Appropriate bond strength between bracket and tooth surface is one of the most important aspects of orthodontic treatments [1,2]. Bonding of MIM monoblock metal bracket to enamel started in the mid 1960s [3,4]. Only auto-polymerizing materials were available at the time. Bonding of orthodontic brackets with visible light-cure adhesives was first reported by Tavas and Watts [5]. The light-cure adhesives were widely accepted due to their advantages in comparison with other chemical-cure adhesives. These advantages include high primary bond strength, better physical characteristics because of air inhibition phenomenon, user friendly application, extended working time for precise bracket placement and better removal of adhesive excess; but they have three major disadvantages namely being time-consuming, hindering light transmission and polymerization shrinkage [6,7]. Since then, several new methods using different composites and light-curing units have been introduced for this purpose. The halogen lamp, also known as quartz halogen and tungsten halogen lamp, has been used as light-curing unit for many years [8,9], and is the most common source of visible blue light for dental applications. This lamp contains a blue filter to produce light of 400–500 nm wavelength [10]. The wide spectrum of action, easy use and low-cost maintenance are the most favorable characteristics of halogen light curing systems [9]. Despite their popularity, halogen light curing units have several disadvantages. For example, their light power output is 1% of the total electric energy consumed [11,12]. Moreover, the lamp, reflector and filter wear out gradually [13]. Halogen bulbs have a restricted useful lifetime of about 40–100 hours [13,14]. The power density of light curing unit decreases with increase in distance. The other drawback of application of halogen bulbs is prolonged curing time [15,16]. Over the past several years, other light sources such as xenon plasma arc, argon laser, and light-emitting diodes (LEDs) have been introduced in orthodontics [17]. According to the results of previous studies [1,18–20], the shear bond strength (SBS) values of orthodontic brackets in curing with halogen lamps and plasma arc are the same but plasma light reduces curing time per tooth from 20–40 seconds to two seconds. Also, argon laser curing unit provides better SBS than halogen lights. But xenon plasma arc and argon laser are too expensive [18]. Mills [19] introduced LED light curing units as a polymerizing light source in 1995. At present, LED sources are among the most reliable light source categories for bracket bonding [8,20]. Light cure resins set when irradiated with light at wavelengths of 460nm and 480nm in the blue end of visible spectrum with an intensity of 300mW/cm 2 [21]. Also, LED is an effective transducer of electrical power into visible blue light and does not produce a lot of heat [8]. The advantages of LED light curing units include lifetime of several thousand hours without significant degradation of light flux over time, resistant to shock and vibration and no need for filter to produce blue light [22–24]. Moreover, LED light curing units consume little power and can be run on rechargeable batteries, allowing them to have a lightweight ergonomic design [25]. The new LED curing units were launched simultaneously with the advancement of technology. First, these curing units generated light with an intensity of approximately 800–1000YmW/cm 2 , reducing the required light exposure time to 10 seconds [26,27]. Currently, some high-power LED curing units are able to emit light radiation with intensity of 1600–2000YmW/cm 2 , allowing shorter exposure times of six seconds for metal brackets [28]. In this study, the effect of conventional and high-power models of LED units on SBS of metal and ceramic brackets to tooth surfaces was evaluated.

Forty sound bovine maxillary central incisors were used in this study. After extraction, the teeth were cleaned and immersed in 0.5% chloramine solution at 4°C for one week. They were divided into four groups of 10 teeth in each group. Next, teeth surfaces were etched with 37% phosphoric acid (Reliance; Itasca, IL, USA) for 20 seconds. After etching, the teeth were washed with water spray for approximately 10 seconds. The sample size (n=8 minimum samples for each group) was calculated with a power analysis in order to provide a statistical significance of alpha=0.05 and a standard deviation of 4.2 MPa using Minitab software. Sampling method in the study was consecutive. Bracket model and the type of light curing unit used for teeth were determined randomly.

Group A: After checking correct conditioning of the enamel, metal brackets (American Orthodontics, Sheboygan, WI, USA) with a nominal base area of 11.3mm 2 were bonded with Transbond XT (3M ESPE, St. Paul, MN, USA), applying a uniform layer of adhesive primer on the etched enamel, and resin cement on the base of brackets. Brackets were placed in place and were pressed against the surface of the tooth. Excess cement was carefully removed with a dental probe, and the adhesive was high-power light-cured (2700mW/cm 2 ; Dentlight LLC, Plano, TX, USA) for four seconds (two seconds from mesial and two seconds from distal).

MIM bondable metal bracket with a nominal base area of 15.1mm 2 were bonded to the etched enamel and other steps were performed similar to group A. The adhesive was high-power light-cured for three seconds (1.5 seconds from mesial and 1.5 seconds from distal).

Group C: Metal brackets (American Orthodontics, Sheboygan, WI, USA) with a nominal base area of 11.3mm 2 were bonded to the etched enamel and other steps were performed similar to other groups. The adhesive was light-cured conventionally (600 mW/cm 2 ; Dr’s light, Good Doctors Co., Ltd., Incheon, South Korea) for 20 seconds (10 seconds from mesial and 10 seconds from distal).

Group D: Ceramic brackets (American Orthodontics, Radiance Plus, Sheboygan, WI, USA) with a nominal base area of 15.1mm 2 were bonded to the etched enamel and other steps were performed similar to other groups. The adhesive was light-cured conventionally for 20 seconds (10 seconds from mesial and 10 seconds from distal). The samples were mounted in a metal mold containing auto-polymerizing acrylic resin and thermocycled for 2,500 cycles between 5–55°C for 20 seconds at each temperature with 20 seconds of transfer time. Rectangular wires were used to match the central alignment of teeth in acrylic resin. All samples were subjected to SBS test in a universal testing machine (7060; Zwick Roell, Ulm, Germany) at a crosshead speed of 3 mm/minute (Fig. 1).

The results were obtained in kilogram-force, converted to Newtons and then to megapascals (MPa). After failure, the samples were observed under a stereomicroscope (SMZ 800; Nikon, Tokyo, Japan) at ×20 magnification to score the amount of remaining adhesive using the adhesive remnant index (ARI) [29]: 0=No adhesive remained on the tooth; 1=Less than 50% of adhesive remained on the tooth; 2=50% or more of the adhesive remained on the tooth surface; 3= 100% of the adhesive remained on the tooth, with a distinct impression of bracket mesh, corresponding to failure at the bracket-adhesive interface. Data were statistically analyzed using SPSS version 22.0.0 (SPSS Inc., Chicago, IL, USA).

The mean, standard deviation, minimum and maximum values of SBS of metal and ceramic brackets to tooth surfaces using two models of light-curing units were computed and reported. The SBS data were analyzed using one-way ANOVA, followed by Tukey’s post hoc test. Failure mode data were subjected to Kruskal-Wallis nonparametric test, followed by LSD post hoc test. Statistical significance was set at alpha=0.05.

Objective. Clinical comparison of the survival rates between stainless steel and ceramic brackets over a 12-month period. Materials and Methods. The study involved 20 consecutive patients with diagnosed malocclusion that required two-arch fixed appliance treatment. The participants were randomly divided into two 10-member groups. Group 1 was treated with Abzil Agile (3M Unitek) stainless steel brackets; group 2 was treated with Radiance (American Orthodontics) monocrystalline ceramic brackets. All the sapphire brackets were bonded by the same operator. Over the next 12 months, all bracket failures were recorded with each appointment. The received data were processed statistically using the Mantel–Cox test, Kaplan–Meier method, and Cox hazard model. Results. A total of 381 brackets were bonded, 195 of which were metallic brackets and 186 were ceramic ones. In the 12-month observation period, there were 14 metal (7.2%) and 2 ceramic bracket (1.1%) failures. The overall failure rate was 4.2% (n = 16). The majority of failures (14 brackets; 87.5%) occurred during the first 6 months of the experiment, 12 (83%) of which were metal brackets and 2 (100%) were ceramic brackets. The statistical analysis revealed significant differences between the groups (). Conclusions. Metal brackets demonstrated significantly higher failure rates than ceramic brackets for both 6- and 12-month observation periods (). The 6% difference between the brackets is clinically significant as it corresponds to one additional failure within 12 months

Bond strength of orthodontic brackets is an important factor which can influence the treatment with the use of fixed appliances. Bracket failures may potentially increase the total treatment time and financial costs of the therapy. The optimal bonding force between the bracket and enamel surface should be sufficient to enable a durable bracket position during treatment and to prevent the enamel from iatrogenic damage during the debonding procedure. Bond failures may be caused by numerous factors, including masticatory forces, forces produced by orthodontic appliances, aging of the orthodontic adhesives, mistakes during any step of bonding protocol, or some conservative dentistry therapies performed prior to bonding, such as topical fluoride varnish applications, or bleaching [7–9]. The range of the desired bonding force has not been determined yet. On the basis of their in vitro study, Reynolds and von Fraunhofer [10] stated that the minimum bond strength of 6–8 MPa is considered appropriate, whereas Bishara [11] suggested that the bonding strength ought to exceed 13.5 MPa. Recently Gauge [3] assumed that the ideal orthodontic adhesive should withstand forces over 20 MPa. The majority of the studies that evaluated the bond strength of zirconia brackets were carried out as in vitro experiments under ideal laboratory conditions that may not reflect all clinical conditions. In vitro experiments provide information about initial bond strength to the enamel but cannot serve as predictors of bracket survivability [12–14]. Therefore, more accurate guidance on the clinical relevance of adhesion protocols is provided by in vivo tests, which assess the failure rate of the enamel-boding agent-bracket interface during treatment.