Dental Hypotheses

: 2022  |  Volume : 13  |  Issue : 2  |  Page : 27--35

Evaluation of Friction and Surface Characteristics of Two Types of Self-Ligating Bracket Gate: An In Vitro Study

Hussein A Kanbar, Dina H Obaid, Ali I Ibrahim 
 Department of Orthodontics, College of Dentistry, University of Baghdad, Baghdad, Iraq

Correspondence Address:
Ali I Ibrahim
Department of Orthodontics, College of Dentistry, University of Baghdad, Baghdad


Introduction: Frictional forces generation during orthodontic treatment with fixed appliances impedes appropriate tooth movement; hence, research is ongoing to explore “frictionless” techniques. This in vitro study compares Damon Q and Pactive self-ligating metallic brackets in terms of friction and surface characteristics of the bracket gates when using CuNiTi archwires during leveling and alignment stage and examines the effects of aging conditions on frictional force generation. Methods: A total of 108 metallic self-ligating brackets (Damon Q and Pactive) were investigated for frictional resistance with round 0.014″ and rectangular 0.014″∗0.025″ CuNiTi archwires post exposure to water storage and acidic attack aging conditions. The bracket gate surface characteristics were evaluated using scanning electron microscope (SEM). Results: There was no statistically significant difference (P > 0.05) in friction generation between the two bracket systems when coupled with 0.014″ CuNiTi archwire, but the Pactive brackets yielded significantly higher frictional forces (P < 0.05) when coupled with 0.014″∗0.025″ archwire. The SEM findings revealed nonsignificant differences (P > 0.05) between the surface characteristics of the bracket gates. Conclusions: Damon Q brackets generate low frictional forces, suggesting better performance than Pactive brackets during the first phase of orthodontic treatment. A modified scoring system was developed for an objective description of bracket surface characteristics.

How to cite this article:
Kanbar HA, Obaid DH, Ibrahim AI. Evaluation of Friction and Surface Characteristics of Two Types of Self-Ligating Bracket Gate: An In Vitro Study.Dent Hypotheses 2022;13:27-35

How to cite this URL:
Kanbar HA, Obaid DH, Ibrahim AI. Evaluation of Friction and Surface Characteristics of Two Types of Self-Ligating Bracket Gate: An In Vitro Study. Dent Hypotheses [serial online] 2022 [cited 2022 Aug 10 ];13:27-35
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Full Text


The success of orthodontic treatment using straight wire appliances depends on the ability of the archwire to slide through the brackets and tubes at various stages of treatment.[1] The main problem encountered to achieve proper orthodontic tooth movement is the generation of frictional forces, which are the forces that oppose movement between two surfaces[2] (supplementary text 1).[3]

 Materials and Methods


Two types of metallic self-ligating brackets of upper right central incisor with a slot size 0.022∗0.028″ (standard torque prescription for the Damon Q brackets and Roth prescription for the Pactive brackets) were investigated: 56 Damon Q (Ormco Corporation, USA) and 56 Pactive brackets (IOS company, USA), coupled with two gauges of CuNiTi archwires (round 0.014″ and rectangular 0.014∗0.025″). The straight ends of the archwires were cut and used for the friction test: 36 pieces of 0.014″ and 36 pieces of 0.014∗0.025″ CuNiTi archwires (Damon Q, Ormco Corporation, Orange, CA, USA).


Preparation of the experimental blocks for testing the friction

Three brackets from each system were fixed on a plastic block using a cyanoacrylate adhesive (Soma Kimya Co., Turkey). Each block had three squares, with the central one positioned 2 mm higher than the other squares; the distance between the midpoints of the squares was 11 mm to mimic a segment of the dental arch that is unaligned,[4] as shown in (supplementary figure 1A).[5] A straight stainless steel wire jig of 0.021∗0.025″ gauge was used to align the brackets on the plastic blocks (supplementary figure 1B)[5] to eliminate the torque and the tip (factors affecting friction force).

Study design

The study was approved by the Research Ethics Committee of the College of Dentistry, University of Baghdad (Reference number: 588/588422).

Data analysts and biostatisticians were blinded toward the different types of brackets study models used. A simple, single-blinded, randomized allocation was employed for 36 study models with a minimum sample of nine models per subgroup, which was required to verify a significant difference in frictional forces between subgroups calculated based on a previous study[6] with an effect size of 0.35 and 80% power, two-tailed test at 5% level of significance using G-power version The friction was assessed for four subgroups; each was subjected to two bracket/archwire combinations, as demonstrated in (supplementary figure 2).[7]

Sample exposure to aging conditions

Samples were subjected to two aging conditions: acid challenge and water storage. For the acid challenge, the samples were exposed to an acidic solution that was prepared daily by gradually adding 3.5 mL of 1 molar hydrochloric acid solution (Thomas Baker Co., India) to 500 ml of distilled water until the acidity was set on a pH value of 2.5, using a protocol of three sessions per day, 5 minutes each, with equal intervals between sessions (2 hours) for 30 days. To simulate the wet oral environment, samples were placed in distilled water at 37°C (pH = 6) for the rest of the day.[8] For the water storage, the samples were immersed in distilled water and stored inside the incubator at 37°C for 30 days; the distilled water was replenished daily.[8]

The friction test

Friction was assessed using the Instron (H50KT Tinius Olsen, England) testing machine with a load cell of 10 N. The model was held by the machine’s lower part (the fixed part), while the upper part (the load cell) clamped the free end of the wire[9] (supplementary figure 3).[10] Following the data entry, each wire was pulled through the bracket slot over a distance of 5 mm at a speed of 5 mm/minute[2] until a 5 mm length of the wire was entirely pulled through the bracket. Each of the four subgroups involved nine models; each model was tested by pulling the round 0.014″ followed by the rectangular 0.014″∗0.025″ CuNiTi archwires. Meanwhile, a plastic syringe (china) was used to drip distilled water on the bracket/wire combination during the friction test. Only 3 mL/min of distilled water was dripped in each test for standardization purposes.[11] Frictional forces were displayed on the computer screen of the testing machine (QMat 4.53 T series software, England) and both the static and kinetic frictions were calculated.

Assessment of surface characteristics of bracket gates

The inner gate surfaces of 18 randomly selected brackets, with three “as-received from the manufacturer” brackets from both companies and three brackets of each artificial aging subgroup (post friction test) were analyzed with a scanning electron microscope (SEM) (Inspect F 50, Holland) at four magnifications: 150 ×, 300 ×, 5000 ×, and 10,000 ×. The images obtained were examined using a scale for quantitative classification described by Agarwal et al.,[12] but were modified in this study for a more objective description of surface characteristics, as shown below:

Score 0: Smooth surface (flat surface with no pits, no scratches, and no surface irregularities)

Score 1: Relatively smooth surface (flat surface with dispersing pits or mild surface irregularities)

Score 2: Relatively rough surface (when the surface has pits with scratches or grooves)

Score 3: Rough surface (the surface has pits, scratches/grooves, and marked surface irregularities)

Two independent orthodontists scored the SEM images, and the outcomes were examined with the kappa interrater test. The percentage of agreement was 90%, which means almost perfect agreement.

Statistical analysis

The statistical analysis was conducted using the SPSS 23 statistical package of social sciences (SPSS Inc., Chicago, IL, USA) at a level of significance of P < 0.05. A parametric test (two samples independent t test) was used for frictional force analysis, while a nonparametric test (Mann–Whitney U test) was utilized to examine the difference in bracket gate surface scores.


Normally distributed data were found according to the Shapiro–Wilk test; hence, parametric tests were used as follows.

Descriptive statistics

The descriptive statistics (mean, standard deviation, and minimum and maximum values) of the frictional force, measured in grams (g), of each subgroup are shown in (supplementary figure 4).[13] For both types of wires, the mean values of the frictional forces were higher with the Pactive brackets than with Damon brackets post both aging conditions (acid attack and water storage). Moreover, in two types of brackets, the acid challenge yielded an elevation in the frictional forces generated when these brackets were coupled with either wire type (round 0.014″ or rectangular 0.014∗0.025″ CuNiTi) (supplementary figure 4).[13]

Inferential statistics

Comparison between the two bracket systems

Independent samples t test showed a statistically insignificant difference between the mean values of both static and kinetic frictional forces of the two bracket systems when these brackets were coupled with the round CuNiTi wires in both aging conditions (water storage and acid challenge). However, there was a statistically significant difference between the mean values of the frictional forces (static and kinetic) of Damon Q and Pactive brackets when coupled with the rectangular CuNiTi wires in both aging conditions, as the Pactive brackets yielded significantly higher mean values of the frictional forces (supplementary figure 5).[14]

Comparison between the two aging conditions within the same bracket type

For the Damon brackets, the Independent samples t test revealed significantly higher mean values post brackets exposure to acid challenge than water when coupled with round wires, but there was no statistically significant difference when coupled with the rectangular archwires (supplementary figure 6).[15]

For the Pactive brackets, both archwire types yielded higher mean values of the kinetic frictional forces following exposure to acid challenge than water storage, but there was a statistically nonsignificant difference between the effects of acid and water storage on the static frictional forces (supplementary figure 6).[15]

SEM findings for the assessment of surface characteristics of the bracket gates

The SEM images were assessed according to the modified scoring system developed in this study. The “as received from the manufacturer” and “post-aging” Pactive brackets consistently yielded rougher surfaces than Damon Q brackets by recording higher scores (more pits, grooves/scratches, and surface irregularities) as shown in (supplementary figures 7, 8, and 9)[16]–[18] respectively. However, a statistically nonsignificant difference was revealed by the Mann–Whitney U test between the two bracket systems’ surface characteristics of the bracket gates (supplementary figure 10).[19]


Comparison of frictional forces between the two bracket types

Results showed no statistically significant difference in static or kinetic friction between the brackets under both aging conditions when coupled with round CuNiTi archwire in contrast to a statistically significant difference when using the rectangular archwire as the Pactive brackets produced significantly higher friction and this can be attributed to their flexible clip versus the sliding clip design of Damon brackets; as with the smaller round archwire, the clip acts passively, yet with the larger rectangular wire, the clip of the Pactive system encounters deflections yielding higher friction. This finding agreed with the results reported by Phaphriya et al.[20] Additionally, the Pactive bracket has a wider slot mesiodistally, which might increase the area of surface contact, hence exacerbating the friction.[21]

The available methods for evaluating metal bracket surface characteristics are relatively few, subjective, and lack a thorough description of the surface changes induced by orthodontic archwires.[12] Therefore, a modified scoring system was developed in the current study premised on a thorough evaluation of the surface changes as seen by the SEM images. The “as received from the manufacturer” and “post-aging” Pactive brackets demonstrated relatively higher scores with more pits, grooves/scratches, and surface irregularities on the bracket gate surface than Damon Q brackets. This finding can be attributed to the accuracy of the brackets manufacturing process, such as bracket milling or electro-polishing procedures that might initially leave surface defects. Moreover, acidic attacks could have exacerbated the corrosion, pitting, and grooving of these surfaces, a finding that has been confirmed previously.[22] However, pertaining to the bracket gate surface characteristics, statistically nonsignificant differences were found between the bracket systems, which may be caused by the bracket sample size that is small and used for SEM analysis (N = 3 per each artificial aging subgroup). According to the abovementioned findings, the null hypothesis regarding friction generation is rejected, while there is insufficient evidence to reject the null hypothesis regarding the bracket gate surface characteristics.

Comparison between the effects of the two aging conditions on the same bracket system

With either type of brackets, exposure of brackets to acid challenge resulted in significantly higher frictional force mean values than exposure to water storage. This may be attributed to the corrosive effect of the hydrochloric acid solution on the bracket gate surface that can lead to metal ions release, causing many surface defects, a finding that has been reported previously.[22] On the other hand, these surface defects might have been induced by bracket milling, pickling, or electro-polishing processes and may accelerate the corrosion in the presence of acidic attacks, as mentioned by previous studies.[23] Thus, the null hypothesis assumed in the current study is rejected, as there were significant differences between the effects of the two aging conditions on the same bracket system.

 Limitations of this study

The outcomes are based on in vitro models using limited numbers of brackets rather than full-arch models which may produce imprecision in data (supplementary text 1).[3]

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.












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