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  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 . 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, 17–19] (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  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 .
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 [26–28] 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 . 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 . MBT recommends 0.022-in.-slot height than 0.018-in.-slot height . Regarding the use of 0.022-in.-slot, 54 % of orthodontists preferred the 0.022-in.-slot size . 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, 31–37] 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.