In Vitro Study Of The Effects Of Er:YAG Laser Irrigation Technology On The Dentin Smear Layer,Microhardness And Fracture Resistance Of Teeth

Objective To explore the effects of Er: YAG laser shockwave enhanced emission photoacoustic streaming(SWEEPS),passive ultrasonic irrigation(PUI)and conventional syringe irrigation(CI)techniques to activate two fluids(3% NaOCl and normal saline)on the dentin smear layer,dentin microhardness and fracture resistance of teeth.Methods The single-rooted premolars that met the inclusion criteria were collected,leaving a standardized root length of 13 mm after crown truncation.Eight samples were randomly selected without root canal preparation,other samples were extracted the pulp,and the root canal was expanded to V3(35/0.04)according to the standard techniques,divided into NaOCl group and normal saline group(NS)according to different irrigation agents,and then divided into three subgroups(CI,PUI and SWEEPS)according to terminal irrigation.(1)Smear layer: Thirty samples were selected and randomly divided into six groups(n=5).The samples were divided into two halves longitudinally along the buccal lingual surface of the long axis of the tooth,one half for the experimental group and the other half for the control group.The grooved marks at10 mm,6 mm and 2 mm from the apical hole indicate the upper,middle and apical thirds of the root canal,respectively.Scanning electron microscopy was used to observe the smear layer removal.(2)Dentin microhardness: A 1mm thick dentin section was intercepted 8～10mm from the root apex,a total of seventy-two section samples were polished and divided into six groups(n=12)for terminal irrigation.The microhardness before(initial microhardness)and after irrigation(final microhardness)was measured by Vickers microhardness tester,and the difference was calculated before and after irrigation.(3)Fracture of resistance: Fifty-six samples were randomly divided into a control group(no preparation)and six experimental groups(n=8)for terminal irrigation.The root canal was filled and allowed to stand for a week,then embedded and placed in a universal testing machine.The specimen was vertically loaded from the center of the specimen to the root fracture at a speed of 1mm/min,and the maximum fracture force was recorded.The data were analyzed using one-way ANOVA and the Kruskal-Wallis test methodResults(1)Smear layer: In the upper section,the score of the NS + CI was the highest,and there was no statistically significant difference with NaOCl + CI,but it was significantly higher than that of the other groups(P<0.05).The NaOCl + SWEEPS score was significantly lower than other groups(P<0.05).In the middle section,NaOCl+ SWEEPS had the lowest score,and there was no statistically significant difference compared to NaOCl + PUI,which was significantly lower than other groups(P<0.05).In the apical section,NaOCl + SWEEPS had the lowest score,with significant differences(P<0.05),followed by NaOCl + PUI and NS + SWEEPS.Intra-group comparisons showed that there was no statistically significant difference in the total scores of the three NaOCl + SWEEPS positions.In the other experimental groups,the total scores of the upper segment were significantly lower than those of the apical segment,and there was no significant difference from the middle segment.(2)Dentin microhardness: There was no significant difference in the initial microhardness between the groups,and the microhardness of the dentin after irrigation was significantly lower than that before irrigation(P<0.05).The microhardness of NaOCl + SWEEPS decreased the most,followed by that of NaOCl + PUI,and there was significant difference between the two groups which was significantly higher than that of all other groups(P<0.05).(3)Fracture resistance: The fracture resistance of the control group was significantly higher than that of all the experimental groups(P<0.05).There were no significant differences in the maximum load forces of the samples in the experimental groups.Conclusion The activation of NaOCl solution by SWEEPS and PUI techniques can effectively remove the dentin smear layer in the upper and middle segments,but adversely affects the microhardness of the dentin.In the apical segment,SWEEPS removed the smear layer better than PUI.The fracture resistance of the roots after mechanical preparation was significantly reduced,but the terminal irrigation scheme did not affect it.