Influence of pulse duration and water/air cooling ratio on the efficiency of Er:YAG 2940 nm laser in debonding of porcelain laminate veneers: An in vitro study

Abstract Objective To determine the effectiveness of different pulse durations (PD) and the water/air (W/A) cooling ratio of the Er:YAG 2940 nm laser that are required for debonding porcelain laminate veneers (PLV), by investigation of the needed time for PLV debonding (DT) and the changes in dental pulp temperature. Materials and Methods Thirty‐six extracted noncarious human maxillary premolars were prepared for receiving PLV. Samples were randomly assigned to six different groups, based on PD and the W/A ratio: Groups A (50 µs, 1:1), B (50 µs, 3:3), C (100 µs, 1:1), D (100 µs, 3:3), E (300 µs, 1:1), and F (300 µs, 3:3). Veneers were debonded using laser irradiation by the same parameters (270 mJ, 15 Hz) with noncontact application mode. Results All 36 veneers were debonded. Samples of the 50 and 100 µs PDs showed significantly shorter DT (7.4−17 s) than that of the 300 µs which showed significantly the longest DT (104 s) among all other groups (p < .001). However, the highest elevation of pulp temperature was observed in Group E (300 µs, 1:1) which reached (3.4°C). Conclusion Using the 50 or 100 µs PD of the Er:YAG laser was more efficient than 300 µs in reducing DT of PLVs with minimal change in pulp temperature. W/A cooling ratio had minimal influence on the DT of PLV.

with the development of a PLV debonding technique, using an Er:YAG laser (Erbium-doped: yttrium aluminum garnet) at a wavelength of 2940 nm, and Er,Cr:YSGG laser (erbium, chromium-doped yttrium, scandium, gallium and garnet laser) at a wavelength of 2780 nm (Bader & Krejci, 2006;Olivi & Olivi, 2015). These lasers have been successfully used for debonding orthodontic ceramic brackets, owing to their high photonic absorption in water molecules, residual monomers, and adhesive bonding cement containing water that is also present in the PLV bonding layer (Azzeh & Feldon, 2003;Oztoprak et al., 2010;Tocchio et al., 1993).
The emitted laser beam penetrates the full thickness of PLV and causes thermal softening of the adhesive resin layer (Olivi and Olivi, 2015). This technique is still not well described in the scientific literature, and experimental in-vitro studies in this field are insufficient. This is mainly due to difficulties related to the application of removal force on the well-adapted cervical margin of PLV, which should be applied during or directly after laser application, just before the cooling down of the targeted adhesive resin layer (Iseri et al., 2014).
Very little data have been published in this field with different veneer removal techniques. Morford et al. (2011) investigated the efficiency of the Er:YAG laser in debonding PLVs using one technique with many different laser parameters; Iseri et al (2014)  laser energy and frequency to get the best choices of parameters to be used for debonding of PLV without elevation of dental pulp temperature over a threshold of 5.5°C, considered as critical and irritating for dental pulp (Zach & Cohen, 1965). Other laser parameters like pulse durations (PD) and water/air (W/A) cooling ratio have not been studied yet.
The aim of this study was to determine the efficiency of Er:YAG laser for debonding PLVs without damaging the underlying sound tooth structure, by using different parameters of PD and W/A cooling ratio; associated areas of investigation were the time required to effect debonding and the changes in the degree of dental pulp temperature. Laser irradiation was applied perpendicular to the surface of each PLV and completed using a horizontal scanning motion.

| MATERIALS AND METHODS
The pulse duration parameters of the laser device were super short pulse (SSP = 50 µs), medium-short pulse (MSP = 100 µs), short pulse (SP = 300 µs), long pulse (LP = 600 µs), and very long pulse (VLP = 1000 µs); each pulse width value was applied on two samples with different W/A cooling ratios of 1:1 and 3:3. Even after laser application for a period of 150 s, debonding of PLVs did not succeed when the PD was LP or VLP, whether the W/A cooling ratio was either 1:1 or 3:3. In consequence, those two PD parameters (LP and VLP) were excluded from the main study.
Thirty-six samples then were randomly assigned to six different groups (A, B, C, D, E, and F) based on the two studied factors: PDs (SSP, MSP, and SP) and W/A cooling ratio (1:1 and 3:3). Each group comprised six samples.
All porcelain veneers of the examined groups were completely and easily removed using the Er:YAG laser, using the aforementioned parameters. The results were entered into the statistical program SPSS (V.22). In addition to standard descriptive statistical calculations (mean, standard deviation, median), the one-way analysis of variance test was used in the comparison between the study groups, followed by the post-hoc Tukey HSD test that was utilized to identify the differences; statistical significance was established at p < .05.
The fit surface of the debonded veneer and associated prepared surface of the tooth were examined stereoscopically at a ×20 magnification (stereoscope 1999; MEIJI Techno, Japan), to evaluate whether the failure modes occurred adhesively in the veneer/cement (V/C) interface, or in the enamel/cement (E/C) interface, or cohesively within the resin cement layer (Mak et al., 2002) (Figure 3).

| RESULTS
Based on the results of the aforementioned pilot study, PLVs were successfully debonded when the pulse duration was SSP, MSP, and SP, but when the pulse duration was LP and VLP (whether W/A cooling ratio was either 1:1 or 3:3), debonding did not succeed until 150 s of laser application.
The mean values of debonding time (s) and temperature change (°C), and the respective standard deviation and standard error values are shown in Table 1.
Results of the GLM test revealed a significant difference between groups for each tested factor (debonding time and temperature change) with p < .001 (Table 2). Shapiro−Wilk normality test is used with each test and showed that the data are distributed normally (for temperature p = .06405 > .05 and time p = .8724).
Time and temperature were affected by PD and W/A cooling ratio and the interaction between them (p < .05, Table 2

| DISCUSSION
Published study data relating to the debonding of PLVs using erbium laser are still few and not well covered when compared with those that focus on the debonding of orthodontic ceramic brackets (Azzeh & Feldon, 2003;Ma et al., 1997;Nalbantgil et al., 2014;Oztoprak et al., 2010;Tocchio et al., 1993). This may in part be due to difficulty to what happens clinically, to test the debonding process in a laboratory setting; this has greater similarity to clinical reality than the application of shear forces by Instron device to test shear bond strength, as seen in most of the previous studies designed for debonding porcelain veneer or orthodontic ceramic brackets (Oztoprak et al., 2010;Tocchio et al., 1993). Using a specially designed cervical prominence of the veneer (1 × 1) at a 45°angle, a loading force of 15 N was applied. The 15-N force was selected to be the median among measured root scaling force values, which was determined previously from 8 to 23 N (Villanueva et al., 2007).
In order for the force to be applied, a 1 kg weight should be hung by a chain with a ring hooked onto the cervical prominence of the veneer at an angle of 45°. The equivalent weight determined is based on the following equation [Weight = Force × cos45/acceleration of gravity (9.8)] which results in 1.08 kg.
Access to the pulp chamber was carried out just before the time of laser application to avoid pulp tissue dehydration, which may affect the measuring process of pulp temperature (Nalbantgil et al., 2014;Walinski et al., 2021).
Similar parameters of laser energy, frequency, and application mode were gained from outcomes of our previously published research, performed using similar methods (AlBalkhi et al., 2017). This study was designed as a complement to the previous study published in 2017(AlBalkhi et al., 2017, which aimed to test two variables that had not been studied before (PD and W/A cooling ratio), to establish an ideal criterion for using the Er:YAG laser in debonding PLVs. The parameters were selected according to the results of a pilot study, which was performed on 10 samples. The

| CONCLUSION
Within the limitations of this study, it was concluded that both SSP duration (50 µs) and MSP duration (100 µs) were efficient, fast, and safe parameters of Er:YAG laser for the removal of lithium disilicate PLV, when using the same laser energy and frequency parameters of noncontact application mode, and irrespective of a W/A cooling ratio of 1:1 or 3:3.
The W/A cooling ratio had minimal influence on the debonding time and pulp temperature when shorter PD were used (SSP, 50 µs, MSP, 100 µs). All selected parameters in this study did not raise the dental pulp temperature beyond a threshold of 5.5°C, throughout the time duration of PLV removal using Er:YAG laser irradiation.

ACKNOWLEDGMENT
The authors would like to acknowledge Dr. Ahmed Abdullah from Abertay University, UK, for his statistical advice. This study was carried out with personal funding.

CONFLICTS OF INTEREST
The authors declare no conflicts of interest.

This study was approved by the Damascus University Ethical
Committee under decision no. 163 WD. We certify that the study was performed in accordance with the ethical standards as laid down in the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards. This article does not contain any studies involving animals performed by any of the authors.

INFORMED CONSENT
A consent form was obtained initially from all patients who underwent tooth extraction and provided those teeth that were used in this study.

AUTHOR CONTRIBUTIONS
Mohand AlBalkhi designed the study, collected sample, methodology (prepared samples for the test), collected data (measurements) then entered them into the statistical software (SPSS) and made analysis, wrote most parts of the manuscript, and edited according to the CEDR journal. Omar Hamadah shared in the design of the study, methodology (applied laser on samples), wrote an interpretation of the statistical analysis, revised the manuscript, and edited the manuscript.

DATA AVAILABILITY STATEMENT
The data sets used and/or analyzed during the current study are available from the corresponding author on reasonable request.