| Dental caries is the progressive diseases in dental hard tissue caused by a variety of complex factors in oral cavity, a large number of clinical and epidemiological surveys show that the dental caries prevalence rate of all ages in Chinese remains high for many years. Bacterial factor is the major cariogenic factor in dental caries, Streptococcus mutans is one of the main cariogenic bacteria. The direct reason of dental caries is the acid metabolic activity caused by Cariogenic bacteria in the mouth. Therefore, we can achieve the purpose of preventing and controlling caries by interfering growth and metabolism of cariogenic bacteria effectively. Aiming at the defect of preventing dental caries, to explore a safe and effective new method of dental caries prevention is very important.Photodynamic therapy (PDT) is an effective antimicrobial therapy, more and more attention of its effect on inactivating bacteria are attracted. It can treat bacterial diseases effectively without drug resistance and in favor of the prevention of dental caries. The domestic and foreign scholars methods have applied PDT on Streptococcus mutans in vitro and oral biofilm in artificial caries, and achieved a good antibacterial effects. The topic of this subject is to discuss the prevention and safety of dental caries in the rat caries model of PDT by the optimizing and selecting the appropriate parameters of PDT.Objective:Using HMME-PDT to kill S. mutans in vitro by the semiconductor laser with the wave of 532nm and 630nm respectively.Optimizing photosensitizer incubation time, photosensitizer concentration and various laser parameters to choose the best laser, photosensitizer incubation time and laser exposure parameters. Establishing rats caries model for PDT, to investigate the killing effect on S.mutans in rats oral cavity and the preventive effect on caries and the safety of oral soft tissue by PDT of different energy densities, providing the theoretical and experimental basis on PDT clinical practice.Methods: 1. S.mutans was recovered in the standing-temperature cultivator at 37℃and anaerobic cultured for 24h, and prepared concentration of standard bacteria solution with 1×108CFU/ml and set aside.2. S.mutans in 96 pores plate was treated with HMME-PDT by the semiconductor laser with the wave of 532nm and 630nm. Chose the appropriate laser,photosensitizer incubation time and various laser parameters, to provide the basis of rats caries experiment according to plate count results.3. Establishing rats caries model by inoculating tandard solution of S.mutans in the rat oral cavity and feeding diet 2000#4.Dividing 56 Wistar rats into seven groups randomly:energy density of 8.4J/cm2,12.6J/cm2,16.8J/cm2 PDT groups and the photosensitizer group, simple laser group,0.2% NaF positive control group,0.9% normal saline negative control group. Elected the semiconductor laser with wavelength 532nm and power density 140mw/cm2 as light source,40μg/ml HMME as photosensitizer for PDT in accordance with the above groups weekly. Sampled the plate colony count until 56d at the end of the experiment. Observed the killing effect of S.mutanss in dental caries each group according the result of plate colony count; observed the caries preventive effect by Keyes points; observed the oral soft tissue in rats by histopathologic examination.Results:1.The colony counts of S.mutans in 96 plates which was treated with PDT in vitro showed that the best combination of this experiment with photosensitizer concentration 40μg/ml of HMME, incubation time 5min, the semiconductor laser of wavelength of 532nm and power density 140mw/cm2.2.Plate colony count of the inhibition of HMME-PDT with different energy density on S.mutans in rats showed that compared with 0.9% saline negative control group, the difference of S.mutans in HMME group was no significant (P＞0.05); the number of S.mutans (CFU/ml) decreased (P＜0.05) in both laser irradiation group and 0.2% NaF group,sterilization were 50.38% and 38.93%;the number of S.mutans was reduced significantly (P<0.05);in every PDT treatment groups, the sterilization rate in energy density of 8.4J/cm2,12.6J/cm2,16.8J/cm2 PDT group were 65.06%,96.97%, 97.52% respectively. The sterilization rate of 12.6J/cm2and 16.8J/cm2 PDT group increased significantly (P＜0.05) compared to the 8.4J/cm2 PDT group, while there was no significant difference between the 12.6J/cm2 and 16.8J/cm2 PDT group (P＞0.05).3. Keyes points results of fissures in rats under the stereomicroscope showed that compared with the 0.9% NS negative control group the difference in the experimental group, the NaF group and the laser group were significant statistically (P＜0.05), the difference in the photosensitizer group was not significant(P＞0.05). The score decreased by the increasing energy density in each PDT groups which were compared with each other, but the difference was not significant(P＞0.05).4. There were changes of cheek, tongue mucosa, gingival mucosa samples in every PDT groups compared with the blank control group in varying degrees. The number of congestion, edema of buccal, lingual mucosa cases in 8.4J/cm2 and 12.6J/cm2 PDT group were less than 16.8J/cm2 PDT group. The number of these cases in 16.8J/cm2 PDT group increased significantly; there have been slight infiltration of inflammatory cells in few samples.The phenomenon of thickened keratinized layer and bleeding in gum appeared occasionally. The phenomenon of congestion and edema were the main pathological response in all samples. Conclusion:1.The inhibitiont of HMME-PDT on S.mutans in vitro was significantly. The sterilizing result was related with the photosensitizer incubation time,concentration and the exposure time and power density.2. The prevention of HMME-PDT on rat dental caries significantly, but the effects were related to the energy density closely.3. Different energy density of PDT can cause different degrees of changes of oral soft tissues. With the energy density increasing, the pathological changes of congestion, edema in oral soft tissues and gums occurred easily. We should choose the ideal energy density of PDT in the follow-up experimental and clinical application to reduce the damage of oral soft tissue.
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