Methods:
The study was an in vitro experimental study conducted at Orthodontics & Dentofacial Orthopedics Unit, Department of Dentistry, Tribhuvan University Teaching Hospital, Maharajgunj Medical Campus, Maharajgunj, Kathmandu, Nepal in co-ordination with Nepal Bureau of Standards and Metrology, Balaju and Nepal Agricultural Research Council (NARC), Khumaltar. Ethical clearance was obtained from Institutional Review Board. One hundred twenty human premolars extracted for orthodontic treatment purpose were used in this study and non-probability convenience sampling technique was applied. Inclusion criteria were; human premolars with extraction time less than 4 months, intact buccal surface and stored in distilled water, while the exclusion criteria were; those with developmental defects, cracks caused by the extraction forceps, dental caries and the teeth subjected to any pretreatment chemical agent.
Custom fabricated mould was used to make acrylic block (Rapid repair, Dentsply India Pvt. Ltd.) and the teeth thus collected were mounted on an acrylic block such that the roots were completely embedded into the acrylic up to the cemento-enamel junction leaving the crown exposed. The labial surfaces of the teeth were kept perpendicular to the bottom surface of the mould [17]. Each sample was assigned number 1 to 120 and randomly divided into 4 groups (Fig. 1):
Group I : Control (new brackets, assigned with “C”)
Group II : Flaming group (assigned with “F”)
Group III : Flaming with sandblasting group (assigned with “S”)
Group IV : Flaming with ultrasonic cleaning group (assigned with “U”)
Before bonding, the buccal surfaces of the teeth were cleaned with fine pumice powder in water using a cup [7, 8, 10]. The buccal surface of each tooth was etched for 30 seconds with 37% phosphoric acid gel [18–20]. Each tooth was then rinsed with a distilled water spray for 5 seconds and dried with oil free air till the etched tooth will appear chalky white.[13, 16, 17, 21] A thin coat of light cured adhesive primer Orthosol (Enlight, Ormco corp. USA) was applied to acid-etched enamel. Enlight light cure adhesive resin was applied on the 0.022” slot MBT stainless steel double mesh premolar bracket base (Standard, Leone co, Sesto, Florentine, Florence, Italy) having a surface area of 11.6 mm2, which was then placed on the teeth with a reverse tweezers near the centre of the buccal surfaces. Light curing was done with Rainbow LED curing light for 10 seconds [13]. The light intensity measured with radiometer (CM300-2000, APOZA, Taiwan) was 830 mW/cm2. Group II, Group III and Group IV brackets were subjected to recycling and Group I brackets were stored in distilled water until final debonding with universal testing machine to measure shear bond strength.
Debonding of brackets in Group II, Group III and Group IV was done using peeling type of force before recycling as recommended by Zachrisson and Büyükyılmaz [22]. Recycling of brackets in Group II were done using flaming with reducing zone of the flame of the Gas microtorch (RS Pro, Nozzle, MT 790) for 5 seconds then quenched in water at room temperature and dried in an air stream (Fig. 2). Group III brackets were subjected to flaming for 5 seconds, quenched in water at room temperature and dried in an air steam as described above followed by sandblasting with 50 µm aluminium oxide abrasive powders using Bio-Art sandblaster (Rua Teotônio Vilela, Brazil). The distance between the bracket base and the handpiece head was fixed at 10 mm [8]. Each bracket was sandblasted for 25 seconds under 5 bar (72.5 psi) line pressure [8] (Fig. 3). In Group IV brackets, flaming was done using the same protocol followed by ultrasonic cleaning using ultrasonic cleaning solution from Gemoro ultrasonic parts cleaner solution solvent fluid, USA in an ultrasonic cleaning unit (Confident dental equipments Ltd, India) for 10 minutes [8] (Fig. 4).
Composite was removed from tooth surface with sixteen fluted tungsten carbide bur in unidirectional movement [23] with water cooling system until there was no visible adhesive remaining on tooth surface [22]. All recycled brackets were bonded to teeth using standard bonding procedure as described above. All samples were stored in distilled water until final debonding was done. Final debonding was done immediately after 24 hours of bonding to standardize shear bond strength in a universal testing machine (AG-IC/100 KN, Shimadzu, Japan) (Fig. 5) available at the Nepal Bureau of Standards and Metrology, Balaju at a crosshead speed of 0.5 mm/min[17]. The force required to dislodge the brackets was measured in Newton, and the shear bond strength (MPa) was calculated by dividing the force values with the bracket base area of 11.6 mm2
After bond strength testing, all specimens were collected and visually examined with a stereomicroscope (Olympus SZX12, Olympus corp., Japan) at 10X magnification to assess the adhesive remnant index [17, 24] available at Nepal Agricultural Research Council (NARC), Khumaltar. The adhesive remnant index was used to evaluate the amount of resin remaining on the tooth after debonding. At the beginning of the experiment, assessment of intraobserver reliability was done for which the entire procedure was performed by single person and the observation of shear bond strength and ARI scoring was also done by the same observer. Twenty percentage of samples from each group were randomly selected, altogether comprising 24 in number (6 in each group), and subjected to respective method of recycling. Shear bond strength was recorded using universal testing machine (T1). Same procedure was repeated after 2 weeks of first observation and shear bond strength was recorded (T2). The data were processed and analysed using the Statistical Package for the Social Sciences software version 21.0 (SPSS Inc. Chicago, Illinois, USA).