Self-ligating brackets (SLB) were reintroduced in the clinical orthodontic practice in late 90s [1
]. Some advantages were described, such as faster movements, less chair time, and less extraction need. Recent systematic reviews describe only these advantages: Less time needed to ligate, less incisor protrusion, and greater transversal changes in the molar region [2
Knowledge of the friction concept with the use of self-ligating brackets, and its clinical evidence, may be critical to the orthodontist to be able to select the better system for treating patients. Resistance to sliding is divided into three main components: Friction, binding, and notching [4
]. Friction is the resistance produced between two surfaces: The slot and the arch wire, making a movement in the same direction but in the opposite way [5
]. In general terms, there are two types of friction: Dynamic and static. Dynamic friction is directly proportional to the normal ligated strength, which works perpendicularly to the direction of movement between the surfaces in contact with the arch and the bracket. On the other hand, static friction is the one which is opposite to any other applied force, in that, as soon as this force exists, the movement begins. The dynamic is usually lower than the static, and it is opposed to the tooth movement [4
]. In general terms, dynamic friction is produced throughout the movement, while the static friction is produced to start the movement. The nature of friction has many factors involved [6
]. There are biological, mainly in the oral environment [8
], physical, and mechanical factors that take place during tooth sliding. Besides the contact of the arch wire and the slot of the bracket, other complementary variables are involved, such as: The design and material of the bracket, and the size, shape, and material of the wire. Additional products have been designed in order to reduce values of friction during treatment, known as friction reducer agents (FRA) [9
]. It is very difficult to know the clinical performance of several different arch wires and brackets combinations offered by the industry. As a previous step, in vitro studies may be helpful to simulate clinical situations with different materials. Even those results must be checked clinically to confirm presumed advantages.
The main objective in this study is: To simulate, in vitro, the effect of different variables which can affect arch movement in conventional and self-ligating brackets. To sum up, static friction depends on its variables, such as type and design of the bracket, arch wire section, kind of ligature, and use of an FRA in the wet state.
This is an in vitro study simulating clinical conditions comparing static friction values among different bracket/wire combinations and the use of an FRA. In the literature there are many papers comparing friction values [6
]. Some of them compare static and dynamic friction. Here, we rejected studying dynamic friction, due to the difficulty to measure it in a simple, reproducible, and comparable way [13
]. In accordance with that, some authors have claimed that the study of dynamic friction is not relevant because it rarely happens in dental sliding [15
]. On the other hand, consensus about the importance and measurement of static friction in the literature is found [19
]. Again, it must be emphasized that the main limitation of this in vitro study is that the majority of friction studies are designed along these lines to avoid the difficulty of achieving friction values from a living patient [20
]. However, some authors suggest that intraoral simulations are not comparable to what really happens in a patient’s mouth [4
]. The bracket setting was in a replicable half arch disposition, similar to other previous studies [14
], avoiding only one or two bracket tests. A full arch bracket disposal was not achieved, due to the difficulty of simulating the sliding and recording static friction values. Simulation was done in a wet state with artificial saliva [20
In the analysis of the results, each bracket has a different behavior. Lower static friction values were found in PSL (<1 N/mm), with no differences in the use of hybrid wires or FRA (p
-value > 0.05). Higher values were found with ASL (2.53 ± 1.23–4.36 ± 0.32 N/mm), with differences among combinations and lower static friction with the use of FRA. Differences between PSL and ASL were similar to those found in the literature [13
]. In CL, most of the values were higher than ASL (2.43 ± 0.15–8.01 ± 1.08 N/mm). Two interesting things to point out about the expressed values of metallic ligatures are: First, static friction values of metallic ligatures with CL brackets and rectangular or hybrid wires were higher than those obtained with elastic ligatures. Second, the standard deviation values were greater than those obtained with elastic ligatures. Higher values with elastic ones should be expected, but already in the literature it is affirmed that friction depends on ligation strength and the friction coefficient [21
]. Elastic ligatures have more surface in contact with the wires. Hence, higher values should be expected. However, due to the great variability and operator sensibility of metallic ligatures, the increase of ligation strength could be responsible for the higher values in metallic ones [23
]. Results obtained with rectangular stainless steel wires were similar to other authors’ comparisons of PSL and CL brackets with elastic ligatures, they affirm that bracket wire combination is not important for angulations higher than 7º, as, at that time, binding and notching are responsible for friction values [4
Hybrid wires are designed to decrease the values of friction by diminishing the surface of contact between wires and brackets to reduce binding and notching. Regarding the interpretation of static friction reduction using hybrid section wires, we can affirm that: Using hybrid section wires with self-ligating brackets, active or passive, does not reduce static friction values. Hence, it would not be justified to use hybrid wires during the closing spaces phase of the treatment. In conventional brackets with elastic and metallic ligatures, hybrid section wires reduce static friction values significantly [26
]. Hence, its use could be justified, since with conventional brackets, elastic ligature, and hybrid section wires show static friction values as low as active self-ligating brackets with rectangular section wires. Nevertheless, the use of this combination must be tested in a clinical trial to confirm these results.
The application of FRA has the purpose to reduce surface and contact points among brackets, wires and ligatures, composing a surface layer that works as a contact shock absorber. From the interpretation of the results of this in vitro study, it seems that only in ASL is a significant reduction of values found with the rectangular or hybrid section with an FRA. However, no effect is found in the passive ones, in PSL, very low values are found with or without FRA application. In the CL group using elastic ligatures, FRA application reduces significantly static friction values with rectangular section wires. While with hybrid section wires, behavior is irregular and an evident reduction was not observed. Using metallic ligatures, the reduction of static friction comportment is similar with hybrid wires but reduction using rectangular wires shows significant decrease, achieving values similar to ASL. These results are similar to the only published paper [9
] about FRA, where both ligation methods and wires of different section are compared over an 45º and 60º inclined plane. Showing no differences between elastic and metallic ligatures in 0.020”, 0.019 × 0.025”, and 0.021 × 0.025” wires.
The comparison of different variables; such as arch wires, brackets, and the environment in artificial conditions in an in vitro study allows to test different situations that are extremely difficult to do clinically. Obviously, this is the prime limitation of these studies. However, on the other hand, it gives the practitioner the option to test his own appliances. Second, it helps to design clinical studies in a more specific way, comparing variables tested in an in vitro study beforehand.
Caution should be had in order to avoid the direct extrapolation from in vitro results to clinical practice. However, seen from the results of the present study, it seems that different bracket/wire/FRA combinations can be used to reduce static friction values and could be tested clinically. With passive SL brackets it would not be necessary to use hybrid section wires or an FRA to obtain low friction values. While with active SL brackets, the use of FRA, in in vitro conditions, reduces significantly the friction values but takes in to account that hybrid section wires are not effective. In metallic CL brackets with a metallic ligature, in in vitro conditions, the use of FRA is very effective combined with rectangular section wires to reduce friction. In the case of using elastic ligatures, hybrid section wires would be a good option. Therefore, the practitioner can take into consideration this information with his own brackets, keeping in mind this is an in vitro simulation of a certain clinical condition and results may be misleading. Although the results of the present study show advantages in static friction values in SL compared to CL brackets, we have to point out that many other variables, such as mastication forces, corrosion, temperature changes or plaque accumulation were not considered and cannot be comparable to reality during orthodontic movement [4
]. Furthermore, by analyzing the findings of this study, the clinical advantages of SL brackets cannot be affirmed and are still unclear, according to recent systematic reviews. These reviews only showed advantages in SL brackets in shortened chair time and less protrusion of the mandibular incisor, but no benefits in arch expansion, space closure time or orthodontic efficiency could be demonstrated [2
]. Hence, clinical research is mandatory to confirm current results.
This in vitro study simulated different combinations of brackets, arch-wires, and a friction reducer agent. Results showed that higher static friction values are found in CL compare to ASL and PSL brackets, in the latter, lower values were found. CL brackets using metallic ligature show the highest static friction values with a great variability. Use of HY wire does not reduce static friction values in ASL and PSL, while in CL brackets with elastic ligatures, values were reduced significantly. Use of an FRA reduces static friction values in ASL but not in PSL. In the case of CL reduction, the effect is higher with SS than with HY wires, and with metallic ligatures, the values descend to ASL data. The direct extrapolation of the results might be inaccurate, since all these findings should be tested clinically to be validated.