Friction between Archwires of Different Sizes, Cross-Section and Alloy and Brackets Ligated with Low-Friction or Conventional Ligatures

Friction is the resistance to motion that occurs when an object moves tangentially against another.1,2 In orthodontics, factors capable of influencing frictional resistance (FR) between the bracket and the archwire have been evaluated with the use of in vitro models.1–6 Recently, importance has been given to the type and force of ligation7 and to the use of self-ligating brackets.8 

Self-ligating appliances and other low-friction systems seem to induce arch development and dentoalveolar expansion of the maxillary arch during initial phases of therapy with superelastic nickel-titanium (NiTi) archwires,8,9 and to reduce treatment time, especially in extraction cases.1 Recently, a new method was reported whereby low friction was generated with the use of nonconventional elastomeric ligatures, referred to as low-friction ligatures, that can be used on conventional brackets (Figure 1)9 instead of conventional ligatures, when low friction is needed.

In 2006, Franchi and Baccetti10 tested the forces generated by three sizes of wire (0.012-in, 0.014-in, and 0.016-in superelastic NiTi) with two types of elastomeric ligature (conventional and low friction) at different amounts of upward canine misalignment (1.5, 3, 4.5, and 6 mm) in a segment of five stainless steel 0.022-in preadjusted brackets.

These investigators found significant differences between conventional and low-friction ligatures for all tested conditions, with the exception of 0.014-in and 0.016-in wires at canine misalignment of 1.5 mm. However, in their study, only the size of the conventional ligatures employed was specified (inside diameter of 1.3 mm and thickness of 0.9 mm)—not the size of the low-friction ligatures. In addition, only round archwires were considered.

In 2007, Camporesi et al,11 using the Franchi and Baccetti10 model, evaluated the frictional force generated by preadjusted 0.022-in ceramic brackets with low-friction esthetic ligatures and confirmed what had been found with metal brackets in 2006.

Recently, Tecco et al12 evaluated the FR generated by conventional ligatures, self-ligating Damon SL II brackets (Ormco, Glendora, Calif), self-ligating Time Plus brackets (American Orthodontics, Sheboygan, Wis), and low-friction ligatures coupled with various SS, NiTi, and beta-titanium (titanium molybdenum alloy [TMA]) archwires, using a new experimental method to investigate the FR generated during the sliding of an archwire along a group of 10 aligned brackets.13 When coupled with 0.016-in NiTi, low-friction ligatures generated greater FR compared with Damon SL II (P < .001); when coupled with 0.016 × 0.022-in NiTi, low-friction ligatures generated significantly greater FR (P < .001) compared with all self-ligating brackets. It was only when low-friction ligatures were coupled with 0.019 × 0.025-in stainless steel (SS) or 0.019 × 0.025-in NiTi that low-friction ligatures generated significantly lower FR than was produced by self-ligating brackets and conventional ligatures. No difference was observed among the low-friction ligatures, the conventional ligatures, and the self-ligating brackets when coupled with a 0.017 × 0.025-in TMA archwire.

In a study that examined the differences between conventional and low-friction ligatures, low-friction ligatures were observed to generate a lower FR than conventional ligatures only with 0.019 × 0.025-in SS and NiTi, but this result contrasted with what was stated by the manufacturer. Because low-friction ligatures are available in different sizes (small, medium, and large), the purpose of this study was to compare the FR generated by various archwires coupled with small, medium, or large low-friction ligatures or with conventional ligatures, using the same apparatus (Figure 2),13 because FR also could be due to the different sizes of ligatures. From a clinical point of view, the importance of this investigation lies in the fact that a clinician who is looking for low friction needs to know the right clinical protocol by which low-friction ligatures can be used.

The null hypothesis was that no statistically significant difference in FR was noted when round or rectangular archwires were used in conjunction with low-friction ligatures (small, medium, or large) or conventional ligatures.

Details of the brackets, ligatures, and archwires tested are given in Table 1. Only upper left central incisor brackets were used.

The low-friction ligature has a built-in elastic device that transforms the slot of the bracket into a tube and closes off the archwire in the slot (Figure 1). This elastic device is more rigid and is rather stiff when compared with the soft and elastic surfaces of conventional ligatures. Size differences between the ligatures are described in Table 1.

In this study, all ligatures were applied with a Module Applicator (Leone, Firenze, Italy) that was used according to the manufacturer's recommendations. One operator ligated each bracket/wire combination for each of the runs. The testing model (manufactured by Myrmex Laboratory, Foggia, Italy) was described previously13 (Figure 2).

On the testing model, 10 aligned brackets of the same group were bonded with the use of a cyanoacrylate adhesive (Loctite 416; Henkel Loctite Corp, Rocky Hill, Conn). Alignment of the brackets was obtained through the preliminary insertion of a 0.021 × 0.028-in SS archwire into the slots of the brackets, without ligation, before bonding on the metal bar. After definitive bonding of the brackets was completed on the metal bar, the 0.021 × 0.028-in SS archwire was accurately removed. For each group, a single model was used to test the same bracket/archwire combination 10 consecutive times. For each test, a new archwire was employed. The FR was evaluated for each type of archwire after each test run. A total of 400 tests were performed, and all tests were conducted in the dry state at an ambient temperature of 34°C.

For these evaluations, a mechanical testing machine (Model Lloyd 30K; Lloyd Instruments Ltd., Segensworth, UK) was employed with a 10-lb tension load cell that was set on a range of 1 lb and was calibrated from 0 to 1000 g (Figure 2). Each archwire was gripped by crimping brass fittings on the distal ends. The load cell registered the force levels required to move the wire along the 10 aligned brackets; these levels then were transmitted to a computer hard disk. The archwires moved at a crosshead speed of 0.5 mm/min. Each run lasted approximately 5 minutes. Load values of FR were calculated in centi-Newtons (cN). After each test, the testing machine was stopped, the bracket/archwire assembly was removed, and a new assembly was placed. This process, which was completed for 10 evaluations for each bracket/archwire combination, allowed sliding of the wire along the 10 brackets and recording of the FR. Finally, the bracket/archwire assembly was removed, and a new assembly was placed. This was done for 10 nonrepeated evaluations for each bracket/archwire combination (n = 10). A randomized sequence was performed for each type of archwire.

Descriptive statistical information, including mean and standard deviation, was calculated for each bracket/archwire combination. For statistical analyses, data were recorded as the differences in FR observed among the four groups of ligatures. Results for the two types of ligatures were compared with one-way analysis of variance (ANOVA) testing (P < .05) that was completed with the use of statistical software (Statistical Package for the Social Sciences [SPSS] for Windows, version 12.0; SPSS Inc., Chicago, Ill).

Means and standard deviations of the FR for each bracket/archwire combination and the differences between them are presented in Figure 3.

When coupled with round archwires, low-friction ligatures showed a statistically significantly lower FR when compared with conventional ligatures (P < .05) (Figure 3). When coupled with 0.016 × 0.022-in NiTi and SS, no significant difference was observed among the four groups (Figure 3). When coupled with 0.017 × 0.025-in archwires, low-friction ligatures showed a statistically significantly higher FR than was seen with conventional ligatures (P < .05) (Figure 3). When coupled with 0.019 × 0.025-in NiTi, low-friction ligatures showed a statistically significantly higher FR than did conventional ligatures (Figure 3), but no difference among the four groups was observed with the 0.019 × 0.025-in SS (Figure 3).

In this study, low-friction and conventional ligatures demonstrated different trends of results for archwires of various cross-sections (round or rectangular) and sizes (Figure 3).

Franchi and Baccetti10 observed that the amount of released force by conventional and low-friction ligatures with 0.014-in and 0.016-in round archwires was not significantly different at 1.5 mm of canine misalignment but became significant at higher levels of canine misalignment.

Franchi and Baccetti10 concluded that when a slight amount of tooth alignment is needed (1.5 mm), differences in the performance of conventional and low-friction ligatures are minimal, but they become significant for correction of a misalignment of greater than 3 mm. By comparing our protocol with the “minimal amount of alignment need” from Franchi and Baccetti10 (because we used a straight wire), we could complete their results; we observed a statistically significant difference between the two types of ligatures, also with aligned brackets, when 10 brackets were tested instead of five. Our results are in accord with those of Franchi and Baccetti for round archwires, but they are not comparable for rectangular archwires, because Franchi and Baccetti investigated only round archwires.

Our findings seem to indicate that the design of low-friction ligatures allows low friction only when they are coupled with round archwires and not when they are coupled with most rectangular archwires.

In another recent study, Tecco et al,12 when comparing the friction of low-friction ligatures with that of conventional ligatures and two types of self-ligating brackets, found that, coupled with 0.016 × 0.022-in NiTi, self-ligating brackets (Time and Damon SL II) generated significantly lower friction (P < .001) when compared with conventional and low-friction ligatures. Thus, Tecco et al,12 in their study, observed no significant differences between conventional and low-friction ligatures, when coupled with 0.016 × 0.022-in NiTi. This last finding is consistent with our findings in that we observed no significant differences among the four groups (conventional and low-friction ligatures) with this type of archwire.

Our findings about rectangular archwires may be explained by the design of low-friction ligatures. Their shape, with the elastic device built in to transform the slot into a tube and to close off the archwire in the slot (Figure 1), is more rigid and rather stiff when compared with the soft and elastic surfaces of conventional ligatures and could generate greater FR with the greatest rectangular archwires. This may be considered an advantage in some situations; for example, during retraction of upper anterior teeth on a 0.019 × 0.025-in SS archwire, lower friction is desired in the lateral segment of the dental arches, but during the final phase of stabilization, greater friction is desired for all teeth. Thus, the clinician can choose the type of friction needed, thereby changing the type of elastic ligature required.

However, the results observed with rectangular archwires could be related to the vertical dimension and the type of alloy in the archwires. The larger contact area between the wire and the slot and the surface texture of the wire surface are factors that can affect the magnitude of frictional forces, in that friction was observed to increase with an increase in wire size.14 For example, the surface roughness of SS appears to be the smoothest among various wires, followed by Ni-Ti15; TMA seems to produce greater frictional force than is produced by NiTi.16,17 Kapila et al4 showed that the wire size–alloy interaction significantly influenced the magnitude of friction, in addition to bracket size (0.022 produced an increase in friction with respect to 0.018 in). For the NiTi alloy, austenitic-active alloy wires with low-stress hysteresis and lower stiffness were found to generate less friction than was produced by Sentalloy wires.18 

In this study, the use of several types of archwires led to the inclusion of many variables that could have influenced the frictional force, such as arch size (6 thickness), arch cross-section (round and rectangular), and wire surface roughness (SS, NiTi, TMA).

In this study, the absence of differences among the three sizes of low-friction ligatures, in terms of friction (Figure 3), seems to suggest that the different sizes proposed by the manufacturer (small, medium, and large) do not cause differences in terms of FR. However, it must be noted that the lack of a significant difference among the three sizes could be associated with the small sample size included in each group (n = 10); future studies should seek to clarify this point.

A high standard deviation (SD) was observed in our findings, most often with the 0.017 × 0.025-in TMA archwire and low-friction ligatures, suggesting that the special design of low-friction ligatures is associated with a less predictable result in terms of FR, although high SD may be related to the small sample size.

A high SD also was observed with SS and NiTi 0.019 × 0.025-in archwires for both conventional and low-friction ligatures, suggesting that in these cases, the specifically designed apparatus could have played a role. For example, very small misalignments of the brackets could have influenced the FR, especially when rectangular archwires were employed. The small interbracket span could have influenced the FR, most often with rectangular archwires, although no certain explanation can be proposed.

In this study, the 0.017 × 0.025-in wires (Figure 3) showed decreased friction with TMA, through NiTi until SS, although no statistical evaluation of these data was performed. This observation is consistent with the findings of several other studies,2 which reported that TMA generates greater friction than is produced by SS and NiTi for all bracket/archwire combinations, probably because of adherence between archwire and slot material.

The findings of this study are affected by the limited power associated with the small sample size. The study was carried out under controlled laboratory conditions in a passive frictional configuration, and not in an active configuration. The active configuration includes some misalignment of brackets, as is described in previous reports.2,13,19–24 

In addition, no attempt was made to evaluate the effects of time and oral environment on the amount of force released with different types of elastomeric ligatures.25 Finally, in this study, the height of the bracket slot was not measured and was not regarded as a variable capable of influencing friction. However, this variable can influence the FR,26 because when the archwire contacts the clip (and this depends also on the height of the bracket slot), the FR is influenced by archwire size in that the higher the slot height, the greater is the FR generated by the archwire that contacts the ligature.

• Low-friction ligatures offer lower frictional resistance than is produced by conventional ligatures only when coupled with round archwires—not when coupled with rectangular archwires.