A different Laser powers and their Assisted in Ceramic Veneers Removal: (An in vitro Study)

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INTRODUCTION
Ceramic veneer (CV) are the most durable, reliable, and suggested restorative material for anterior teeth, since the 1980s (1) , especially with the continuous progress of adhesive techniques (2) .In esthetic dentistry, forming a life like smile is a continuous challenge which is dependent on factors including, tooth substrate, luting cement, shade matching, ceramic type, and technical skills (3) .Many new developments are made to aesthetic dental procedures related to the manufacture, cementation, maintenance and removal of ceramic laminates (4) .As all dental restorations, the CV may need to be removed at variable intervals duo to many causes such as dental decay, sensitivity, pain, lack of aesthetics, periodontal response problems, in addition to failures during the cementation process, or malposition (1,5,6) .These conditions shoves the dental professionals to remove restorations, that may require cutting the veneers or even drilling them completely giving the difficulty of their removal, that may lead to full destruction of the restorations (7) .Because of high bond strength of resin cement both to the tooth structure and ceramic material, in addition the lack of contrast between them, this may lead to veneer destruction combined with tooth destruction underlying veneer (8,9) .
Ultrasonic was another conventional method used for veneer removal through special scalar tips (10) .The drawbacks of using ultrasonic are: CV may get fractured, time consuming approach, greater possibility to damage the substructure, tips can inadvertently move away from the boundary and the long axis of the teeth/ veneer, the periodontal tissues, and heat created may be have untoward effect to the teeth structures (11,12) .So, it is motivating to study debonding options without the damaging neither the tooth nor the fixed prostheses (7) .The use of a laser eliminates the problems of debonding that are associated with the conventional methods (13)   .The use of laser has been successfully applied for ceramic orthodontic brackets debonding from tooth surfaces by Strobl et al. (14) .After a few years, the use of lasers to remove ceramic veneers and crowns was reported (15,16) .Lasers such as Er Cr: YSGG, 2,780 nm wavelength could be used for harmless removal of all ceramic fixed prosthesis (4) .The procedure is relatively fast and there is no iatrogenic hazard for the underlying teeth substrate.Laser was suggested to be used as an alternative method for removal of ceramic veneers (1) .
Studies have suggested that erbium laser's energy may be transmitted through the ceramic and selectively absorbed by water rich substances molecules such as residual monomers in the luting cements, resulting in reduced SBS (17)(18)(19) .Therefore, the aims

Specimen's collection and preparation
The Freshly extracted mandibular incisors (n=30) from (2 _ 3) years old bovine were obtained from a local abattoir to be used in the study.Criteria of the teeth selected were that they had no crown cracks or fractures, enamel defects or caries (5,20) .To assess that the crowns were without cracks, the teeth were visually examined with light transillumination.They were cleaned from any attached soft tissues and scaled by using scaler then polished with fluoridefree pumice (Bilkim LTD/ Turkey) (2) .
Crowns were cut at the cementoenamel junction with a carbide disk, the root part discarded and the pulp tissues in the crown part were removed with barbed broach and the canal space cleaned with normal saline and dried (21) .The teeth were stored in distilled water at room temperature until further processing, not more than one week (13)   .Autopolimerizing cold cure resin (Veracril Cold Curing Dental Polymer, New Static , Antioquia-Colombia) cold cure resin was mixed according to the manufacturer instructions and poured in a cylindrical plastic tube of 2.5cm in diameter and 2.75cm in height, after which the specimens were embedded in acrylic resin (Figure 1) such that the outer surface of the enamel was facing upward (5,16) .
After the completion of polymerization, the outer surface of the teeth was abraded with abrasive discs to make a flat surface within enamel of 8mm in diameter in the middle of the middle third of the labial aspect and smoothed under running water by using 200,400, and 600 grit silicon carbide papers consecutively (English Abrasives, Holland) to obtain a standardized flat surface for bonding procedures.Each carbide paper was used, approximately for 20 s .Then samples were immersed in dental ultrasonic bath for 3 minutes in order to remove any abrasive grain present in the specimen (8) .The specimens were stored in distilled water at room temperature (22) .
Then, washed thoroughly with water (20 sec.) and ultrasonicly cleaned in distilled water for 5 min. (23), dried with oil-free air, after which monobond Plus silane (Ivoclar, Vivadent ® , Schaan, Liechtenstein) was applied onto the internal surfaces of the discs for 1 min.and allowed to air dry (21) .

Conditioning of teeth surfaces
The prepared surfaces of the teeth were dried, etched with 37% phosphoric acid (Etchant gel, Pegasus, Altrincham, UK) for 30 seconds, rinsed thoroughly with water and air dried (20) .For all groups, universal adhesive system ( VivaPen, Ivoclar _ Vivadent, Schaan, Lichtenstein; LOT: Z006G9) was applied to the prepared tooth surface and scrubbed with micro brush on the surface for 20 sec, air thinned,

A B C
and cured for 10 seconds by LED light curing unit (Henan, China) at a light intensity of 1200mW/cm 2 (24) .

Ceramic discs cementation
Resins cement (Choice™2 LC, Bisco, USA) (LOT no.1900004751; composition: Bis-GMA, > 90% Strontium glass, Amorphous silica) translucent shade was used according to manufacturer instructions and applied to the internal surface of the veneer.The veneer was then seated into place on the prepared tooth and the ceramic disc was subjected to a constant load by using a glass slide applied on the ceramic disc with a 100 grams over it for 1 min in order to standardize the thickness of resin cement layer (25) .An initial cure was made for one second in order to remove excess cement.The final cure of the cement will be accomplished with 40 second with the curing light in contact with the glass slide (26) .After cementation procedure, the samples were stored at 37°C for 24 hours in 100% humidity in an incubator (Electromag; Italy) (27) .

Specimens distribution:
Thirty disc specimens from (IPS Emax ® ) were divided randomly into three groups (n=10) according to the treatment received as follows:

Control group (Cont.):
No laser irradiation for the ceramic discs.

Laser group 5Watt (L5):
The specimens irradiated with laser at 5 watt power.and an air level of 90% (1) .In this study, the application tip of the laser was placed at 1millimeter distance from ceramic discs (non-contact mode) by using a surveyor (Gerdent, Dental materials Co.; Syria).

Laser settings
During irradiation, a scanning method perpendicular to the disc surface was used from incisal to cervical border of the disc (one cycle).Each disc surface was irradiated 6 times during the whole 60 seconds irradiation period (average 10 sec. /cycle).

Testing of "shear bond strength"
The "SBS was measured using Universal Testing Machine "(Gester, Gester International Co.; China) to evaluate the shear force needed for veneer disc debonding, for the control and laser irradiated groups.The specimens were placed in a specially designed mounting jig that allowed it to be loaded parallel to the adhesive interface (Figure 3).The force was applied at the adhesive interface using a crosshead speed of 0.5 mm/min. in an incisogingival direction.The force required to deboned the veneers was registered in newton (N) and then transformed into Megepascals (MPa), as explained with the following equation: (28)

Mode of failure:
A stereoscope (Optika Microscopes; Italy) at 10X magnification was used for failure mode examination that was categorized according to the Adhesive Remnant Index (ARI) scores, as follows: (29) No adhesive remaining adhered to enamel= Score 0 Less than half of the adhesive remaining adhered to enamel= Score 1 More than half of the adhesive remaining adhered to enamel= Score 2 All the adhesive remaining adhered to enamel= Score 3 The images of each sample after debonding were taken using Optika digital camera (Italy) attached to the microsope.Imagej computer program was used to measure the surface area of the remaining adhesive resin cement on the tooth surface (30) (Figure 4) represent score 1,2, and 3 respectively.

RESULTS
The result of normality test are listed in .5151.000

DISCUSSION
Bovine teeth were used in this research as it can be considered as a suitable substitute for human teeth for bond strength tests (30) .
Bovine teeth are easier to handle due to their large size producing standardization and high reproducibility for shear bond strength tests (31) .
The Er,Cr:YSGG emits a pulse beam of energy at a wavelength of 2780nm (32) .This wavelength, which falls into the midinfrared (IR) range resembles the peak absorption range of the water curve that lies in the mid-infrared spectrum (33) .In recent years, the usage of lasers in prosthetic applications such as the removal of allceramic restorations with different parameters (e.g.laser power, water/air ratio, energy and frequency) have been increasing, being one of the important topics (27) .Er, Cr: YSGG laser have several benefits in debonding laminate veneers because of the operator's ability to alter the settings (power, energy, frequency, air /water percentages, the irradiation method, and time) which could aid in protecting the tooth surface and avoiding increasing the pulp chamber temperatures (34) .The depth of preparation of the tooth surface, the type of ceramic material and the thickness of the restoration should be taken into consideration when choosing the laser parameters for debonding (27) .Most of the studies used different power parameters, by changing laser energy, frequency, application duration and mode. (4,8,17,22,35)rford et al .Iseri et al. (16) in their laboratory study, used Er:YAG laser; 100 mJ.However, a higher frequency was used, 50 Hz, the used tip (1 mm in diameter) was placed perpendicularly at 2 mm distance from the porcelain laminate veneer (PLVs) made from lithium disilicate discs (5 mm in diameter, 0.7 mm in height).The authors concluded that a laser-aided debonding using a scanning method was effective for debonding laminate veneers and an Er:YAG laser effectively reduced the SBS of laminate veneers, facilitating their removal (16) .Gurney et al . (35) in their laboratory study, used a "lithium disilicate" crown restorations, which was cemented with resin cement, an Er,Cr:YSGG laser was used at different wattages "(3, 3.5, 4, and 5 W) with 30, 60, and 90 second applied time", using 60% air, 30% water, and 25 Hz, with the aim of determining the most effective power and exposure time for the removal of lithium disilicate crowns.
The authors found that using 3.5 and 4 Watts of power over 60 seconds could result in the removal of the restoration without producing pulp damage (35) .
During laser irradiation of ceramic veneer for debonding, laser energy is absorbed by residual monomer in the cement, and degradation of cement can occur by one of the three assumed mechanisms, as explained by Tocchio (37) : thermal softening, thermal ablation, and photoablation.Photoablation and thermal ablation happens when high energies are used (27) .The Er,Cr:YSGG laser works on the principle of micro explosion during tissue ablation resulting in microscopic and macroscopic irregularities, and softening of luting cement (22) .As an established technique for veneer removal, it avoids the physical contacts with veneers, therefore reducing fracture risk of veneer (3) .In our study, no ceramic disc was fractured during laser irradiation or during SBS test which indicates that the use of laser for assisted removal of CV restoration may preserve the veneer restoration and can be rebounded if needed, which agrees with other studies (3,17,21,35) .Kellesarian et al. (38) stated that the main purpose of debonding of all CV restorations by lasers is to decrease the bond of the resin to the ceramic substrate.
The scanning method by moving the hand piece horizontally over the entire surface of the CV restoration with a specific time, frequency and specific energy will reduce the SBS needed for veneer removal, and prevent temperature rise (13) .The reduction in the SBS required to deboned ceramic discs is in agreement with other studies (6,7,27,28,36) .As there is significant difference between the control and the laser irradiated groups, the first null hypothesis was rejected.The result of this study in that there was no significant difference between the two laser powers in reducing the SBS agree with the result of Morshedi et al. (27) .
Since there is no significant difference between the two powers used (3, and 5 watt), the second null hypothesis was accepted.
In debonding studies, the mode of failure is an important index of where the failure occurred and evaluates the probable risks of enamel damage (1) .After laser treatment, bond failure occurred commonly at the resin-ceramic interface or within the resin that means preserving the tooth structure from iatrogenic damage, which is opposite to traditional debonding methods where the failure is more likely at the enamel-resin interface (34) .The result of our study for the mode of failure of laser irradiated groups mostly scored 2 and 3, indicating that the outer surface of the resin cement was softened by laser and the debonding location was between resin/ceramic interface, compared with the control group that scored 1 in addition to score 2 indicating that the debonding location was mostly between the tooth and resin cement.This could increase the damage risk to the tooth, which is in agreement with other studies (21,28,36) .It is therefore tempting to speculate that erbium lasers are a probable tool and valued for the removal of all ceramic fixed dental prosthesis (38) .

CONCLUSION
Within the limitations of the current in vitro study, it can be concluded Er,Cr:YSGG could be a safe, effective, fast and harmless method for debonding Emax ® veneers.Since there was no significant difference in the reduction of SBS when 3 and 5 watts laser powers were used, 3watts may be advocated.

REFERENCES
study was accepted by Research Ethics Committee board (University of Mosul, College of Dentistry, REC reference No. (UoM.Dent/ A.L.25/21).

Figure ( 2
Figure (2): A: Ingot of E-max press ® ; B: Measuring the thickness of ceramic discs using digital caliber, C: Measuring the diameter of ceramic disc.
(36)   used an Er:YAG wavelength at a low repetition rate of 10 Hz, and energy setting of 133, 217, 316, 400, 503 mJ (1.33 W -5W) with a small pulse duration of 100 µsec, the average irradiation for removal time was 106 ± 59 sec.The authors performed the Fourier-Transformed Infrared Spectroscopy (FTIR) after the irradiations and verified that the Er:YAG laser was not powerfully absorbed by the ceramic materials so, might be passed through the laminate.The authors concluded that the use of the laser results in a safe method for the laminate removal(36)    that mean the laser energy not absorbed by ceramic materials or poorly absorbed, therefore transmitted through the ceramic and reach the residual monomer and water that present in luting resin that lead to reduce the shear bond strength or debonding the veneer completely by one of the three assumed mechanism thermal softening, photoablation and thermal ablation.Oztoprak et al.(6)  used the Er: YAG laser 5 W, 50 Hz, 100 mJ/pulse, with a 1 mm diameter tip at a 2 mm distance from the surface of the laminates during 3, 6, and 9 second for debonding lithium disilicate veneer restoration luted with variolink veneer resin cement.The authors concluded that the irradiated ceramic laminates had lower shear bond strength(6)  .

Table ( 1
) which showed that normal distribution of the data   0.05.

Table ( 4
): Duncan Multiple Analysis Rang Test for the test groups