High microbicidal effect of peroxynitric acid on biofilm-infected dentin in a root carious tooth model and verification of tissue safety.

OBJECTIVES: Root-caries, which frequently occurs in elderly people, is more difficult to treat than caries in a tooth crown, especially in filling restorations. To overcome this difficulty, it is essential to find a strategy for sufficiently sterilizing the infected dentin; however, techniques for sterilizing carious pathogens inside the biofilm, called dental plaque, have not yet been established. Recently, dental applications of plasma sterilization technology have attracted attention. The mechanism of plasma sterilization became clear, and revealed that peroxynitric acid (PNA) is an effective sterilization substance. Highly concentrated PNA solutions can be chemically synthesized in large quantities without using plasma technology. We thought that the application of PNA solution could be a novel treatment for root caries, and examined the microbicidal effect and safety of PNA. METHODS: A sterilization experiment was performed using an extracted tooth model infected with Streptococcus mutans. Subsequently, a biofilm of S. mutans and Candida albicans was formed on a plate or a dentin slice, and sterilization experiments were performed in comparison with chlorhexidine. Furthermore, a toxicity test of PNA was performed using an epithelial tissue model. RESULTS: In the infection model, sterilization was achieved with a 22 mM PNA solution in only 10 s. In the biofilm model, a 22 mM PNA solution showed a higher microbicidal effect than 2.0% chlorhexidine. In the toxicity test, 2.0% chlorhexidine was toxic, but a 220 mM PNA solution showed no toxicity. CONCLUSIONS: PNA is an unprecedented disinfectant that has high microbicidal activity on biofilm and is safe for tissues.

Residual polymerization stresses in human premolars generated with Class II composite restorations.

The objective of this study was to assess the influence of filling techniques on residual polymerization stresses in resin composite restorations of the tooth. Flat planes were ground in buccal enamel surfaces of extracted human premolars, followed by preparing Class II cavities. Indentation cracks were introduced in the planes and crack lengths were measured mesio-distally (x-direction) and cervico-incisally (y-direction). Cavities were filled with a resin composite and an adhesive using three methods; one with bulk filling and two with differing incremental filling techniques. The x- and y-tensile stresses were calculated from crack lengths measured repeatedly over 360 min after filling. Elastic modulus and polymerization shrinkage of the composite were also measured. Filling technique and time after fillings were statistically significant only for the y-stress. The incremental techniques generated smaller stresses than the bulk filling. The stresses developed for 60 min after filling, while the modulus and the shrinkage stopped developing within 10 min and 2 min after irradiation, respectively. The incremental technique, in which the proximal portion of the cavity was filled first, was effective in decreasing the residual tensile stress generated by the polymerization of resin composite.

Plasma-treated water eliminates Streptococcus mutans in infected dentin model.

Non-mechanical procedures for removing caries-infected dentin are warranted in dentistry. We previously demonstrated the marked sterilization effect for direct irradiation of low-temperature plasma using dentin model infected with Streptococcus mutans. However it requires 180 s of intraoral plasma irradiation to eliminate bacteria. We alternatively investigated whether plasma-treated water (PTW), i.e., pure water exposed to plasma in an atmosphere, has a same bactericidal activity with the plasma irradiation. In the infected dentin model, the viable S. mutans counts recovered by bur at depth of 0.8-2.4 mm from the cavity floor were 104-106 CFU/round bur. After PTW application for only 10 s, the count was significantly decreased to below the detection limit (2.5 CFU/round bur) or 3.0±5.0 CFU/round bur. Since the bactericidal activity of PTW is rapidly deactivated at body temperature (37°C), PTW is likely to be biocompatible and holds significant potential for non-mechanical procedures for removing caries-infected dentin.