The effect of temperature change on the load value of Japanese NiTi coil springs in the superelastic range

Abstract

Japanese nickel-titanium (NiTi) coil springs exhibit superelastic properties similar to the properties of Japanese NiTi wires. The springs exert a constant light, continuous force over a long range of increasing or decreasing activation. This property enables the springs to be used for physiologic orthodontic movement of teeth. A constant load value can be maintained only at constant temperatures. However, external influences affecting the mouth, for example when eating or drinking, cause fluctuations in temperature, which can be considerable. In this study, the effect of temperature change on the force delivery of NiTi springs in their superelastic range was investigated. Japanese NiTi closed coil springs were heated and cooled between 20 degrees C and 50 degrees C, while held in constant extension. During this procedure, the force was continually recorded. Commercially available stainless steel closed coil springs were tested in the same way. For all the springs examined, load values were found to increase with rising temperatures and to decrease with a drop in temperature. This relationship between temperature change and load was more pronounced in the case of NiTi than in the steel springs. The force measured at 37 degrees C was about twice as high as at 20 degrees C for one type of NiTi spring. On cooling, the superelastic springs showed unusual behavior. Immediately after the temperature started to drop, a rapid decrease in force occurred to levels below those found at rising temperatures. Such a nonlinear decrease in load was not observed in the stainless steel springs tested. The findings demonstrate that only minimal changes in temperature can cause significant changes in the force delivery of superelastic NiTi springs.