cells (U2OS), permanent human keratinocytes (HaCat) and primary human gingival fibroblasts (HGF) were exposed to eluates and cell viability was measured by MTT assay and was compared to untreated cells. The results were expressed as the mean +/- SD and were analysed by Mann-Whitney U-test (p<0.05).

RESULTS: When exposed to eluates obtained at pH 7, the cells showed an even greater viability than untreated cells, suggesting a proliferative response to a small amount of released ions. On the contrary, the results revealed a statistically significant decrease in U2OS, HaCat and HGF viability after exposure to eluates obtained at pH 4. Among the cell lines tested, HGF showed the most significant decrease of mitochondrial activity. Titanium grade V miniscrews caused the most toxic effects when immersed at pH 4, more than stainless steel devices.

CONCLUSIONS: The results suggested that the four miniscrews are biocompatible. Our experiments with eluates obtained at pH 4 caused significant cell viability reductions. Interestingly, different cell lines can produce different responses to miniscrew eluates.