Nickel-titanium spring properties in a simulated oral environment

A study of nickel-titanium springs was undertaken to determine whether their mechanical properties are affected by prolonged exposure to a static, simulated oral environment. Stainless steel springs and polyurethane elastic chains were also studied for comparison. The springs were elongated to twice their rest length and held at that length while immersed in artificial saliva at body temperature for 0, 2, 4 or 6 weeks. Plots of force vs. deformation were made as the springs were stretched from rest length to three times that length and then relaxed back to rest. Nickel-titanium springs suffered no degradation of their spring properties in the simulated oral environment. In contrast, stainless steel springs became slightly more compliant to stretching, and polyurethane elastics lost a large portion of their force-generating capacity. These findings confirm and extend earlier reports which indicate that nickel-titanium is a preferred material for many orthodontic applications.