Superelastic nickel titanium alloy retraction springs--an experimental investigation of force systems

Abstract

The purpose of the present investigation was to study the mechanical characteristics of canine retraction springs made of superelastic nickel titanium (NiTi) alloys. A modified Burstone T-loop was used to construct an experimental canine retraction spring 10 mm in height and 10 mm in length. Twenty-five NiTi T-segments were hand made from the superelastic orthodontic alloys Ormco NiTi and Soar Sentalloy (dimensions 0.016 x 0.022"). The T-segments were equipped with arms made of rectangular standard steel wire (0.017 x 0.025"). The following geometrical and mechanical parameters of the retraction springs were analysed: radius and bending angles of the T-segments, distalizing force and M/F ratio during activation and the force/deflection rate of the springs. The error in the geometric parameters was in the range of 5-10 per cent, irrespective of the alloy used to produce the T-segments. On the other hand, the force systems of the springs were strongly influenced by the alloy and the batch under investigation. There were differences in the distalizing force of up to 100 per cent, i.e. at the beginning of the unloading plateau the distalizing force varied from 0.4 to 2.5 N. The force/deflection rate varied between a value of 0.06 and 0.15 N/mm, whereas the moment/force ratio reached values of 6.5-7.0 mm. Within a single batch, a reproducibility of these mechanical properties of approximately 5 per cent could be obtained. These results confirm that each orthodontic device made of superelastic NiTi alloys has to be calibrated individually. The manufacturers should pay more attention to keeping the material properties of their NiTi alloys constant.

Nickel titanium T-loop wire dimensions for en masse retraction

“…13,14,19 Thus, the 0.016" 3 0.022" T-loop produced forces below what is supposedly deemed necessary for en masse retraction (Table 7) but still considered greater than the force deemed necessary for canine retraction. 9 Another T-loop made with the same brand of NiTi we used (Sentalloy, Dentsply GAC International Inc) out of 0.016" 3 0.025" wire 9 showed an even smaller force than we did, probably due to its larger height (10 mm) than ours (6 mm) and because of more extensive heat treatment, which decreases the pseudoelastic plateau of NiTi materials. 8 The 0.018" 3 0.025" T-loops, on the other hand, delivered forces ranging from 470 gf to 224 gf (Table 7), mostly adequate for en masse retraction and higher than what has been documented in the other T-loops made of the same size wire.…”

“…23 That large stress concentration induces a stress-induced martensitic transformation that will make that region more flexible rather than more rigid; moreover, further increasing the angulation in that region will cause a different effect than the desired one because higher stresses will make the load/deflection rate of NiTi even lower. 7,21 A solution for that problem has been shown by Bourauel and colleagues 9 with parallel tubes and stainless steel base wires, but that would still allow stress relaxation to occur in that specific region, because there was a 90u bend in the stainless steel base wire after the tubes. We have found a different solution, which was the addition of a crimpable cross tube in that particular area, making it more rigid and adding stainless steel base wires, preactivated by curvature, rather than NiTi base wires, which would probably not produce enough moments.…”

“…Even though the use of NiTi T-loops has already been shown in attempts to maintain the force upon deactivation, [9][10][11][12] it appears that they were designed for canine retraction. Because recent studies have shown that there is no difference in the rates of anterior retraction and loss of anchorage between that and en masse retraction, [13][14][15] it makes sense that the latter method would be clinically more practical and save chair time.…”

“…Because recent studies have shown that there is no difference in the rates of anterior retraction and loss of anchorage between that and en masse retraction, [13][14][15] it makes sense that the latter method would be clinically more practical and save chair time. The issue is that previously described available NiTi T-loop9 designs produce low forces for that purpose (below 250 gf at the beginning of deactivation), either because they were made from a small dimension wire, such as 0.016" 3 0.022", 9 or because they were possibly not activated enough. [10][11][12] Another issue with the proposed NiTi T-loops is that they are too large, with heights above 8 mm, which could cause impingements clinically.…”

Three-dimensional analysis of an orthodontic delta spring

“…Examples include orthodontic springs made with superelastic alloys, such as the modifi ed T-loop (Bourauel et al, 1997), the delta spring (Ferreira et al, 2005) and triangular loops (Katona et al, 2006). Satisfactory analysis of an orthodontic spring requires the observation of various parameters, including the center of resistance (C res ) and center of rotation (C rot ) (Raboud et al, 1997).…”