Friction and orthodontic mechanics: clinical studies of moment and ligation effects

Abstract

<h2>Abstract</h2> The efficiency of tooth movement associated with orthodontic sliding mechanics can be compromised by friction between arch wire and tube or bracket slot. The described studies measured the apparent coefficient of static friction (μ_a) during sliding along an arch wire. Known moments were applied to simulate tooth tipping. Measurements tested whether intraoral vibration during gum chewing reduced friction. The first experiments measured intraoral friction associated with the sliding of 4 and 8 mm stainless steel (SS) tubes along a SS auxiliary arch wire, in 10 subjects. Mean intraoral μ_a for 4 and 8 mm tubes were 0.12 (SD = 0.04) and 0.17 (SD = 0.05), respectively. Analysis of variance (ANOVA) showed that μ_a was significantly higher (<i>P</i> < 0.001) for the longer tubes. Although intraoral vibration decreased μ_a compared with bench-top tests (<i>P</i> < 0.001), frictional resistance was never completely eliminated. The second experiments studied the effects of ligation. Ten professionals performed exercises to characterize average tight and loose SS ligation forces. These ligation forces (F_{N Ligation}), and those associated with elastic ligation, were reproduced by a calibrated operator in a modified intraoral device, where SS orthodontic brackets slid along a SS auxiliary wire. Ten subjects chewed gum with the device in place. Nested ANOVA and Tukey Honest Significant Difference tests determined the effects of ligation type and environment. No significant differences (<i>P</i> > 0.01) were found between ex vivo and intraoral μ_a values for tight and loose SS ligation. Intraoral values for μ_a were significantly greater than ex vivo values (<i>P</i> < 0.001) for elastic ligation. Overall, the results suggested that vibration introduced by gum chewing did not eliminate friction. Tipping moments and ligation forces were equally significant in determining frictional forces. As well, there was considerable intraoperator variation in F_{N Ligation} for SS ligatures. Variations in clinical ligation forces are likely to be equal or greater than these experimental data and have potential to affect treatment efficiency during orthodontic sliding mechanics.