Characterisation of NiTi orthodontic archwires characteristic functional properties

The aim of this paper was to analyse the characteristic properties of different commercially available NiTi archwires (diameter 0.014 ''). Characteristic properties were determined by using DSC analysis for identification of phase temperatures and a tensile test for obtaining the mechanical properties, while semi-quantitative EDS analysis was used for their chemical composition. Analysis of phase temperatures showed that nickel- titanium orthodontic archwires, in the austenitic phase, were exhibiting superelastic effect in the oral environment (T=37 degrees C). The uniaxial stress-strain curves showed different values for the beginning and the end of loading due to the stress and transformation range. The investigation of the chemical composition showed an equiatomic NiTi alloy. The aim of the first phase of biocompatibility assessment of Ni-Ti archwires was to investigate the potential cytotoxicity on a model of rat thymocytes, where a direct contact exists between the archwire and the cells. The test was chosen because of the high sensitivity of thymocytes to proapoptotic stimuli and because the test includes the culture of non-adherent, non-proliferating cells, such as thymocytes, which is much more convenient for the evaluation of a material's cytotoxicity in direct contact, compared to the adherent cells such as fibroblasts. The experiment was designed to cultivate the Ni-Ti archwire samples with rat thymocytes, using different surface-over-volume ratios of the archwires in complete medium. The cytotoxic effect was evaluated by measuring the apoptosis. The aim of the second phase of the study was to investigate further the cytotoxic effects of Ni-Ti archwires in the short-term (24h) and long-term (48h) cultures of rat's thymocytes, which are highly sensitive to pro-apoptotic stimuli, where a direct contact exists between archwires and the cells.