Bactericidal Effect Of PIPS-Er:YAG Laser With Different Apical Terminal Working Width On E.faecalis In Experimentally Infected Root Canals

Microbial invasion and infection inside the root canal system is the major cause of pulpitis and apical periodontitis. So the success of root canal therapy depends on the elimination of microorganisms in the root canals. Refractory periapical disease is mainly caused by facultative anaerobic gram-positive bacteria, especially E.faecalis. E.faecalis has been frequently isolated from treated root canals,which is attributed to its ability to form biofilms in root canals, to invade dentinal tubules and to endure several adverse challenges.Because of the complexity of root canal system and intracanal microrganism, chemomechanical preparation does not reliably remove all the microrganism in the root canals.So some scholars advocate a larger apical enlargement to remove more infected dentine.Recently,large taper nickel-titanium instruments such as Pro Taper, GT, Mtwo are becoming more and more widely used clinically. Larger apical preparation size and taper contribute to the elimination of bacteria,yet they can weaken root structure and reduce the root fracture resistance at the same time.Therefore,we aim to find a method which can eradicate the bacteria at the same time maximize the preservation of tooth structures.In recent years, PIPS-Er:YAG laser has been gradually introduced into endodontics and showed its excellent bactericidal effect. In our previous study, PIPS-Er:YAG laser combined with sodium hypochlorite had achieved optimal bactericidal effect. But the study model had overenlarged the root canal,thus can not reflect the clinical application effect practically.This experiment aims to investigate the bactericidal effect of PIPS-Er:YAG laser with different irradiation time and apical working width on Enterococcus faecalis in root canals by establishing root models infected with Enterococcus faecalis in minimally instrumented root canals(15#/0.04), at the same time the impact of apical working width on the root fracture resistance is evaluated.And we expect providing a more effective and minimally invasive method of root canal preparation and disinfection for clinical application.The experiment was divided into the following four parts:Experiment one:Evaluate the bactericidal effect of different apical terminal working widths on Enterococcus faecalis in root canals.The samples were randomly divided into following six groups:the root canals of chemomechanical preparation groups were enlarged to 20# ? 25# ? 30# ? 40# respectively,then they were treated with 5.25% Na Cl O for one minute;The samples of sodium hypochlorite group and normal saline group were treated with 5.25%Na Cl O or normal saline for one minute respectively. After treatments,the numbers of bacteria of the specimens of each group in the different depth of root canal wall were counted.The bactericidal effects of all groups were analyzed and compared.The results showed that:the bactericidal efficiencies of sodium hypochlorite group and chemomechanical preparation groups were superior to the normal saline group(P < 0.05); There are significant differences statistically in comparison between any two groups(P<0.05),except that the bactericidal efficiencies between sodium hypochlorite group and 20# group had no significant statistical difference(P>0.05); Enlarging the root canals to 40#/0.04 chemomechanically can get 93.56% and 85.12% reduction of bacteria at depth of 100~200?m and 200~300?m respectively.The results indicated that increasing the apical terminal working width can improve the removal of Enterococcus faecalis from root canals. But a complete elimination of Enterococcus faecalis in the deep layers of root canal wall was not achieved even when the root canals were enlarged to 40#/0.04 combined with chemical irrigation with sodium hypochlorite.Experiment two: Investigate the effect of apical terminal working width on the root fracture resistance.The samples were randomly divided into following five groups: the root canals of four experimental groups were enlarged to 20#?25#?30#?40#(0.04taper) respectively;the control group was without any treatment.Then the maximum compressive load of all the samples were evaluated and analyzed.The results showed that: there are significant differences statistically in comparison between any two groups(P<0.05),except that the resistance values between control group and 20# group,and between 25# and 30# groups had no significant statistical difference(P>0.05);The mean reductions of each group in fracture resistance in comparison with the former group were 0.8%?43.6%?10.6%?61.4% respectively,among which the highest reduction rate was found in the 40# group(61.4%),followed by 25# group(43.6%).The results indicated that the increase of apical terminal working width can reduce the root fracture resistance, especially when the apical width was increased from 30# to 40#,or from 20# to 25#.Experiment three:Evaluate the bactericidal effect of PIPS-Er:YAG laser with different irradiation time on Enterococcus faecalis in root canals.The samples were randomly divided into following five groups: Er?YAG/Na Cl O group were further divided into three subgroups by irradiation time:Er?YAG/Na Cl O 30s?Er?YAG/Na Cl O 1min?Er?YAG/Na Cl O 2min; The samples of sodium hypochlorite group and normal saline group were treated with 5.25%Na Cl O or normal saline respectively. The results showed that: the bactericidal efficiencies of Er?YAG/Na Cl O groups were superior to sodium hypochlorite group and normal saline group at different depths( P<0.05);The bactericidal efficiencies of Er?YAG/Na Cl O groups were similar on the surface and at 0 ~ 100?m depth(P>0.05),and the bacterial reduction of three groups reached more than 98%; But at depths of 100~200?m and 200~300?m, the bactericidal efficiency of Er?YAG/Na Cl O 1min and 2min groups were superior to 30s(P<0.05),and that of 1min and 2min group were similar(P>0.05).The results indicated that PIPS-Er:YAG laser can enhance the bactericidal effect of sodium hypochlorite and a combination of both can effectively eliminate the Enterococcus faecalis in the superficial layers of the root canal wall.The increase of Er?YAG laser irridation time can improve its bactericidal effect, nevertheless that cannot eradicate the Enterococcus faecalis in the deep layers of the root canal wall in our minimally invasive samples(15#/0.04).Experiment four:Investigate the bactericidal effect of PIPS-Er:YAG laser with different apical terminal working width on Enterococcus faecalis in root canals.The samples were randomly divided into four groups:(1)Er?YAG laser combined with mechanical instrumentation group were divided into four subgroups and the root canals were enlarged to 20#?25#?30#?40# respectively,then they were treated with Er:YAG laser and 5.25% Na Cl O for one minute.(2)Er:YAG/Na Cl O group were treated with Er:YAG laser and 5.25% Na Cl O for one minute.(3)The sodium hypochlorite group were treated with 5.25% Na Cl O for one minute.(4)The nomal saline group were treated with normal saline for one minute.The results showed that: the bactericidal efficiencies of 20#/Er:YAG/Na Cl O group and 40#/Na Cl O group(experiment one)were similar at depth of 100~200?m and 200~300?m(P>0.05);The 30#/Er:YAG/Na Cl O group get 99.81% and 99.05% reduction of bacteria at depth of 100~200?m and 200~300?m respectively. The results indicated that the bactericidal effect of treatment with Er:YAG laser and Na Cl O after enlarging the root canals to 20#/0.04 was equal to that of enlarging the root canals to 40#/0.04 chemomechanically.Treatment with Er:YAG laser and Na Cl O after enlarging the root canals to 30#/0.04 can get a more than 99% reduction of bacteria in the deep layers of the dentinal tubules(100~200?m and 200~300?m).In conclusion, PIPS-Er:YAG laser can eliminate the Enterococcus faecalis in the deep layers of the root canal wall effectively at the same time maximize the preservation of tooth structure.Thus that will reduce the risk of root fracture for teeth after root canal therapy.Therefore, PIPS-Er:YAG laser can be a prospectively complementary method for minimal invasive root canal treatment.