Tough to Torque? Variation in the accuracy of wire size and shape may play a role.

Lombardo et al. 2015

BY HENRY W. FIELDS

STUDY SYNOPSIS
Several factors affect the expression of the third order (torque) configuration of archwires upon teeth.  These include the bracket slot size, wire dimension, and the bevel of the edge of square and rectangular wires. These variables have been examined in the past, but this recent study combined all three aspects for 43 archwires of different wire sizes and types using a digital gauge and micrographs measured with digital image software. The wires came from six manufacturers. Each wire was examined using 3 samples.
 
The results demonstrated that there is substantial variation within each wire for each manufacturer.  Wire width and height were both under and over-sized—many significantly so. Some manufacturers displayed sizing tendencies, but all had significant differences from the ideal nominal sizes. Corner radiuses were present (not 90 degrees) in all of the samples, and often there were four different radii for one wire (each corner was a different shape). 
 
The wire material affected sizing as well.  Coated wires were the most true to size, followed by  NiTi and TMA. Steel and tempered steel were the most undersized.

WHAT THE PROFESSOR THINKS
These results, along with the knowledge that most bracket slots are oversized, leads to more problems than expected in expressing torque in tooth movement. The undersized wires and corner bevels lead to a considerable amount of play (rotation before expressing a moment) between the bracket and rectangular wire. This difference can be considerably larger than ideal and up to 3 times what is expected.

Unfortunately, since the cross-sectional size and shape of commercially-available wires are highly variable, straightwire clinicians are likely to encounter difficulty in torquing unexpectedly. . . and more frequently than desired.

For the clinician who wants more ideal torque, the options are 1) to increase the wire size, which is perhaps a better option when using TMA wires so that harsh forces are not produced 2) purchase high torque brackets for potential problem cases or 3) consider torquing auxiliaries or archwire bends.

Article Reviewed: Lombardo L, Arreghini A, Bratti E et al. Comparative analysis of real and ideal wire-slot play in square and rectangular archwires.  Angle Orthod 85: 848-58, 2015 (Sept).

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