INTRODUCTION

Bonding with visible light-cured (VLC) composite resins has become popular in orthodontic bonding mainly because of the extended time they allow for bracket placement.12 Halogen lamps have been widely used as the main curing units for composite resins. Light is emitted from a white halogen bulb, which is filled with iodine or bromide gas and contains a tungsten filament. When connected to an electric current, the tungsten filament glows.3 This produces a very powerful constant light but also a considerable amount of heat despite the placement of appropriate filters between the light source and the light guide of the halogen units.4

Light-emitting diode (LED) technology has been proposed as an alternative for curing VLC dental materials. This is an attempt to overcome the limitations inherent to the conventional halogen-based curing units, such as the undesirable infrared and ultraviolet radiation; the degradation of the bulb, filter, and photoconductive fibers over time; and the limited effective lifetime. Rather than a hot filament, as used in halogen bulbs, LEDs use junctions of semiconductors (p–n junctions) to produce light by electroluminescence. The semiconductor usually consists of gallium nitride (gallium nitride LEDs); thus, the light produced is emitted in the blue region of the visible spectrum so that no filters are required in these curing units. Furthermore, it has been reported that LED-based curing units have an expected lifetime of several thousand hours, and by contrast with conventional halogen-based units, they undergo little degradation of light output over this time.5 In addition, they are characterized by low power consumption and, therefore, they are suitable for portable use.6

There are no studies in the orthodontic literature examining the clinical performance of LED curing units. In vitro studies have found no statistically significant difference in the shear bond strength between halogen and LED lamps when the same polymerization time was used.7–9 However, the scientific integrity of bond strength protocols and the clinical relevance of these studies have been questioned.1011

The aim of this study was to compare the clinical failure rate of two different photopolymerization sources, a LED lamp and a conventional halogen lamp, over the first nine months of orthodontic treatment.

MATERIALS AND METHODS

The polymerization sources used in this study were the LED lamp Ortholux LED Curing Light (3M Unitek, Monrovia, Calif) and the halogen lamp Ortholux XT Curing Light (3M Unitek). Thirty consecutive patients with Class I, Class II, or Class III malocclusions who received comprehensive orthodontic treatment by means of fixed appliances (straight wire) at the Postgraduate Orthodontic Clinic of the authors' institution constituted the sample of this study. The patients were selected according to the following criteria: (a) presence of complete permanent dentition, (b) absence of occlusal interferences, (c) no need of extractions or orthognathic surgery, and (d) absence of visible dental enamel imperfections and/or restorations on the buccal surface of teeth. Gender, age, and race were not factors in the selection process. Approval by the institution's ethical committee and written parental informed consent were obtained.

A total of 600 edgewise brackets (0.018-inch slot, Roth prescription; Mini, Forestadent, Pforzheim, Germany) were directly bonded in all teeth except first and second molars after the following procedures were undertaken. The teeth were isolated (Nola lip/cheek retractor kit, Nola Specialties, Hilton Head Island, SC), pumiced with a fluoride-free paste (Clean Polish, Hawe-Neos Dental, Bioggio, Switzerland), rinsed, and dried with an oil-free syringe. The dried enamel surfaces were etched with 37% orthophosphoric acid (Monolok2 Rocky Mountain Orthodontics, Denver, Colo) for 30 seconds, rinsed, and dried with an oil-free syringe until a chalky appearance emerged. Light-cured Transbond XT adhesive primer (3M Unitek) was applied on the etched enamel surface and polymerized for 10 seconds as recommended by the manufacturer. Transbond XT adhesive paste (3M Unitek) was applied directly to the bracket base, each bracket was placed on the tooth with bracket placement pliers, and the excess adhesive material was removed with an explorer. Polymerization was performed with one of the two light curing units, according to the manufacturer's guidelines, with the light-guide tip placed as close as possible to the bracket-adhesive interface.

A split-mouth design was used. In 15 patients, the maxillary left and mandibular right quadrants were cured with the LED lamp (10 seconds for each bracket-adhesive interface: 5 seconds on the mesial and 5 seconds on the distal), and the other quadrants were cured with the halogen lamp (20 seconds for each bracket-adhesive interface: 10 seconds on the mesial and 10 seconds on the distal). In the other 15 patients, the quadrants were inverted. The split-mouth design was randomly alternated from patient to patient so that the two lamps were equally distributed on the left and right sides.

The initial wire placed in all patients immediately after bonding was 0.014-inch nickel-titanium, followed by various types of round and rectangular nickel-titanium and stainless steel wires according to the treatment plan. The number, location, and date of bracket failures were recorded for each light-curing unit over the first nine months of the treatment. Only first-time bracket failures were recorded. All bonding and clinical procedures were performed by the first author (Dr Koupis).

Bond-failure rates during the nine-month period were estimated for each light-curing unit. Failure rates were analyzed with respect to light-curing unit, tooth location, and patients' gender using χ² test statistics. All statistical analyses were performed at α = .05 level of significance.

RESULTS

Table 1 shows the number and rate of failures over the nine-month period. Both light-curing units showed sufficiently low bond failure rates. The overall failure rate recorded with the halogen unit (3.33%) was not significantly different (P = .0307) from the failure rate for the LED unit (5.00%).

Table 1.  Total Failure of Brackets Light-cured with the Two Lamps Used in the Study

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Table 2 presents the distribution of failures per dental arch, side, intra-arch location and gender. Higher bond failure rate was found in the mandibular dental arch compared with the maxillary (P = .008), in the posterior segments (premolars) compared with the anterior (P = .037), and in boys compared with girls (P = .040). No statistically significant difference was found between right and left segments (P = .838).

Table 2.  Distribution of Failures per Arch, Side, Tooth Type and Patient's Gender

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DISCUSSION

The present study focused on the failure rate of two different polymerization sources using a split-mouth design in a controlled clinical environment under identical conditions with respect to brackets characteristics. All bonding procedures were performed by the same clinician to eliminate interexaminer variability, and special attention was paid to moisture control by using an appropriate isolation system. Only first-time bracket failures were recorded, and the respective teeth were excluded from the sample.

The failure rate for the halogen lamp (3.33%) was lower than the rate (5.7%) found by Elaut and Wehrbein12 and agrees with that (3.41%) reported by Pettemerides et al13 These two studies used similar methodology and the same adhesive agent as the present study, but the latter used the same halogen source for 20 seconds for each bracket.13

There are no clinical studies in the literature evaluating the efficiency of LED lamps in orthodontic bonding. Previous in vitro studies found that LED and halogen lamps provided comparable shear bond strength when bonding orthodontic brackets with equivalent polymerization time.278

The overall failure rate recorded with the halogen unit was not significantly different (P = .0307) from that found with the LED unit. However, the duration of the polymerization with the halogen lamp was twice as long as that with the LED unit. This effect may be explained by the good correlation between the absorption spectrum of the camphorquinone—the photoinitiator used in Transbond XT adhesive resin—and the emission spectrum of the LED lamp, as has been previously noted.514

The distribution of failures between the two dental arches has shown considerable variation. Previous studies reported more failures in the lower dental arch, while others found no statistically significant difference between failures in the mandibular and maxillary arches.15–23 In the present study, more failures were found in the mandibular arch than in the maxillary arch. This difference may be attributed to occlusal interferences between the brackets of the lower and the upper dental arch during the first phase of orthodontic therapy, the gravity of the bolus of food, and the presence of more initial crowding in the lower dental arch—especially in the anterior segment—in most of the patients of this sample.

Regarding correlation between failure rate and dental arch left or right side, previous studies reported contradictory results.1924 The variability observed between these studies could be assigned to differences in mastication habits between patients, pressure during tooth brushing between right- or left-handed patients, as well as moisture control and handling of materials and bonding procedure between right- and left-handed operators. In the present study, no statistically significant difference was found in failures between the right and the left side, despite the fact that the operator and most of the patients in the sample were right-handed.

This study found more failures in premolars than in anterior teeth and this agrees with the results of previous studies.15–192123–27 This finding has been attributed to higher masticatory forces exerted on posterior teeth,19232829 access difficulties during bonding,15–17192526 and differences in the micromorphology and structure of the superficial enamel layer between posterior and anterior teeth.192330 Because more failures were found in boys than in girls (P = .040), it seems that the boys who participated in this study were less attentive to diet and care of the fixed appliances during the observation period.

CONCLUSIONS

• The present clinical study showed no significant differences in total bond failure rate between stainless steel brackets cured with a LED lamp and those cured with a halogen lamp, despite the different polymerization time used by the two sources.

• Therefore, LED curing units can be considered an advantageous alternative to conventional halogen sources in orthodontics because they enable a reduced chair-time bonding procedure without significantly affecting bond failure rate.