This study was approved by the regional ethics committee (OMUKAEK 2016/193). The study included 35 patients with a mean age of 14.4 years. They were selected according to the following criteria: (1) no visible WSL on the buccal enamel surfaces of the teeth, (2) permanent dentition, (3) no restoration of the buccal surfaces of the teeth, and (4) good oral hygiene at the beginning of the fixed orthodontic treatment. The study did not include the teeth that were extracted according to the patient’s orthodontic treatment plan.
All teeth were cleaned and polished before the fixed appliances were bonded. In the Clearfil (CF) group, teeth were acid etched with 32% phosphoric acid (Scotchbond™ Universal Etchant, 3M Dental Products, Germany) for 10 s, rinsed, and dried. Then an antibacterial and fluoride-releasing self-etching primer (Clearfil Protect Bond, Kuraray Medical, Okayama, Japan) was used according to the manufacturer’s instructions, and adhesive-coated brackets (APC Plus Victory series, 3M Unitek, Monrovia, CA, USA) were bonded. Clearfil Protect Bond has two properties: long-term fluoride release and 12-methacryloyloxydodecylpyridinium bromide (MDPB), which has an antibacterial effect. Although Clearfil is a self-etching primer, the manufacturer suggests applying 35% phosphoric acid to the uncut enamel and letting it remain for 10 s, before washing and drying. In the Transbond (TB) group, the teeth were acid etched with 32% phosphoric acid for 30 s, rinsed, and dried. Then, a conventional primer (Transbond XT Primer; 3M Unitek, Monrovia, CA, USA) was applied to the etched enamel surfaces, and the same adhesive-coated brackets were bonded. The Transbond XT primer has no fluoride-releasing property. The adhesive remaining around the bracket margins was removed. All bonding procedures were performed by the same investigator (A.A.O) who was blind to which material was used on which side during and after the procedures.
Another investigator (O.S.) provided all patients with oral hygiene instructions and brushing training. The patients were also prescribed with fluoride toothpaste. After the fixed orthodontic appliances were removed, the adhesive remaining on the enamel surfaces was removed using a carbide-finishing bur. The mean duration of orthodontic treatment was 16 months.
Standard clinical photographs were taken before and after orthodontic treatment according to the American Board of Orthodontics [16]. In addition, photos taken of each tooth individually before and after the orthodontic treatment showed the buccal surfaces of the premolars, canines, and all incisors. The same examiner (A.A.O) used those individual photographs to examine all WSLs presence both before and after orthodontic treatment. Patients with enamel hypoplasia, demineralization before the treatment, and other developmental alterations were excluded from the study. All examinations were conducted using the open-source image processing software (Image J, version 2.0, National Institutes of Health, Bethesda, MD, USA). The images that showed WSLs were scaled according to the mesiodistal measurements of each tooth as measured on an orthodontic model (Fig. 1). Then the actual areas of the WSLs were measured using the same software. The reliability of these measurements was assessed by having the same investigator recalculate the measurements of 10 patients selected at random. The mean error was 0.11 mm2 for the lesion areas.
The presence and severity of the WSLs were also recorded by visually assessing the photographs. The lesions were scored as follows: 0 = no WSLs, 1 = slight WSLs, 2 = severe WSLs covering more than one third of the surface, and 3 = WSLs with cavitation.
The clinical failure rates were also recorded during the fixed therapy. Each patient was recalled every month (4–5 weeks) for a routine appointment. First-time bracket failures were recorded. If a bracket broke, a new one was bonded at the first appointment after the failure. New brackets were bonded using the bonding protocols used at the beginning of the treatment for that patient’s investigation group. However, new brackets were not included in the bond-failure section of the study.
A split-mouth design was used to bond the brackets, allowing each patient to be his or her own control. Each patient’s oral cavity was divided into four quadrants. In 18 randomly selected patients, the teeth on the maxillary right and mandibular left quadrants were used as control sides (which received conventional primer), and the maxillary left and mandibular right quadrants of the dental arches were used as treatment sides (which received antibacterial and fluoride-releasing self-etching primer). For bonding the other 17 patients’ brackets, the quadrants were inverted. To eliminate any bias, the sides bonded with Clearfil Protect primer (CF group) and those bonded with Transbond XT primer (TB group) were alternated on each consecutive patient.
Statistical analysis
Statistical analyses were performed using a software package (IBM SPSS version 23, Chicago, IL, USA). Wilcoxon’s test was used to compare the areas of the WSLs between the groups. Bracket survival rates during orthodontic treatment were evaluated with the Kaplan–Meier test. Differences in bracket survival curves by primer type, tooth type, dental arch, and patients’ sex were evaluated with the log-rank test. Chi-square tests were used to analyze the relationship between frequencies of WSLs and primer type. The level of significance was set at P < 0.05.