In-vitro bond strengths and clinical failure rates of metal brackets bonded with different light-emitting diode units and curing times


The purpose of this study was to compare the clinical failure rates and the in-vitro bond strengths of metal brackets bonded with different light-emitting diode (LED) devices and curing times.


Forty patients were included in the clinical part of this study. A split-mouth design was used, with the adhesive in group 1 cured for 10 seconds with an LED unit (Elipar S10; 3M Unitek, Monrovia, Calif), and the adhesive in group 2 cured for 3 seconds with another LED unit (VALO Ortho; Ultradent Products, South Jordan, Utah). Bond failures during 12 months of orthodontic treatment were recorded. In-vitro performance of the brackets was also compared by bonding brackets to extracted premolars and using the same light units and curing times (n = 20 for each group). The adhesive remnant index was used to determine the bond failure interface.


Clinical bond failure rates were 2.90% for the Elipar and 3.16% for the VALO curing units. The difference in bracket failure rates between the 2 LED devices was not statistically significant. No statistically significant difference was found between the in-vitro bond strengths of the groups.


Our findings regarding long-term clinical survival rates and in-vitro bond strengths indicate that bracket bonding can be safely accomplished in 10 seconds of light-curing with an Elipar LED and 3 seconds of light-curing with a VALO LED.


  • Bond failure rates after high-intensity and conventional light-emitting diode curing were compared.

  • In-vitro results were compared with clinical bond failure data.

  • Three-second high-intensity and 10-second conventional curing had similar failure rates.

Light-cured materials developed for bracket bonding have come into widespread use in clinical orthodontic practice based on several advantages, including better bracket positioning, shorter working time, and easy removal of excess material. Curing devices have developed in line with emerging technologies in dentistry. Although conventional halogen lights have for decades been the main devices used in light-curing, their curing times of 20 to 40 seconds per bracket are an inconvenience for clinicians. Alternatives to halogen lights include plasma arc and light-emitting diodes (LEDs). Compared with the 300 to 400 mW intensity of conventional halogen lamps, most LEDs emit energy at levels up to 1600 mW, and plasma arc lamps up to 2000 mW.

Various studies have compared halogen lights, LED devices, and plasma arc lamps in terms of clinical bond failures of orthodontic brackets. Sfondrini et al evaluated bond failures of metal brackets cured with conventional halogen and plasma arc light-curing units and found no significant difference in the failure rates between the 2 units. Similarly, Krishnaswamy and Sunitha assessed the clinical performance of conventional halogen units and LEDs during 15 months and found no statistically significant difference in bond failure rates and mean survival times. Furthermore, Pandis et al compared the long-term failure rates of brackets bonded with high-intensity LED and plasma arc curing units and found that LED lights had higher bond failure rates than plasma lamps. An in-vitro study comparing the shear bond strength values of brackets cured with halogen, plasma arc, and LED light sources found no differences between any of the light sources. Although there is no clear evidence to suggest any significant differences in the bracket failure risk between the 3 curing light systems, most studies have emphasized that plasma arcs and LEDs significantly reduce the curing time of orthodontic brackets, with plasma arcs requiring only 6 seconds and LED units 10 seconds for bracket bonding.

Recently, a new LED curing light unit (VALO Ortho; Ultradent Products, South Jordan, Utah) has become commercially available for orthodontic bonding. This product has 3 curing modes including an extra power mode with 3200 mW per square centimeter of light intensity. Because of its high-intensity light waves, it is claimed to cure adhesive materials in 3 seconds. Currently, no studies have evaluated the performance of this LED curing unit.

Long-term clinical studies could provide more relevant data on actual clinical bond failures than in-vitro studies, which might not be a proper guide for clinical practice. Therefore, in this study, we aimed to investigate and compare clinical bond-failure rates of brackets bonded with 2 LED curing units with different emission intensities and curing times. We also evaluated the in-vitro bond strengths of the same curing protocols using a universal testing machine.

Material and methods

This study was approved by the regional ethics committee (ethics number B.30.2.ODM.0.20.08/1129) of Ondokuz Mayıs University, Samsun, Turkey. Our subjects were 40 patients with a mean age of 14.3 years (13 boys, 27 girls). They were selected according to the following criteria: (1) permanent dentition with fully erupted teeth; (2) no morphologic crown anomalies; and (3) no restorations that might affect bracket bonding on buccal enamel surfaces.

All teeth were cleaned and polished with pumice before bonding. A total of 759 metal brackets (Gemini; 3M Unitek, Monrovia, Calif) were included in the assessment of bond failures during orthodontic treatment. All brackets were bonded using Transbond XT adhesive resin (3M Unitek) according to the manufacturer’s instructions. A split-mouth design was used, and the patient’s oral cavity was divided into 4 quadrants. In 20 randomly selected patients, the maxillary left and mandibular right quadrants of the dental arches were cured using an Elipar S10 LED light-curing unit (3M Unitek) for 10 seconds, and on the other side the teeth were cured with a VALO Ortho LED light-curing unit for 3 seconds. Each of the 5 teeth was exposed for 3 seconds followed by a 2-second rest. The quadrants were inverted when the other 20 patients’ brackets were cured. The sides cured with the Elipar and VALO units were alternated with each consecutive patient to eliminate any bias that might have been introduced because the clinician (A.Z.O.) was right handed. The specifications of the LED light-curing units are given in Table I .

Table I
Specifications of the LED light-curing units
Light intensity (mW/cm 2 ) Wavelength (nm)
Elipar S10 1600 430-480
VALO Ortho 3200 395-480

The patients were recalled every 4 weeks for orthodontic treatment. First-time bracket failures were recorded, and broken brackets were replaced at the first appointment after the bond failure. New brackets replacing failed brackets were not included in the clinical study.

Bond strengths of the different LED devices were also investigated in vitro using 40 extracted premolars collected from patients who had undergone extraction therapy. The sample size was determined using the data from Dall’Igna et al. A sample size of 20 teeth per group would give more than 99% power to detect significance differences at the 0.05 level. The teeth were stored in distilled water until use. Extracted teeth were embedded in casts, with the buccal surfaces exposed and aligned with the horizontal plane. Enamel surfaces were acid-etched with 37% phosphoric acid for 30 seconds, rinsed, and dried. Gemini metal brackets were bonded to the teeth with Transbond XT adhesive resin according to the manufacturer’s instructions. The samples were randomly divided into 2 groups, and those in group 1 were cured with the Elipar curing unit for 10 seconds, and those in group 2 were cured with the VALO curing unit for 3 seconds. The samples were stored in distilled water at 37°C for 24 hours before the debonding tests. Bond strength was tested using a testing machine (LRX; Lloyd Instruments, Fareham, Hampshire, United Kingdom) set at a crosshead speed of 1 mm per minute.

The samples were scored using the 4-step adhesive remnant index (ARI) according to the method of Årtun and Bergland, as follows: 0, all adhesive on the bracket base; 1, more than half of the adhesive on the bracket base; 2, less than half of the adhesive on the bracket base; and 3, no adhesive on the bracket base.

Statistical analysis was performed using SPSS for Windows (version 15.0; SPSS, Chicago, Ill). Bracket survival rates were evaluated with the Kaplan-Meier test. Differences in bracket survival curves by LED device, tooth type, dental arch, and patients’ sex were evaluated with the log-rank test. In-vitro bond strength was evaluated using paired samples t tests. In-vitro ARI scores were assessed using the chi-square test. The level of significance was set at P <0.05.


Table II shows the sample characteristics and distributions of bracket types, curing units, and sexes. Of a total of 759 brackets, 23 bond failures occurred over the 12-month evaluation period. Bond failure rates for the Elipar and VALO curing units were 2.90% and 3.16%, respectively. The difference in bracket failure rates between the 2 LED devices was not statistically significant. No significant difference was observed between the dental arches and sex in the bracket failure rates. Bond failure rates were higher for premolars (6.03%) than for incisors (0.95%) and canines (1.88%) ( Table III ). The effects of curing unit, sex, dental arch, and tooth type on bracket survival rates are shown in the Figure .

Table II
Identification of the samples
Number Percentage
Patients 40
Female 27 32.5
Male 13 67.5
Brackets 759
Bonded with VALO 380 50.1
Bonded with Elipar 379 49.9
Maxillary brackets 375 49.4
Mandibular brackets 384 50.6
Maxillary incisors 159 28.4
Mandibular incisors 158 20.8
Maxillary canines 80 10.5
Mandibular canines 80 10.5
Maxillary premolar 136 17.9
Mandibular premolar 146 19.2

Apr 6, 2017 | Posted by in Orthodontics | Comments Off on In-vitro bond strengths and clinical failure rates of metal brackets bonded with different light-emitting diode units and curing times
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