Ethical approval

Approval to conduct this study was granted by the Institutional Review Board at the University of Minnesota on June 5, 2013 (code number 1305M33841).

Trial design and any changes after trial commencement

This was a single-center, single-operator, split-mouth randomized controlled trial with a 1:1 allocation ratio. There were no changes after trial commencement.

Participants, eligibility criteria, and setting

Consecutive patients presenting for comprehensive orthodontic treatment at the postgraduate program in orthodontics at the University of Minnesota, who were willing to participate, were recruited from September 2013 to April 2015 using the following inclusion criteria: (1) full permanent dentition including incisors, canines, premolars, and first molars; (2) teeth with sound, noncarious buccal enamel and no pretreatment with chemical agents such as hydrogen peroxide; and (3) no previous orthodontic treatment with fixed appliances. Subjects were excluded if they (1) were unable to give informed consent, (2) had craniofacial anomalies, (3) required extractions or orthognathic surgery as part of their treatment, or (4) had congenital enamel defects. Informed consent was obtained from all study participants. For a minor, consent was obtained from a parent or guardian, and assent was obtained from the minor.

Patients could withdraw from the study at any time without any compromise to the agreed treatment. Such dropouts were accounted for during data analysis and were not replaced. All data were recorded as agreed until the withdrawal date of each patient.

Interventions

The patients had their maxillary incisors, canines, and premolars bonded with adhesive precoated ceramic orthodontic brackets (Clarity Advanced Ceramic Brackets; 3M Unitek) using a system with a flash-free adhesive (APC Flash-Free Adhesive Appliance System; 3M Unitek) on 1 side and a system with a conventional adhesive (APCII Adhesive Appliance System; 3M Unitek) on the other side. These systems use identical brackets and differ only with regard to the adhesive coated on the brackets. The side allocation was randomized as detailed below.

Before bonding, the patients’ teeth were polished using a fluoride-free prophylaxis paste (Topex Prep & Polish; Sultan Healthcare, Hackensack, NJ) on a rubber cup attached to a low-speed handpiece, etched with 35% orthophosphoric acid (Temrex, Freeport, NY) for 30 seconds, rinsed thoroughly with water to ensure complete removal of the etchant, air-dried, and primed using a light-cure adhesive primer (Transbond XT Light Cure Adhesive Primer; 3M Unitek). The teeth were then bonded according to the predetermined side allocation using a direct bonding technique. The bonding sequence was standardized and started with the left second premolar, progressed around the arch, and ended with the right second premolar for each patient. Excessive adhesive around brackets precoated with the conventional adhesive was removed with a dental explorer. The adhesive was light-cured through the bracket for 3 seconds per tooth with a new light-emitting diode polymerization device (Ortholux Luminous Curing Light; 3M Unitek). The distance between the exit window and the adhesive was maintained at less than 5 mm to obtain optimum polymerization. The time taken to bond each tooth was timed to the nearest second using a digital stopwatch. Timing was started after enamel surface preparation and stopped before light curing. In addition, the total time taken to bond each quadrant was timed to the nearest second.

Bond failure was recorded at standardized appointment intervals of 4 weeks. In addition, the patients were instructed to call and visit the clinic immediately in case of breakage. In case of a bond failure, the adhesive remnant index (ARI) was scored to categorize the failure mode (ie; cohesive adhesive fracture, fracture at the bracket-adhesive interface, or fracture at the adhesive-enamel interface), and the patients were asked about a possible explanation for the bond failure. Remaining adhesive was removed from the tooth surface using a tungsten carbide finishing bur (H 283-21-012; Brasseler, Savannah, Ga) in a low-speed handpiece without water spray, the enamel surface was etched and primed as detailed above, and the tooth was rebonded with a new bracket as per the assigned protocol. All brackets associated with the study were bonded by 1 operator (T.G.) with 18 years of orthodontic experience. The patients were treated by 12 orthodontic residents under the supervision of 13 orthodontic faculty members. In cases of incorrectly placed brackets, compensating bends were placed in the archwire to avoid repositioning of brackets associated with the trial.

Outcomes (primary and secondary) and any changes after trial commencement

The primary outcome was bonding time (average per tooth for each patient and per quadrant).

The secondary outcomes were bracket failure rate for flash-free adhesive and conventional adhesive within 1 year for each patient, time to first-time failure of a bracket, and bond failure type using the ARI. The ARI is a point-based system ranking: 0, no adhesive left on the tooth; 1, less than half of the adhesive left on the tooth; 2, more than half of the adhesive left on the tooth; and 3, all adhesive left on the tooth.

There were no outcome changes after trial commencement.

Sample size and power of the study

Calculations were based on primary outcomes. With a sample size of 45, a paired t test will have 81% power to detect an effect size of 0.43 at a level of significance of α = 0.05.

Interim analyses and stopping guidelines

Not applicable. Patients with repeated bond failures caused by habits or trauma would be excluded from further analysis.

Randomization (random number generation, allocation concealment, implementation)

A randomization scheme with equal distributions of the 2 side allocations was generated using the online randomization tool at http://www.graphpad.com/quickcalcs . Allocation concealment was achieved with sequentially numbered, opaque sealed envelopes containing the side allocation of the adhesives, prepared before the trial. An independent person was responsible for opening the next envelope in sequence and implementing the randomization process.

Blinding

Blinding of the operator at bracket bonding was not possible because of the nature of the intervention. However, outcome assessment was blind for the secondary outcomes because these were recorded by different operators, and it was not possible to distinguish between the adhesives when recording bracket failures or scoring the ARI. Blinding of the patient could have been achieved but was not considered relevant.

Statistical analysis

Descriptive statistics were used to summarize the data. A Mann-Whitney rank sum test was used to compare ages between the male and female patients. Paired t tests were used to compare bonding times (average per tooth and quadrant) between the adhesive types after the data had been tested for normality (Shapiro-Wilk test). Bracket failure rates were expressed as percentages of failed brackets for each adhesive. A two 1-sided t tests test was used to test for equivalence between mean bracket failure rates. An equivalence test is a statistical hypothesis test whose inferential goal is to establish practical equivalence rather than a statistically significant difference. The adhesives were considered equivalent if the confidence interval for the difference between bracket failure rates fell within a margin of equivalence of ±5%. A difference of 5% was considered clinically meaningful, since this would signify 1 additional bracket failure per patient. A Kaplan-Meier plot and a Cox regression model with a robust sandwich covariance matrix (for within-patient correlations) were used to compare bracket survival times between adhesive types. A Wilcoxon rank sum test was used to compare ARI scores upon bond failures between the adhesives. Statistical analyses were performed using SAS software for Windows (version 9.3; SAS Institute, Cary, NC) with P values less than 0.05 considered statistically significant.

Participant flow (includes flow diagram, early stopping, and time periods)

Fifty patients were approached for inclusion in the study; 45 patients met the inclusion criteria, agreed to participate, and received the allocated interventions. Two enrolled patients were later excluded from further analysis because their treatment plan was changed to include extractions or they moved away from the study site ( Fig 1 ).

CONSORT flow chart showing patient flow during the trial.

Baseline data (including baseline table)

Study sample demographics are shown in Table I . The average age of the participants at baseline was 19.7 ± 9.3 years.

Numbers analyzed for each outcome, estimation and precision, subgroup analyses

Bonding times are based on 45 patients. Means and standard deviations of bonding times are shown in Table II . The bonding times were statistically significantly shorter with the flash-free adhesive than with the conventional adhesive, both per tooth ( P <0.001) and per quadrant ( P <0.001). Compared with the conventional adhesive, the average bonding times per tooth and per quadrant, when the flash-free adhesive was used, were 37.3% and 32.9% shorter, respectively. Means and standard deviations of bonding times by tooth type are shown in Table III .

Bracket failure rates are based on 43 patients. The bracket failure rates at 1 year were 3.7% for the flash-free adhesive and 0.9% for the conventional adhesive ( Table IV ) resulting in a difference in the failure rate between the adhesives of 2.8%. The numbers of bracket failures by adhesive and tooth types are shown in Table V . With the exception of 1 patient who had 2 failures, all failures came from different patients. All failures were unnoticed by the patients. Equivalence testing showed statistical equivalence of the adhesives with regard to bracket failure rates (90% confidence interval, 0.004-0.049; P = 0.047).

Table IV

Bracket failure rates at 1 year

Study group	Total (n)	Failures (n)	Censored		Failure rate
			n	Percentage
Flash-free adhesive	215	8	207	96.3	3.7%
Conventional adhesive	215	2	213	99.1	0.9%
Overall	430	10	420	97.7	2.3%

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