The Effect of Autoclave Cycles on the Mechanical Properties and Surface Roughness of NITI Archwires: Invivo Study.

Three groups of 0.016 x 0.022 inch NiTi archwires (American Orthodontics, Wisconsin, USA) will be investigated in this study. The first group of wires will be investigated in the as-received form. The two other groups will be installed in the patients mouth for 4 weeks, and then retrieved, cleaned using ultrasonic cleaner, dried and packaged. The second group will be autoclaved through the rapid cycle (4 mins, 134oC) and the third group will be autoclaved through the universal cycle (20 mins, 121 oC). The autoclave used in this study is (Anthos A-17 Autoclave, Italy).

Each wire will be cut into 3 segments; 2 straight distal segment and one anterior curved segment. The distal segments will be used for tensile testing to derive the mechanical properties and the anterior segment will be used to measure the surface roughness.

The mechanical properties (load deflection curve) will be measured with a universal testing machine (Model 4466, Instron Corp, Canton, Mass) with a 1000-kg load cell at 37°C. Each wire segment will be placed in the jaws of the machine. A grip is positioned at each end of the wire with a distance of 40 mm between the jaws. The machine will be operated in the tensile mode with a crosshead speed of 5 mm per minute. Three characteristics of each sample will be quantified from the readouts: ultimate tensile strength (kg/mm2), elongation rate (%), and modulus of elasticity (kg/mm2).

Atomic force microscopy (AFM) is a non-invasive method for three dimensional analysis of surface roughness. Anterior archwire segments will be evaluated using the AFM (Anton Paar GmbH-Tosca200 AFM, Germany) employing the tapping mode 500 micrometer increment with a rate of 1 line per second. Average surface roughness (Sa) in nanometers will be obtained by software analysis (Mountains 8.2 Software-Digital Surf, Besancon, France). The average surface roughness of each specimen will be measured at three different sites and the mean (Sa) will then be calculated.