Palatal finger springs in removable orthodontic appliances--an in vitro study

Palatal finger springs are often used in removable orthodontic appliances to tip teeth in a mesiodistal direction. There is general consensus that a force of 30-50 g is required to tip a single-rooted tooth, with an activation of about 3 mm for a spring with a load/deflection rate (LDR) of 15 g/mm. The purpose of this report is to establish the magnitude of forces for finger springs made from different types of wires (i.e. those from different manufacturers and of different diameters and lengths). The findings were that springs made from ordinary 18/8 stainless steel wire showed a lower LDR than previously reported. The 18/9 stainless steel springs gave yet a lower LDR. Definite specifications are given for forming springs. For example, a Unitek 0.51 x 17 mm spring can be used when the traditional activation of 2-4 mm is applied, but should increased activation be required (advocated in this report), the spring length would have to be increased to 20 mm to keep the force within the specified range.