Abstract
ABSTRACTObjective: Evaluate changes in the force system of T-Loop Springs (TLS) preactivated by curvature, due to stress relaxation. Methods: Ninety TLSs measuring 6 x 10 mm, produced out with 0.017 x 0.025-in TMA® wire and preactived by gradual curvature, were randomly distributed into nine groups according to time point of evaluation. Group 1 was tested immediately after spring preactivation and stress relief, by trial activation. The other eight groups were tested after 24, 48 and 72 hours, 1, 2, 4, 8 and 12 weeks, respectively. Using a moment transducer coupled to a digital extensometer indicator adapted to a universal testing machine, the amount of horizontal force, moment and moment-to-force ratios were recorded at every 0.5 mm of deactivation from 5 mm of the initial activation, in an interbracket distance of 23 mm. Results: The horizontal forces decreased gradually among the groups (p< 0.001) and the moments showed a significant and slow decrease over time among the groups (p< 0.001). All groups produced similar M/F ratios (p= 0.532), with no influence of time. Conclusions: The TLSs preactivated by curvature suffered a gradual deformation over time, which affected the force system, specifically the moments, which affected the horizontal forces produced.
Highlights
The β-Ti T-loop spring (TLS) has been used since the 80’s for space closure due to its alloy and design advantages.[1,2] Even though the specific design produces a high moment of activation, this moment might not be enough to move teeth by translation
Besides the fact that this effect has only been superficially evaluated for β-Ti,[9,11,13,14,15] it has never been studied on TTLs preactivated by gradual curvatures, which might be less sensitive to stress relaxation
This decrease of force over time is in agreement to several other reports that have measured this effect on straight wires, showing it to be time-dependent.9,11,13There were only two studies[14,18] that looked upon this effect in more elaborate configurations, evaluating stress relaxation on TLSs preactivated by bends, in which a decrease of 15.5% on the force levels of TLSs was shown in the first 24 hours
Summary
The β-Ti T-loop spring (TLS) has been used since the 80’s for space closure due to its alloy and design advantages.[1,2] Even though the specific design produces a high moment of activation, this moment might not be enough to move teeth by translation. The addition of a residual moment is needed by increasing the angulation of the extremities of the loops, in a procedure known as preactivation.[3] Templates[4] and other methods[5] of preactivation have been developed to produce specific forces and enough moment-to-force (M/F) ratios, allowing different types of tooth movement. Upon engagement to the brackets, loops are normally loaded in the opposite direction of the preactivation, which could over time cause progressive deformation and force reduction (Fig 1).[6,7] This time-dependent effect is called stress relaxation and has been thoroughly studied in the alloys used in Orthodontics.[8,9,10,11,12,13] Besides the fact that this effect has only been superficially evaluated for β-Ti,[9,11,13,14,15] it has never been studied on TTLs preactivated by gradual curvatures, which might be less sensitive to stress relaxation
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