Temperature Rise During Orthodontic Bonding With Various Light-curing Units—An In Vitro Study

The light-initiated resins have become the most popular adhesives for a majority of orthodontists because of the reduced contamination risk, accurate bracket placement, and easier excess adhesive removal during bonding.1–7 In the 1990s, rapid light-curing alternatives for conventional halogen units, such as Quartz Tungsten Halogen (QTH), Xenon Plasma Arc (PAC), and the light-emitting diode (LED), were introduced in orthodontics, parallel with general dentistry.2–4,6–8 However, concerns have been raised that these new curing lights may induce a temperature rise that could be detrimental to pulp vitality during photoactivation.

It is known that any increase in pulpal temperature exceeding 5°C to 6°C may result in irreversible tissue damage.9 Previous in vitro studies have shown the thermal effect of different light sources in general dentistry.10–16 Goodis et al17 tested six visible light–curing units and found that the lamps do cause a temperature rise within the pulp chamber, and the longer the lamp is used, the higher the temperature rise. Hannig and Bott18 used a Class II restorative preparation on a molar and measured the temperature changes in the pulp chamber with eight different curing units. Their findings indicate that the potential risk for heat-induced pulpal injury during composite resin polymerization is increased with the high-energy light sources when compared with the low-energy light-curing units. Powell et al16 showed that in vitro pulp chamber temperature increase from laser units were significantly lower than that from the conventional curing lights. Tarle et al19 measured the temperature rise in the composite samples with three different light sources, and because of very short exposure time, they found a slight temperature rise with the high-power plasma light.

Such investigations showed that heat generation may vary depending on the light source, exposure time, composite resin thickness–related exothermic reaction, and the distance between the light source and the pulp.10,17,20,21 However, there are several factors in restorative dentistry that vary from those in orthodontic bonding procedures.3 First, in bonding orthodontic brackets or retainers, the distance from the pulp is greater because of the lack of any cavity on the insulating enamel. Second, the orthodontic attachments (brackets, tubes, etc) are present between the light sources and the enamel. Third, the adhesive layer is very thin, pressed between the bracket and the tooth.

Oesterle et al3 carried out an unpublished pilot study and suggested using the xenon plasma light with short durations, especially on teeth that have thin enamel such as the lower incisors. However, the pulp temperature change after curing with different light sources and their effect on tooth type has not been investigated for orthodontic bonding procedures.

The aim of this in vitro study was to investigate the temperature changes in the pulp chamber of a premolar and a lower incisor during bracket bonding using three different light sources.

In this in vitro study, two different types of human teeth (premolar and lower incisor) were irradiated at two different distances (surface and 10 mm) with three visible light–curing units. Twelve experimental groups were prepared (Table 1). A single-root lower first premolar and a lower central incisor were used in all experimental trials. Brackets (Omni Arch, GAC, Bohemia, NY) were bonded using Transbond XT (3M/Unitek, Monrovia, Calif) adhesive and light cured with Halogen light for 40 seconds, with LED light for 20 seconds, and with Plasma Arc light for five seconds. The light units and technical details are shown in Table 2.

Orthodontic bonding was performed without acid etching to enable an easy removal of the bracket and to avoid enamel loss during repeated debonding procedures. In a pilot study, we tested the effect of acid etching on the pulp chamber temperature changes. A different lower premolar was bonded five times with acid etching using the plasma arc, and surface and pulpal temperatures were measured as described above. The mean values of the temperature increases were 1.11 ± 0.21 (with acid etching) and 1.00 ± 0.09 (without acid etching, V group). Mann-Whitney U-test did not indicate statistically significant differences between both values. Therefore, all further measurements were taken without acid etching. Light exposures were performed according to the manufacturers' instructions (Table 2).

For temperature measurements, the roots of the premolar and the incisor were resected and pulp residues were removed retrograde. A J-type 0.36-inch-diameter thermocouple wire (Omega Engineering Inc, Stamford, Conn) was inserted into the pulp chamber of the sample tooth to measure temperature changes. The thermocouple maintained contact with the dentin by a thin layer of silicone heat-transfer compound (ILC P/N 213414, Wakefield Engineering, Wakefield, Mass). Its position in the pulp chamber was checked radiographically. The root stub was then secured with a composite resin, and the sample tooth was mounted in the cover plate of a water bath (37 ± 0.5°C) (Nuve, Ankara, Turkey). The thermocouple wire was connected to a data logger (XR440-M Pocket Logger, Pace Scientific, Mooresville, NC) during the bracket bonding procedure. Intrapulpal temperature changes were recorded every two seconds. The recordings were started at the same time with light curing and were ended when the temperature had started to drop from its maximum level. The collected data, available in both tabular and graphic form, were monitored in real time and transferred to a computer.

Five measurements were obtained from each group. The difference between the starting and the highest temperature readings was taken, and the five calculated temperature changes were averaged to determine the mean temperature increase. The light outputs of the curing units were checked before each testing procedure with a Hilux radiometer (Benlioglu Dental, Ankara, Turkey). Although the handheld radiometers measure the light output within a narrow band, this was used to measure the wavelengths considered effective for curing.22 There was no measurable reduction in light intensity for any light during the experiment.

Analysis of variance (ANOVA) was performed to determine the effect of tooth type, light-curing unit, and distance on the temperature rise. The post hoc analyses were made using the least significant difference (LSD) test to determine which groups were significantly different. SPSS 10.1 (Chicago, Ill) was used for all statistical analyses in which P < .05 was adapted as the critical significance level.

Table 3 shows mean values, standard deviations, and the statistical significances of the temperature increase for all the three variables (light-curing unit, tooth type, and distance). The ANOVA and LSD tests revealed that pulp chamber temperature changes were influenced by the light-source type, tooth type, and the distance from the tip of the light guide to the bracket surface.

Halogen induced significantly higher intrapulpal temperature changes than did the LED and PAC (P = .000). The difference between Halogen and PAC was statistically significant at all but the 10-mm distance. When the light-guide tip was positioned at the surface of the lower incisor, there was no statistically significant difference between LED and PAC. The lower incisor was affected more than the premolar, but the differences were statistically significant only with PAC at two distances (P < .001) and with LED at the 10-mm distance (P < .05). The temperature increased significantly more at the surface than at the 10-mm distance with all light-curing units (P = .000). The graphic presentation of the results is shown in Figure 1.

The thermal effect of light sources is well known, but information about pulp chamber temperature changes during orthodontic bonding procedures is scarce. Publications in restorative dentistry showed that the direct comparison of the light sources' heat effect is very difficult, if not impossible, because of the great variability of investigated materials and exposure times. Tarle et al19 found that high-power plasma light leads to lower temperature increases than do Halogen or LED in composite samples because of very short exposure time. However, the results of other studies showed that a light source with high-energy output (Plasma Arc Light) caused significantly higher temperature changes than with the low-energy output lights (Halogen and LED) under dentin disks.23,24 Hofmann et al,11 using “deflecting disc technique,” recorded that the heating from radiation during polymerization was lower with LED light than with QTH.

Studies on pulpal heat changes were also different in several aspects. Weerakoon et al10 examined the heat changes associated with standardized Class V restorations on the buccal surface of extracted premolar teeth using a curing time of 40 seconds. They concluded that LED lights produced less thermal insult on dental pulp than did the halogen lamps. Hannig and Bott18 studied Class II restorative preparations on a molar and recorded that the risk for heat-induced pulpal injury was increased using the high-energy light source (Plasma Arc Light) at 10 seconds as compared with the low-energy light-curing units (Halogen units) at 40 seconds.

In this study, we used an orthodontic bonding setup to mimic the clinical situation and found the effect of different energy outputs on dental pulp. The thermal effect on the pulp tissue depends on the variations in the enamel and dentin thickness of the pulp chamber wall.10,25 Lower incisors exhibit a higher risk of thermal damage because of thinner enamel and dentin thickness on the labial side.21,26 However, variations within each tooth can also affect the thermal response.25 Accordingly, in this study only one lower premolar and one lower incisor were used to control the thickness of the labial pulp wall and to eliminate any possible structural differences.

To represent the clinical environment, the sample teeth were irradiated at a distance of 10 mm and at the surface.20 Five measurements were obtained from each group. There was no need to increase the number of measurements because of the very low values of the standard deviations (Table 3).

Our results showed statistically significant differences among the three light-curing units tested (Table 3). Halogen with the longest exposure time induced significantly higher intrapulpal temperature changes than did the LED or PAC. These findings conflict with the concern of increasing heat-induced pulpal injury risk with high-energy output lights, but reveal the importance of exposure time.18,23,24 The critical values reported for pulpal injury were not exceeded in any of the experiments of this study.9 However, there are only a few in vivo studies available in the literature reporting on the critical values for pulp injury, and one should be careful in the interpretation of these findings.9,21,25 

Comparison of tooth types revealed that the temperature increase was higher in the lower incisor than in the premolar with all three curing units. These findings were in accordance with the previous results.21,26,27 Statistically significant differences were found only with the PAC at two distances and with the LED at 10 mm. The temperature increase was significantly higher at the closer distance with all light-curing units (Table 3).

However, the temperature values measured in this study cannot be directly applied to temperature changes in vivo. The design of this study did not consider heat conduction within the tooth because of the effect of blood circulation in the pulp chamber and fluid motion in dentin tubules.18 In addition, the surrounding periodontal tissues can promote heat convection in vivo, limiting the intrapulpal temperature rise.27 On the other hand, actual temperature increases might be higher in clinical conditions in younger teeth. Therefore, extensions to this study will consider a histological investigation of the human pulp to evaluate the effects of the light curing during orthodontic bonding in clinical conditions. Future histological investigations are needed to clarify the thermal threshold for pulpal injury, not just to investigate the effect of light curing but also after other orthodontic procedures such as stripping, reshaping, adhesive removal, and thermal debonding.

• Halogen light induced significantly higher intrapulpal temperature changes than did the LED and PAC.

• The temperature increase was significantly higher at closer distances with all light-curing units.

• All light-curing units produced a higher temperature increase in the pulpal chamber of the mandibular incisor than in the premolar at both distances.

• High-energy output lights produced significantly higher heat changes in the incisor than in the premolar.

• Orthodontic bonding with light-curing units did not exceed the critical 5.5°C value for pulpal health.

The authors would like to thank Professor Nazan Alparslan for her assistance with statistical analyses and Dr Aslihan Usumez for providing the data logger.