Evaluation of surface roughness of the bracket slot floor—a 3D perspective study

An important constituent of an orthodontic appliance is orthodontic brackets. It is either the bracket or the archwire that slides through the bracket slot, during sliding mechanics. Overcoming the friction between the two surfaces demands an important consideration in an appliance design. The present study investigated the surface roughness of four different commercially available stainless steel brackets. All tests were carried out to analyse quantitatively the morphological surface of the bracket slot floor with the help of scanning electron microscope (SEM) machine and to qualitatively analyse the average surface roughness (Sa) of the bracket slot floor with the help of a three-dimensional (3D) non-contact optical surface profilometer machine. The SEM microphotographs were evaluated with the help of visual analogue scale, the surface roughness for group A = 0—very rough surface, group C = 1—rough surface, group B = 2—smooth surface, and group D = 3—very smooth surface. Surface roughness evaluation with the 3D non-contact optical surface profilometer machine was highest for group A, followed by group C, group B and group D. Groups B and D provided smooth surface roughness; however, group D had the very smooth surface with values 0.74 and 0.75 for mesial and distal slots, respectively. Evaluation of surface roughness of the bracket slot floor with both SEM and profilometer machine led to the conclusion that the average surface roughness was highest for group A, followed by group C, group B and group D.


Background
One of the important constituents of an orthodontic appliance is brackets. Brackets stay for more time in the patient's mouth, among all orthodontic materials [1]. In fixed orthodontic treatment, some degree of sliding between bracket and archwire occurs and the frictional resistance is encountered. So between the two surfaces, overcoming the friction demands an important consideration in an appliance design [2].
The frictional force to the force with which the contacting surfaces are pressed together is proportional and is affected by the nature of the surface at the interface (rough or smooth, chemically reactive or passive, modified by lubricants, etc.). This is because all surfaces, however smooth they are, have irregularities which are seen as large on a molecular scale, and real contact occurs at the peaks of the surface irregularities. These peaks, known as asperities, between the two surfaces carry the entire load [3,4].
As compared to aesthetic ceramic brackets, metal brackets of stainless steel have a good superficial surface homogeneity and because of which, it has favourable mechanical properties and corrosion resistance. Even though a protective passive layer is present on the SS alloy, the Fe, Cr, or Ni (or all) ions may still be released from the metal surface in the acidic oral environment over the corrosion processes, which increases the risk of tissue damage, aesthetic changes (staining of the tooth by corrosive products) and loss of metal properties [5,6].
Between bracket-wire interfaces, the friction occurs. Some force which is applied is dissipated as friction, and the remainder is being transferred to the supporting structures of the tooth to initiate tooth movement. So if the applied force is of sufficient magnitude, there is an occurrence of biological tooth response [4]. Hence, the amount of friction has a direct relation with the accuracy of bracket slot dimension and bracket slot roughness. This tempted us to evaluate the surface roughness of the bracket slot floor.
The ongoing appliance evolution resulted in two orthodontic bracket sizes that a clinician may choose either 0.018-or 0.022-in. slot. The 0.022-in.-slot size is larger as compared to 0.018-in. slot and facilitates easier wire insertion and less frictional binding during initial alignment and provides increased stiffness during retraction. McLaughlin, Bennett and Trevisi (MBT) itself recommends using 0.022-in. slot and also most of the orthodontists prefer using this slot size, so we decided to use 0.022-in. slot brackets for the study.
Hence, the present in vitro study was conducted to evaluate the surface roughness in three dimensions of the stainless steel bracket slot which can help to determine the clinical performance of the bracket, the accuracy of bracket slot dimension and roughness of the bracket slot.

Aim and objectives
To access and compare the surface roughness of various as-received commercially available conventional 0.022 in. (0.56 mm) slot, MBT prescribed upper right 1st premolar stainless steel brackets.

Methods
A total of 80 pre-adjusted conventional upper right first premolar stainless steel brackets in as-received condition with 0.022 in. (0.56 mm) slot and with MBT prescription (0°tip, −7°torque) from four different manufacturers were taken. In a sample size of 80 brackets, 20 brackets were allotted in each group as The following apparatus were used for the study: Scanning electron microscope (SEM) machine (JSM-6360A, JEOL, Japan, available at Pune, India, with 10-20 kV) ( Fig. 1) Three-dimensional (3D) non-contact optical surface profilometer machine (Taylor Hobson, England, available at Bangalore, India) ( Fig. 2)

Procedure for scanning electron microscopy
From the sample of as-received 80 brackets, 20 brackets-i.e. 5 brackets from each group (groups A, B, C, and D) were randomly selected to analyse the morphological surface of the bracket slot floor (mesial and distal slots), with the help of a SEM machine (Fig. 1). The surface was scanned and viewed on the monitor screen and representative microphotographs at ×5000 magnification [5] of each bracket slot floor were obtained, which was sufficient to have a clear view of the surface characteristics of the slot floor area (Fig. 3). The images which were obtained were saved for the visual evaluation, using a discrete scale quantitative classification containing four scores [6,7]: 0-very rough surface 1-rough surface 2-smooth surface 3-very smooth surface

Surface roughness measurement
Each group was evaluated for the Sa of the bracket slot floor with the help of a 3D non-contact optical surface profilometer machine (Fig. 2).
The measurements were done in sequence of the group's name like group A was evaluated first then group B, C and D, respectively. Also, each time measurement of the mesial slot was done first and then the distal slot for all the samples to maintain a standardized protocol. Each sample was placed on the flat surface of the profilometer machine with the help of a tweezer, under the beam of the white light interferometry which is usually made up of the He-Ne, 633 nm [8].

Configuration settings Scanning speed
The ×1 option was selected, such that the slower the scanning speed of the sample, the greater will be the details of the fringes of the bracket slot floor as shown in Fig. 4.

Measurement setting
The measurement of the slot surface roughness is analysed by software Digital Surf, (TalyMap Platinum software, Leicester, England. Version no. 6.1.6001), from which we can receive 3D and 2D surface texture parameters (Fig. 5). The 3D surface texture parameters and height parameters (ISO25178) are the following (Fig. 6d): Sp-maximum peak height Sv-maximum pit height Sz-maximum height Sq-root mean square height Ssk-skewness Sku-kurtosis Sa-arithmetic mean height From these 3D parameters, we are interested in the Sa value which is an average of the surface heights giving us the average surface roughness of the bracket slot floor in three dimensions.
From the point of two-dimensional parameters, Ra value is of significant importance as it is the mean of the roughness profile. But the Ra value represents the slot in two dimensions. If we draw several lines (profile lines) on the area of the bracket slot floor, we will receive several readings for a single slot of the bracket, which means each bracket slot will have many different Ra values, which is not the case in the three-dimensional Sa value, as it is the average of the whole surface area, the area as a whole which is taken into consideration (rectangular dotted line Fig. 7d).
A template is made ( Fig. 6) in which once the slot surface is scanned, a raw surface view of the slot is received. The raw surface view which was obtained requires a levelling to be done, so that the whole surface is levelled in one form with the help of the software Digital Surf, (TalyMap Platinum software, Leicester, England, Version no. 6.1.6001). Once this is achieved and the threshold is applied, the software eliminates the highest peak and the deepest valley of the slot floor surface and gives the readings which are less biassed by these highest peak and the deepest valley.
An ICC test between examiners was performed. Twenty percent of the samples from each group were re-evaluated in order to evaluate the error of the method.
Likewise, all the 80 samples of stainless steel bracket measurements were completed. One hundred sixty readings of the Sa value were obtained ( Table 3).

Method of statistical analysis
1. SEM analysis-images were analysed, using a discrete scale quantitative classification containing four scores. 2. Surface roughness analysis-all the data collected had normal distribution, so we could apply the parametric tests. Analysis of variance (ANOVA) was used to determine whether the difference between the groups was significant or not. 3. Post hoc comparison test (Tukey's multiple comparison test) was used for inter-group comparison.

Scanning electron microscope results
Surface roughness scores were group A = 0-very rough surface, group C = 1-rough surface, group B = 2-smooth surface, and group D = 3-very smooth surface, respectively. This suggests that group D bracket slot floor were smoother as compared to group A, group B, and group C, respectively.
Surface roughness results Figure 8 shows the mean of the Sa values across the four groups, the surface roughness for both mesial and distal slots was highest in group A, followed by C, B and D, respectively. This indicates that there were differences in the surface roughness across the four groups with group D having the smoothest surface with values 0.74 and 0.75 for the mesial and distal slots, respectively. Table 1 shows the results of the ANOVA test. From the results, we noticed that the F-statistic of the average surface roughness for the mesial slot was 6.898479 and for the distal it was 16.0508, both of these values result in a very low p value (<0.001) which allows us to reject the null hypothesis and conclude that a difference amongst the means across the four groups was present. Table 2 shows the results of the Tukey's multiple comparison test (post hoc test). Thus, surface roughness was highest for group A, followed by C, B and D, respectively. Groups B and D provided smooth surface roughness; however, group D had the very smooth surface with values 0.74 and 0.75 for the mesial and distal slots, respectively.

Discussion
Resistance to sliding (RS) between brackets and archwires during treatment greatly influences the force transmitted to the teeth; to close the extraction space, sliding mechanics are widely used, and they may reduce the orthodontic force as much as 50 % of the bracket [9] and archwire materials. The coefficient of friction is an important factor in RS [10,11], which might depend on the roughness, texture, and/or hardness of the contacting material surfaces [12]. Thus, studies of bracket surface roughness are of great clinical interest.
Previous studies have measured the surface roughness of brackets and archwires using scanning electron microscopy [13,14], a contact surface profilometer [15,16] and atomic force microscopy [11,15,[17][18][19] (AFM). SEM can visualize two dimensionally the surface morphology, and a quantitative information is not being provided regarding the selected area. A contact profilometer allows two-dimensional determination of surface roughness parameter values. However, the sample surface adjacent to the scanning line may be damaged as the measured area is in the form of a line, and in contrast, more advantages are being provided by AFM, such as 3D quantitative and configuration measurements of the selected surface. A 3D non-contact surface profilometer which is available and is based on white light interferometry methods uses He-Ne beam of 633 nm [8] which can successfully allow determination and 3D imaging of surface roughness parameter values. The measurement needs no sample preparation and is non-destructive. So, the test range cover of ∼10 mm can be achieved. So, the surface roughness of the all the specimens were evaluated by using SEM and 3D interferometry profilometer machines, which is non-destructive and much faster compared with a stylus profilometer, and with a larger field, needing no sample preparation, compared with AFM [6].
Hence, all the tests were carried out to analyse quantitatively the morphological surface of the bracket slot floor with the help of SEM machine and to qualitatively analyse the Sa of the bracket slot floor with the help of the 3D non-contact optical surface profilometer machine. In the present study, it was found that the F-statistic of average surface roughness for the mesial slot was 6.898479 and for the distal, it was 16.0508 both of these values result in a very low p value (<0.001) which concluded that a difference amongst the means across the four groups was present. The surface roughness was highest for group A, followed by C, B and D, respectively. Groups B and D provided smooth surface roughness; however, group D had the very smooth surface with values 0.74 and 0.75 for the mesial and distal slots, respectively.
Ceramic brackets, whether they are single crystal sapphire, polycrystalline alumina, or zirconia, relative to stainless steel (SS) brackets, have poor frictional properties [13,20,21]. To combat this, aesthetic ceramic brackets with metallic slot have been developed [22,23]. Compared to stainless steel brackets, titanium brackets have more surface roughness [24,25]. Self-ligating brackets [26][27][28] with low-friction and full bracket engagement of the archwire, which is easy to use, assist good oral hygiene and is comfortable for the patient. But because of its partially reduced torquing capacity and high cost, conventional brackets are widely being used [29]. The above studies reveal that conventional metallic brackets are widely used than ceramic, zirconia, titanium, and self-ligating brackets either due to their properties or cost factor. Hence, the present study evaluated the conventional stainless steel brackets.
The present study was conducted to analyse the Sa of the bracket slot floor of the conventional stainless steel bracket with the help of the 3D non-contact optical surface profilometer machine. The surface roughness was highest for group A, followed by C, B and D, respectively. Groups B and D provided smooth surface roughness; however, group D had the very smooth surface with values 0.74 and 0.75 for the mesial and distal slots, respectively.
In order to select the proper low-friction bracket system, clinicians should consider specific characteristics of slot design [30]. MBT recommends 0.022in.-slot height than 0.018-in.-slot height [31]. Regarding the use of 0.022-in.-slot, 54 % of orthodontists preferred the 0.022-in.-slot size [32]. With this view, we decided to take 0.022-in. MBT slot conventional stainless steel brackets for the present study. Upper-right first premolar brackets were included in this study as to maintain a standardized protocol. Many previous studies [1,2,6,17,19,20,24,[31][32][33][34][35][36][37] have been done on surface roughness. Some of these studies evaluated the friction characteristics, surface roughness, ligation method, etc. To evaluate friction, there is a need to compare any two materials like stainless steel, ceramic, either mono or polycrystalline, titanium, and zirconium. But the present study evaluated the surface roughness of stainless steel brackets itself with the help of the 3D non-contact optical surface profilometer machine and SEM. There was no comparison of friction of the stainless steel brackets with the other materials like wires or ligation technique. So the study was focussed on evaluating the surface roughness in three dimensions of the stainless steel bracket slot which can help to determine the clinical performance of the bracket, the accuracy of bracket slot dimension and bracket slot roughness. The brackets Mini 2000, Ormco Corp. (Glendora, CA) and Mini master (American Orthodontics) have smooth surfaces of bracket slot.
The effects of the oral environment cannot be simulated in an in vitro exploration, and a possible limitation of the present study is the small sample size in each group. In vivo studies with a large sample size in each group will be needed to examine the    intraoral exposure effects on surface roughness in three dimensions of the stainless steel bracket slot.

Conclusions
On SEM evaluation for surface roughness of bracket slot floor, surface roughness for group D was found to be the smoothest of all the groups, while group B is less smooth, group C is rough, and group A was found to have a very rough surface. Measurement of surface roughness of the bracket slot floor with the 3D non-contact surface profilometer machine, led to the conclusion that the average surface roughness was highest for group A, followed by group C, group B and group D, respectively.