Experimental Study On Bonding Efficiency Of Er:YAG Laser Etching And Re‐etching Enamel Surface

Enamel acid etching technique, which has been applied to clinical for recent 40 years, is an important factor that affects orthodontic brackets bonding. It brought revolutionary change to the fixed orthodontic technique. Commonly phosphate acid etching is able to meet the need of clinical bonding strength, but it was found that acid etching has the potential disadvantages of enamel demineralization, lead to increase the prevelance of caries in the process of orthodontic and the enamel around brackets is easy to decay. And after debonding brackets from tooth surface, the resin tags in enamel not only may cause the enamel color change, but also it may be the main reason that affects the rebonding strength. Er: YAG laser which the wavelength is 2940 nm is easily absorbed by the water molecules and hydroxyl apatite, so it is suitable for hard tissues.Some researches have shown that laser etched enamel can reduce enamel demineralization, increase enamel acid resistance and caries resistance. In recent years,researches on the bracket bonding strength after laser etching enamel is increasing,but the conclusions have certain differences.Dunn WJ and Rosalia CB and other scholars found that the shear bond strength after the Nd:YAG laser、Er,Cr:YSGG laser and Er:YAG laser etching enamel were lower than acid etching. While Aleksic and Lan WH showed that the shear bond strength after the Er:YAG laser etching were similar to phosphoric acid etching. Oshagh M used CO2 laser re-etching enamel that the shear bond strength was larger than acid re-etching enamel.In addition, Lee BS and other researchers confirmed that the surface morphology afer Er:YAGlaser etching is similar to phosphoric acid etching,laser may be an alternative method of phosphoric acid etching. Analysis of the literatures found that the causes of inconsistent conclusions may be the researchers used different types of laser and different parameter Settings. The wavelength of 2940 nm is Er: YAG laser and wavelength of 10600 nm is CO2 laser, they are easily absorbed by the water molecules and hydroxide apatite, and therefore they are very suitable for tooth hard tissue. Er: YAG laser is absorbed by water molecules to produce micro blasting energy output by 10 times that of CO2 laser, and the work process with continuous water spray that can control the surface temperature rise, thermal damage of dental pulp tissue significantly lower than that of CO2 laser. By in vitro experiment and clinical observation in vivo, this study compared bonding efficiency of Er: YAG laser etching and re-etching enamel with different working parameter settings with phosphate acid etching.Objective To compare the effect of Er:YAG laser etching and re-etching enamel surface with the 35% phosphoric acid etching. And compare the bracket failure rate after bonding in clinic between phosphoric acid etching and Er: YAG laser etching enamel. Finally, to provide guidance for the clinical application of Er: YAG laser etching.Materials and methods Vitro studies Part 1:Experimental study on bonding efficiency of Er:YAG laser etching enamel 104 extracted premolars were collected from 26 orthodontic patients. The four premolars of each patient were randomly divided into four groups(26 teeth per group),and enamel etching was done respectively with different methods or different parameters: Group A 35% phosphoric acid etching; Group B Er: YAG laser etching with parameters 200 m J, 30 Hz, 6/8water; Group C Er: YAG laser etching with parameters 250 m J, 30 Hz, 6/8water; Group D Er: YAG laser etching with parameters 300 m J, 20 Hz, 6/8water. Scanning electron microscopy(SEM) was used to observe the changed morphology of enamel surface(2 samples per group), and atomic force microscopy(AFM) was used to measure the roughness and peak-peak values(P-P) of enamel surface(4 samples per group, each sample selected five areas that nonoverlapping). At the last, the shear bond strength(SBS) and adhesive remnant index(ARI) were measured and surveyed by universal testing machine and stereo microscope after bracket bonding on the enamel surface(20 samples per group). Part 2:Experimental study on bonding efficiency of Er:YAG laser re-etching enamel 78 extracted premolars were collected from orthodontic patients. First, 35% phosphoric acid was used to etch the enamel surface, and then brackets were bonded to enamel surface. After 24 hours storing in the 37°C water, take off the brackets and all of the remnant adhesive under the naked eye.Then the sample teeth were randomly divided into three groups(26 teeth per group),then the enamel etching were done respectively with different methods or different parameters: Group A 35% phosphoric acid etching enamel; Group B Er: YAG laser etching enamel with parameters 200 m J, 30 Hz, 6/8water; Group C Er: YAG laser etching enamel with parameters 250 m J, 30 Hz, 6/8water. After etching, SEM was applied to observe the morphology change of enamel surface and enamel-resin adhensive interface(2 samples per group), AFM was used to measure the roughness and peak-peak values(P-P) of enamel surface(4 samples per group,each sample selected five areas that non-overlapping). At the last, the SBS and ARI were measured and surveyed after bracket bonding on the enamel surface using universal testing machine and stereo microscope(20 samples per group). Vivo study Part 3: The clinical bracket failure rate of Er:YAG laser etching enamel surface 28 patients treated with fixed appliance from Department of Dentistry of Xuanwu Hospital Capital Medical University were included into this study from July 2014 to July 2015. For each patient, the right or left mandibular teeth randomly used 35% phosphoric acid etching or Er: YAG laser etching(200m J, 30 Hz, 6/8water).Then bonding the brackets. The bracket failure rates of two groups were surveyed and compared within the observation period.Results Vitro studies Part 1:The enamel surface of four groups showed a pattern in which there was a distinct hollowing of prism centers, called typeⅠetching pattern, with a generalized roughening enamel surface. The honeycomb border of group C was the most clears. The roughness value was increased with increasing laser parameters, and significantly statistical difference(P<0.01) was existed among the four groups. The highest peak-peak values of enamel surface were in group D. Group A showed lowest peak-peak values. Group C has the most similar enamel-bonding interface morphology to that of group A. The highest SBS was measured in Group A, group C has no statistical difference with Group A(P>0.05). No statistical difference of ARI scores were found in the four groups(P>0.05).Part 2: There were large amounts of debris on the enamel surface were found in Group A. Group B and C,called type Ⅰ etching pattern, with a generalized roughening enamel surface. Surface roughness was statistically different among the three groups(P< 0.05), Group C showed the highest surface roughness. The enamel surface P-P values group C was the largest, group A and group B enamel surface P-P values had no statistical difference(P>0.05). The SBS of group C was the highest, group A and group B had no statistical difference(P> 0.05). The ARI scores of three groups had no statistical difference(P> 0.05). Vivo study Part 3: Observation period of the study ranges from 6 to 18 months, with the average was 12.71 + 3.71 months. In each group the number of teeth samples was 190. 35% phosphoric acid etching bracket failure rate was 32.9%, while the laser etching group was 34.3%, no statistical difference was found between the two groups(P>0.05).Conclusion 1. Er:YAG laser(250m J, 30 Hz, 6/8water) etching can achieve the most similar surface morphology and enamel- resin bonding interface morphology and shear bond strength compared with 35% phosphoric acid etching. 2. Er:YAG laser re-etching enamel(200m J, 30 Hz, 6/8water and 250 m J, 30 Hz, 6/8water) obtained the similar or higher shear bond strength with 35% phosphoric acid re-etching enamel, and the honeycomb structure of enamel surface is more clear than acid etching. 3. Er:YAG laser(200m J, 30 Hz,6/8water) etching enamel can achieve similar bracket failure rate compared with 35% phosphoric acid etching.