Numerous in-vitro studies have been conducted with various archwire-ligation combinations to evaluate the effect of wire size and ligation method on frictional resistance. The aim of this in-vitro study was to compare the frictional resistance during sliding mechanics with Gianelly-type stainless steel working wires, Leone slide ligature, conventional elastic ligature, and stainless steel (SS) ligature, and a conventional bracket and active and passive self-ligating brackets.
Three ligation methods with Victory (V) brackets—Leone (VLeone), conventional (regular) elastic (VReg), and SS (VSS)—were used with standard SS brackets, and 2 self-ligating brackets—Damon MX (Ormco/“A”, St. Paul, Minn) (DMX) and In-Ovation R (GAC Intl., Bohemia, NY)—were used with 2 rectangular SS wires (0.016 × 0.022 and 0.018 × 0.022 in). Therefore, 5 ligation methods and 2 wire sizes were evaluated with respect to their effects on frictional resistance.
No statistically significant differences were found between the SS ligation method and DMX (neither produced measurable static friction). For the wire size 0.016 × 0.022-in SS, the DMX and SS produced significantly less static friction than the In-Ovation R, VLeone, and VReg. The In-Ovation R produced significantly less friction than both the VLeone and VReg, whereas the VLeone produced significantly less friction than the VReg. For the wire size 0.018 × 0.022-in SS, the overall results were the same, except that the In-Ovation R produced significantly more friction than the VLeone. An increase in wire size (from 0.016 × 0.022 to 0.018 × 0.022 in) led to an increase in friction in all bracket-archwire combinations (except DMX and VSS, which showed no measurable friction at either wire size).
The Leone slide ligature showed less friction at both wire sizes than VReg; however, it showed significantly more friction than both DMX and VSS. DMX and VSS brackets produced no measurable friction with either 0.016 × 0.022-in or 0.018 × 0.022-in wires. An increase in wire size (from 0.016 × 0.022 to 0.018 × 0.022 in) led to an increase in friction in all bracket-archwire combinations (excluding DMX and VSS, which showed no measurable friction at either wire size).
Friction has been defined as the force that opposes the relative motion or tendency toward such motion of 2 surfaces in contact. It is the resistance to motion that occurs when 2 objects move tangentially to each other. Simplistically, some view static friction as an impediment to effective and efficient tooth movement, but a certain amount of resistance to sliding (RS) is required for proper 3-dimensional (3D) expression of a bracket’s prescription. As teeth translate through space, frictional forces develop and can inhibit their movement. The effects of static friction should be carefully evaluated before and during treatment to prevent anchorage loss and unfavorable tooth movement. The concept of frictional forces is important when sliding mechanics are required, since greater frictional resistance requires greater orthodontic forces.
During orthodontic space closure with sliding mechanics, frictional force is generated at the bracket-archwire interface and has a tendency to inhibit the desired tooth movement. Two factors determine the amount of friction during sliding mechanics: the coefficient of friction between contacting surfaces and the forces applied between those surfaces. In mechanisms for space closure, an archwire that is slightly smaller than the bracket slot is often used. This allows for the maximum amount of applied retraction force when moving teeth, even though an ideal force system could only occur in a frictionless environment. Unfortunately, it has been shown that between 12% and 60% of applied force in fixed appliances is lost to friction. Proffit et al agreed that a substantial amount (approximately 50%) of the force necessary to initiate tooth movement is required to overcome the retarding frictional force generated between the various components of a fixed appliance, such as archwires, brackets, and ligatures.
Obviously, forces are required to move teeth, and varying levels of friction are needed throughout treatment. Perhaps lower friction is needed at the start of treatment, with higher friction toward the end of treatment, but, as Thorstenson and Kusy appropriately explained, the amount of clearance between the bracket and the archwire is inversely related to the amount of control over root position.
It is important to elucidate the factors that can contribute to friction. A number of studies have evaluated factors that influence frictional resistance: bracket (geometry) and wire materials (wire stiffness), surface conditions of archwires (roughness) and bracket slot, wire section, torque at wire-bracket interface, type and force of ligation, use of self-ligating brackets, interbracket distance, saliva, and oral functions.
The relative contribution of each factor to frictional resistance is also important. Schumacher et al stated that the amount of friction is mainly determined by the method of ligation. Currently, 2 main types of brackets are available for practicing orthodontists—self ligating and conventional. Self-ligating brackets are considered a ligature-less bracket system, whereas conventional brackets are traditionally ligated with elastic ligatures or stainless steel ties. Also, orthodontic companies have recently sought to develop novel ligatures for use with conventional brackets to reduce friction—eg, the Leone slide ligature.
Two types of self-ligating brackets have been developed: active and passive. Often, these 2 designs are erroneously defined. Active is sometimes wrongly defined as having a spring clip that presses against the archwire, and passive is defined as having a self-ligating clip that does not press against the archwire. However, as Rinchuse and Miles pointed out, this is not an accurate description; both types of brackets press against the wire to some extent throughout treatment, making both designs interactive. But, for consistency with the orthodontic literature, we will use the words “active” and “passive” as conventionally defined.
The claimed advantages of both types of self-ligating bracket systems include increased patient comfort due to the absence of ligatures, improved oral hygiene, less chair time, and shorter overall treatment time. However, there are also certain disadvantages, including difficulty with the full expression of torque, frequent failure of the clips, and brackets that are bulkier and more expensive than conventional brackets.
To address some deficiencies of self-ligating brackets, novel ligatures such as the Leone slide ligature have recently been developed. This ligature is applied to the bracket in the same manner as a conventional elastomeric ligature; however, similar to passive self-ligating bracket systems, the archwire is supposedly free to slide while simultaneously applying the entire force of the wire to the tooth. Comparative studies have not yet been undertaken with the Leone ligature at the wires sizes we used in this study. In the studies of the Leone slide ligature at smaller wire sizes, the results indicate that it produces significantly less friction than do conventional elastomeric ligatures.
Several studies have been conducted to evaluate the difference in RS between self-ligating and conventional brackets. They have mostly focused on the Damon 2, In-Ovation R, and Victory brackets. No study has yet evaluated the new Damon MX (Ormco/“A”, St. Paul, Minn) (DMX) bracket. Therefore, we examined the RS of the new DMX bracket compared with the previously studied In-Ovation R (GAC Intl., Bohemia, NY) and Victory (3M, St. Paul, Minn) conventional brackets. Additionally, the RS of the Leone slide ligature was compared with the RS of these other forms of ligation.
This study also differs from other investigations because we evaluated smaller rectangular stainless steel (SS) wires (0.016 × 0.022 and 0.018 × 0.022 in). Previous studies generally evaluated undersized resilient wires (nickel-titanium) that were ineffective for anterior torque control. When SS wires were tested, they were 0.019 × 0.025 in; however, Gianelly stated that this size is not effective for sliding tooth movement and extraction space closure because of the high friction introduced by the horizontal dimension (0.025 in). Because of these concerns, Gianelly rationalized the merits of the bidimensional bracket design, which allows for full anterior 3D control during space closure. Henao and Kusy indicated that larger wires (0.019 × 0.025-in SS) do not exhibit significant differences in friction between bracket types, and that smaller wires are more effective at elucidating differences in RS. Therefore, the use of smaller rectangular wires has an important clinical perspective.
The aim of this study was to test the null hypothesis that there are no statistically significant differences in drawing forces among 5 ligation systems with 2 sizes of rectangular SS wires.
Material and methods
The methods we used in this study were similar to those of Hain et al. A universal testing machine (model 5564, Instron, Canton, Mass) with a static load cell of ± 100 N was used at a crosshead speed of 2 mm per second over 10-mm lengths of archwire. Five maxillary right second premolar brackets were bonded with epoxy adhesive on a polymethylmethacrylate block and oriented with the long axis of the slot vertical. The software ran on a computer connected with a digital interface card. The load frame was a high-stiffness structure that applied the testing loads to the material specimens. The testing software provided all the test setup, control, and analysis functions. The calibration was set at automatic. The extension was 1250 mm, time was in seconds, and it was a full-scale 100 N. The device was set up to document the amount of static frictional force needed for movement to start through test brackets. This was recorded as the maximum frictional force at initial movement, which represented the peak static frictional resistance.
The sensitivity of the testing machine involved several components. Its load frame has a load-weighing accuracy of ± 0.01% of a full scale. Position repeatability is ± 0.015 mm; position measurement accuracy is ± 0.02 mm; crosshead speed accuracy is ± 0.1% of set speed, measured over 100 mm or 30 seconds, whichever is greater.
Three types of maxillary second premolar brackets were used, with –7° of torque: DMX (angulation, +2°), In-Ovation R (angulation, 0°), and Victory (angulation, 0°). The first 2 are self-ligating brackets, and the last is a conventional bracket. Two types of wires were used: 0.016 × 0.022-in and 0.018 × 0.022-in SS. Straight 10-cm lengths (3M Unitek, Monrovia, Calif) of both wires were tested in combination with each bracket type. The design of the test jigs allowed bracket assemblies to self-align; this eliminated tip and torque as factors affecting frictional resistance. The brackets and archwires were cleaned with alcohol wipes before the modules were tied with a Mathau plier. All modules were ligated 60 minutes before testing to reduce the variation in elastic tension. The SS ligatures were completely tightened and then unwound by 3 turns.
In the first test, the frictional properties of 5 ligation methods were compared with 0.016 × 0.022-in SS wires: 3 different modules were used with Victory (V) brackets (VLeone, VSS, and V-regular (V-reg) and 2 self-ligating brackets: DMX (passive) and In-Ovation R (active). The second tests used the same 5 ligation methods, but the wire size was increased to 0.018 × 0.022-in SS. A new 10-cm length of wire was used for each of the 30 runs of each wire-bracket system (ie, each bracket-archwire combination was tested 30 times with a new archwire each time) to prevent distortion of the archwire surface and produce sufficient power. In all, 300 wire specimens were evaluated.
The results were analyzed by using analysis of variance (ANOVA) in the SPSS statistical program (SPSS, Chicago, Ill). Post-hoc Student-Neuman-Keuls comparison tests were performed to identify significance between group comparisons.