Friction has been studied in a number of ways. In some studies, the wires were pulled through at least one bracket; in other instances [4], a bracket was slid on a wire. Laboratory tests are usually simplified and designed to look at only one or two variables related to the wires tested, and that makes generalization difficult. The bracket-wire interface varies significantly according to the ligation mechanism used [8, 11, 13–18].
Clinicians are not generally interested in knowing the coefficient of friction for a specific type of wire when used with a specific bracket or how much of the resistance to sliding results from friction versus binding. Clinicians need to know the forces applied on the dentition when specific combinations of bracket, wire, and ligation method are used in a malocclusion [14].
In this study, the frictional forces generated by the bracket-wire-ligature system with three types of monocrystalline ceramic brackets with CEL and NCEL during the leveling phase of fixed appliance therapy were analyzed. A testing device similar to the one proposed by Franchi and Baccetti [11] was conceived to re-create clinical conditions for the leveling and aligning phase of the straight-wire technique, i.e., to study the frictional forces generated during the leveling of a displaced tooth by allowing different amounts of vertical misalignment of one bracket (canine bracket) with respect to the four remaining aligned brackets.
Franchi and Baccetti in 2006 [11] observed that when a slight amount of tooth alignment is needed (1.5 mm), differences in the performance of conventional and low-friction ligatures were minimal, but they became significant for correction of a misalignment greater than 3.0 mm. In 2007, Camporesi et al. [12] evaluated the forces available for tooth alignment in the presence of preadjusted 0.022-in. polycrystalline ceramic brackets coupled with low-friction esthetic ligatures and confirmed what had been found with metal brackets in 2006 [11].
In the current study, all monocrystalline ceramic brackets with NCEL produced significantly lower frictional forces than CEL at all amounts of canine misalignment. The results of the present study agree also with those reported by Franchi and Baccetti [11] and Camporesi et al. [12]. It should be noted, however, that the present study measured the frictional forces developed by the bracket-wire-ligature system rather than the forces available for tooth movement (see ‘Appendix’) [11, 12]. The results reported here are also consistent with those of Tecco et al. [13] who found that NCEL showed lower friction when compared with conventional ligatures when coupled with round archwires.
In the current study, no significant differences were assessed among the three types of monocrystalline ceramic brackets when used in combination with NCEL and 0.012-in. SENT wire. In the presence of CEL and 0.012-in. SENT wire, the Inspire Ice brackets showed a significantly greater frictional force when compared with the Pure brackets at 1.5 mm, while the Radiance brackets developed significantly greater frictional forces than all other esthetic brackets at 3.0 and at 4.5 mm. At 6.0 mm of canine displacement, the Radiance brackets with CEL and 0.012-in. SENT wire showed frictional forces similar to those of the Inspire Ice brackets but greater than those of the Pure brackets.
The present study also demonstrated that in the presence of either type of elastomeric ligature coupled with 0.014-in. SENT wire, the Radiance brackets showed significantly greater frictional forces when compared with the other esthetic brackets at all amounts of canine displacement with the exception of 1.5 mm canine misalignment where no significant differences were found.
Obviously, the advantage of an in vitro study such as this is that confounding effects and extrinsic variables are more easily minimized, but there are certain limitations of this study. In particular, the clinical interpretation of these data requires further considerations that modulate these findings. First, the vice-like devices of the testing machine did not allow the brackets contiguous to the misaligned bracket to move. Our results, therefore, relate to a condition that can be defined as absolute anchorage. Second, each test with the machine was performed with new elastomeric ligatures. No attempt was made to evaluate the effects of time and oral environment on the amounts of force released with the different types of elastomeric ligatures [19].