Accurate gingival temperature rise was measured in swine gingiva.
Exposure to polywave LED light caused significant gingival temperature rise.
Higher exposure values increase the probability of gingival lesion.
Rubber dam cannot protect gingiva when higher radiant exposure values are delivered.
The presence of rubber dam may increase the probability of gingival lesion.
This study evaluated the temperature increase in swine gingival temperature after exposure to light emitted by a Polywave ® LED light curing unit (LCU, Bluephase 20i, Ivoclar Vivadent).
After local Ethics Committee approval (protocol 711/2015), 40 pigs were subjected to general anesthesia and the LCU tip was placed 5 mm from the buccal gingival tissue (GT) close to lower lateral incisors. A thermocouple probe (Thermes WFI, Physitemp) was inserted into the gingival sulcus before and immediately after exposure to light. Real-time temperature (°C) was measured after the following exposure modes were applied: High Power (20s-H, 40s-H, and 60s-H) or Turbo mode (5s-T), either with or without the presence of rubber dam (RD) interposed between the LCU tip and GT (n = 10). The presence of gingival lesions after the exposures was also evaluated. Peak temperature (°C) and the temperature increase during exposure over that of the pre-exposure baseline value (ΔT) data were analyzed using 2-way ANOVA followed by Bonferroni’s post-hoc test ( α = 5%). A binary logistic regression analysis determined the risk of gingival lesion development.
Without RD, no significant difference in ΔT was observed among 20s-H, 40s-H, and 60s-H groups, which showed the highest temperature values, while the 5s-T exposure showed the lowest ΔT, regardless of RD. RD reduced ΔT only for the 20s-H group (p = 0.004). Gingival lesions were predominantly observed using 40s-H, with RD, and 60s-H, with and without RD.
Exposure to a LCU light might be harmful to swine gingiva only when high radiant exposure values are delivered, regardless of the use of RD.
Light emitting diode (LED) light curing units (LCU) have become one of the most important tools in the clinicians’ routine. Although earlier LED LCU generations were considered “cool” lights because they generated less heat than did quartz-tungsten-halogen (QTH) lamps , later, more powerful LCU versions were capable of generating as much heat as QTH sources . Most recently, a new generation of LED lamp has been introduced. Different from the previous generations, these polywave ® or “dual-peak” (multi-wave, multi-peak) LCUs emit light a broader wavelength range to activate not only traditional photoinitiators, such as camphorquinone, but also alternative photoinitiators . In addition, seeking more efficiency in the monomer-polymer conversion of resinous compounds and reduction of clinical time, these devices are capable of emitting light over 2000 mW/cm 2 . As a consequence, more heat is generated during the resin polymerization, resulting in a considerable increase in the temperature inside the pulp chamber and within the pulp tissue itself .
Although in vitro and in vivo studies have provided important information about the effects of light emitted by LCUs on pulp temperature and histological changes caused by such a temperature increase within the pulp tissue , the deleterious effects of heat generated from LED LCUs, and exothermic polymerization of resin composites on the pulp, should not be the only concern for the clinician. Some restorative procedures utilizing light-cured resin composites are usually performed in close proximity to soft tissues, when restorations are located near the gingival margin. As a consequence, studies demonstrate the potential of some LED devices to burn soft tissues , affecting fibroblasts and decreasing cell proliferation in vitro , irradiance and exposure duration . In addition, other studies demonstrate that thermal injury can cause bone resorption and even necrosis . For these reasons, although one could expect that the use of rubber dam over the gingival tissue would protect the soft tissue underneath from the blue light and subsequent heat generated during the tooth exposure, it is reasonable to expect that heat generated by these devices may harm gingival tissues as well . In this regard, manufacturers have recommended that clinicians should not expose soft oral tissues at close proximity for long exposure periods . However, no information regarding the thermal effects caused by exposure to LCUs on soft tissues and the protective effect of rubber dam is available in the literature.
In vivo tests on swine tissues have become an alternative research model, because this animal has similar physiology to human tissues, similar pathological conditions, and sufficiently similar tooth anatomy . Most importantly, the width of attached gingiva is also similar to that in humans , and because pigs also have heterodont dentition, swine are also considered an appropriate model to conduct studies of tooth morphogenesis . For these reasons, the pig is an animal of great value as a preclinical model, allowing the analysis of pathologies and oral changes .
Thus, the purpose of this in vivo study was to evaluate the effects of light emitted by a high power, dual-peak LED LCU on the temperature of swine gingival tissue exposed to a LCU light with varying exposure modes, either with rubber dam interposed between the gingiva and LCU tip or not. In addition, the presence of a gingival lesion caused by exposure to the LCU light was also addressed. The research hypotheses were: (1) exposure to light emitted from high intensity LED LCU causes temperature rise in gingival tissue; (2) gingival exposure to a LCU causes a visible lesion, regardless of radiant exposure value; and (3) the use of rubber dam reduces temperature rise in the gingival tissue as well as the potential for development of gingival lesions after exposure to a LCU.
Material and methods
Spectral analysis of light emitted by the LED LCU
The LCU used in the study was a commercially available, Polywave ® unit (Bluephase 20i, Ivoclar Vivadent, Schaan, Liechtenstein). The spectral power values of High and Turbo exposure modes (EM) were recorded using a laboratory grade spectroradiometer (USB 2000, Ocean Optics, Dunedin, FL, USA) connected to a 6-in integrating sphere (Labsphere, North Sutton, NH, USA), previously calibrated using a NIST-traceable light source. The LCU tip end was held 5 mm distant from the aperture of the integrating sphere, either with or without the rubber dam interposed between the LCU tip and the entrance of the integrating sphere, so all light emitted from the unit was captured (n = 5). This distance simulated that observed clinically as imposed by the blue blocker tip ring. Wavelength-based, spectral power emission between 350 to 550 nm during each EM was recorded using software (SpectraSuite v2.0.146, Ocean Optics), which also provided the total emitted power value for that wavelength range. The optical emitting area of the distal end of the light guide was calculated, and this value was divided into the integrated spectral power value to derive the total radiant emittance from the curing light for High and Turbo EM (mW/cm 2 ). This value was then multiplied by the light exposure duration to derive the value of radiant exposure applied to each exposed gingival surface for each light output mode (J/cm 2 ).
This study was approved by the local Ethics Committee for use of Animals in Research (protocol 711/2015). Forty male pigs, having similar body weight (20–30 kg) and obtained from the discipline of Operative Techniques and Experimental Surgery in the graduate program in Medicine at the State University of Ponta Grossa were used. These pigs underwent major surgical procedures previously scheduled as part of the graduate program. Thus, no animal was used for the sole purpose of this study.
Under the care of a veterinarian, the animals received intramuscular administration of Ketamine (14 mg/kg), Xylazine (0.2 mg/kg), and Acepromazine (0.4 mg/kg), followed by endovenous administration of propofol (5 mg/kg). After oro-tracheal intubation, the animals received mechanical ventilation and were monitored with pulse oximetry. Maintenance was performed with inhalation anesthesia using isoflurane at a minimum alveolar concentration of 1.2–1.7%.
Baseline temperature of the gingival tissue
The cervical regions of the buccal surfaces of lower right and left lateral incisors were evaluated, to simulate light exposure on a Class V during a restorative procedure ( Fig. 1 a and b). Two gingival sites were evaluated per pig, resulting in a total of 80 gingival sites. The gingival sites were randomly divided into 10 experimental groups, according to the main factors: LCU exposure modes and exposure durations (4 levels) and use of rubber dam use (2 levels: yes/no),thus 10 gingival sites were evaluated in each group (n = 10).
A sterile needle (0.70 × 25 mm; Precision Glide, BD, NJ, USA) was inserted into the gingival sulcus until the gingival tissue attachment was penetrated, in order to induce a small amount of bleeding, to establish a more accurate temperature measurement within the gingival tissue. A thermocouple probe, coupled to a temperature acquisition system (Temperature Data Acquisition, — Thermes WFI, Physitemp, Clifton, NJ, USA), was inserted into the gingival sulcus and through the small channel created by the needle, to record baseline tissue temperature ( Fig. 1 b).
Exposure to light emitted from a LED LCU
After baseline temperature of the gingival tissue was determined, the probe was removed from the gingival sulcus and the tip of the LED LCU was placed on the cervical region of the buccal surfaces of the lower right and left lateral incisors, simulating light exposure on a Class V preparation during a restorative procedure ( Fig. 1 a and c). The LCU tip area covering the gingiva ranged from half to 2/3 of the tip area, while the remaining portion covered enamel (approximately 1/3 of tip area — Fig. 1 a). The tissue was exposed to the LCU, either with or without the presence of a rubber dam interposed between the LCU tip and the gingival tissue. A new, small piece of blue rubber dam (Lot Number: 300413/LE; Madeitex, Sao Jose dos Campos, SP, Brazil) was attached to the blue light blocker tip before every exposure ( Fig. 1 c). The blue light blocker tips also acted as a spacer, to assure the same distance (5 mm) between the LCU tip and the gingival tissue, during all exposures ( Fig. 1 a).
The following LCU EMs were applied: 5 s Turbo (5s-T: 2204 mW/cm 2 –11.0 J/cm 2 ), 20 s High Intensity (20s-H: 1242 mW/cm 2 –24.8 J/cm 2 ), 40 s High Intensity (40s-H: 1242 mW/cm 2 –49.7 J/cm 2 ), 60 s High Intensity (60s-H: 1242 mW/cm 2 –74.5 J/cm 2 ). After the light shut off, the temperature probe was promptly inserted within the gingival sulcus and the temperature was recorded ( Fig. 1 b). Each gingival site was randomly assigned to only one exposure mode. Peak temperature (°C) and the temperature increase in the gingival tissue site during exposure to the LCU over that of the pre-exposure baseline value (ΔT) were recorded.
Visual presence of gingival lesion following curing light exposure
In order to evaluate the development of a gingival lesion following LCU exposure, 40 additional pigs had only the gingival tissue of both arch sides (80 separate gingival sites) exposed using the same EMs as previously described, either with or without the presence of rubber dam interposed between the gingival site and LCU tip (10 gingival sites for each experimental group), but without any temperature measurement. Approximately 15 min after the exposure, a blinded, calibrated operator evaluated the presence of any gingival lesion ( Fig. 1 d). At the end of all surgical procedures, as scheduled for the graduate discipline, the animals were euthanized. Intravenous propofol was administered at 5 mg/kg, followed by 10 ml of 19.1% potassium chloride.
Temperature rise of rubber dam from light exposure
In order to evaluate the temperature change on the rubber dam when exposed to the LCU, the temperature probe was placed in contact with the rubber dam that was attached to the blue light blocker and was randomly exposed to the same exposure modes (5s-T, 20s-H, 40s-H, and 60s-H). The same procedure was performed without rubber dam. Temperature was measured in real time, while holding the temperature-sensitive portion of the probe in the middle of the projected light path, until values returned to those of the pre-exposure levels, and the peak temperature and temperature increase during exposure to the LCU over that of the pre-exposure temperature value (ΔT) were determined (n = 6).
Peak temperature (°C) and ΔT in the gingival tissue site and in the rubber dam, and the spectral power of LED LCU either with or without the rubber dam interposed between the LCU tip and the entrance of the integrating sphere were analyzed for normal data distribution the Shapiro-Wilk test. All data distributions were considered normal, and the results were analyzed using a 2-way ANOVA (“EMs” and “use of rubber dam”) followed by Bonferroni’s post-hoc test ( α = 5%). Post-hoc power analysis was performed for the statistical analyses of peak temperature and ΔT, as well as for spectral power of LED LCU, either with or without the rubber dam. All statistical analyses were performed using commercial statistical software (Statistics 19, SPSS Inc, IBM Company, Armonk, NY, USA).
A predictive model was built based on a binary logistic regression analysis that included the following variables: rubber dam and radiant exposure values to establish the risk of gingival lesion development. Adjusted Odds ratio (OR) and 95% confidence intervals were reported. For this test, statistical relations were determined to be significant at p < 0.05.
The binary logistic regression, Eq. (1) , was given as:
Logit ( p i ) = β 0 + β 1. x 1 , i + … + β k . x k , i
The probability of the binary dependent variable ( ρ ), considering the predictor variables, was determined as follows (Eq. (2) ):
ρ = 1 1 + e x p − ( β 0 + β 1. x 1 , i + … + β k . x k , i )