Mechanism of dentin hardness measurements using a dentin hardness measuring device with a light-emitting diode.

Significance: Managing caries is imperative in a rapidly aging society. Current diagnoses use qualitative indices. However, a quantitative evaluation of hardness in a clinical setting may lead to more accurate diagnoses. Previously, hardness meter using indenter with light for tooth monitoring (HAMILTOM) was developed to quantitatively measure tooth hardness. Herein, the physical interpretation of dentin hardness measured using HAMILTOM and the dentin hardness measurement mechanism are discussed. Aim: This study evaluates the mechanism of dentin hardness measurements using HAMILTOM physically and compare the invasiveness to dentin by HAMILTOM with those using a dental probe for palpation. Approach: Eleven bovine dentin samples were used to create caries models. HAMILTOM measured the dark areas, and its indentations were observed using scanning electron microscopy. Also, its invasiveness was evaluated by comparing the results with those from dental probe palpation. Results: The indentation areas were smaller than the dark areas in HAMILTOM, which may be due to exuded water from the dentin sample and the elastic recovery of dentin sample. Additionally, the dental probe indentation was deeper than the HAMILTOM indentations. Conclusions: The results demonstrate that the indentation areas were smaller than the dark areas measured by HAMILTOM, which might contain the influence of exuded water and the deformation of dentin sample. Also, HAMILTOM is less invasive than dental probe palpation. In the future, HAMILTOM may become a standard hardness measuring method to diagnose root caries.

Localization of senescent cells under cavity preparations in rats and restoration of reparative dentin formation by senolytics.

Reparative dentin formed by dental cavity preparation (DCP) is frequently used in clinical operations and plays a pivotal role in pulp protection. Recent reports have shown that senescent cells induced by various stressors aggravate many diseases. They can be treated using senolytics, which are drugs that selectively eliminate senescent cells. However, the association between DCP, senescent cells, and senolytics remains unclear. In this study, we established a rat model of DCP and analyzed the spatiotemporal localization of senescent cells in the pulp. The results showed that p21- and p16-positive senescent cells appeared mostly around the pulp horn (PH) under DCP. Furthermore, administration of senolytics (dasatinib and quercetin) successfully eliminated these senescent cells, thereby restoring the volume of reparative dentin formation. These data indicate that senescent cells induced by DCP may hamper the formation of reparative dentin. Senescent cells may be targets for the development of new restorative dentistry therapies.

Effects of a co-stimulation with S-PRG filler eluate and muramyl dipeptide (MDP) on matrix metalloproteinase-1 production by human dental pulp fibroblast-like cells.

The present study investigated the effects of a co-stimulation with surface reaction-type pre-reacted glass-ionomer (S-PRG) filler eluate and muramyl dipeptide (MDP) on matrix metalloproteinase (MMP)-1 production by human dental pulp fibroblast-like cells (hDPFs). S-PRG filler eluate contains 6 ions (F, Na, Al, B, Sr, and Si) released from S-PRG filler. Each S-PRG filler eluate and MDP stimulation enhanced MMP-1 production by hDPFs. The co-stimulation with S-PRG filler eluate and MDP enhanced MMP-1 production more than the MDP stimulation alone. A similar stimulation induced the phosphorylation of ERK 1/2. The increased secretion of MMP-1 and enhanced phosphorylation of ERK 1/2 by the co-stimulation with S-PRG filler eluate and MDP were suppressed by the selective and potent CaSR antagonist NPS 2143. Since strontium binds to CaSR, these results suggest that the enhanced production of MMP-1 by the co-stimulation with S-PRG filler eluate and MDP was due to the effects of strontium.

Demonstration of an optical dentin hardness measuring device using bovine dentin with different demineralization times.

Significance: The increase in root caries is a serious problem as society ages. Root caries is diagnosed by inspection and palpation, which are qualitative. A method to objectively and quantitatively evaluate the progress of root caries in a clinical setting is strongly desired. The root caries could be diagnosed by measuring hardness because dentin becomes softer as the caries progresses. Vickers hardness has been customarily used as an indicator of tooth hardness. However, this method cannot be used to in vivo teeth because the teeth must be dried prior to measurement to make the indentation. A hardness meter using an indenter with light for tooth monitoring (HAMILTOM) is proposed as an optical device. HAMILTOM could measure hardness of teeth in wet condition as a dark area while applying a load to dentins without drying. Therefore, HAMILTOM may realize hardness measurements of in vivo teeth in a clinical setting quantitatively. Aim: The aim of our study is to demonstrate the optical dentin hardness measuring device HAMILTOM using bovine dentin with different demineralization times and to evaluate the correlation between the dark areas measured by HAMILTOM and the Vickers hardness measured by the Vickers hardness tester. Approach: The samples were 20 bovine dentins. They were demineralized by a lactic acid solution with different times and divided into groups 1 and 2 of 10 samples each. In both groups, the dark areas and Vickers hardness were measured for each sample. Group 1 was used to obtain a calibration curve to calculate Vickers hardness from the dark area. Group 2 was used to validate the calibration curve obtained from the dentin samples of group 1. Results: The areas appearing black without a total internal reflection of the indenter measured by HAMILTOM increased as the demineralization time increased. Additionally, the Vickers hardness of group 2 calculated by the dark areas of group 2 and the calibration curve obtained in group 1 and the Vickers hardness of group 2 measured by the Vickers hardness tester were strongly correlated with a determination coefficient of 0.99. Conclusions: The results demonstrate that HAMILTOM may be a suitable alternative to the conventional method. Unlike the conventional method, which cannot be used for in vivo teeth, HAMILTOM holds potential to quantitatively evaluate the progress of caries in in vivo teeth.

Tailoring Silicon Nitride Surface Chemistry for Facilitating Odontogenic Differentiation of Rat Dental Pulp Cells.

Silicon nitride (Si3N4) can facilitate bone formation; hence, it is used as a biomaterial in orthopedics. Nevertheless, its usability for dentistry is unexplored. The aim of the present study was to investigate the effect of Si3N4 granules for the proliferation and odontogenic differentiation of rat dental pulp cells (rDPCs). Four different types of Si3N4 granules were prepared, which underwent different treatments to form pristine as-synthesized Si3N4, chemically treated Si3N4, thermally treated Si3N4, and Si3N4 sintered with 3 wt.% yttrium oxide (Y2O3). rDPCs were cultured on or around the Si3N4 granular beds. Compared with the other three types of Si3N4 granules, the sintered Si3N4 granules significantly promoted cellular attachment, upregulated the expression of odontogenic marker genes (Dentin Matrix Acidic Phosphoprotein 1 and Dentin Sialophosphoprotein) in the early phase, and enhanced the formation of mineralization nodules. Furthermore, the water contact angle of sintered Si3N4 was also greatly increased to 40°. These results suggest that the sintering process for Si3N4 with Y2O3 positively altered the surface properties of pristine as-synthesized Si3N4 granules, thereby facilitating the odontogenic differentiation of rDPCs. Thus, the introduction of a sintering treatment for Si3N4 granules is likely to facilitate their use in the clinical application of dentistry.

Interdental Plaque Microbial Community Changes under In Vitro Violet LED Irradiation.

Oral microbiome dysbiosis has important links to human health and disease. Although photodynamic therapy influences microbiome diversity, the specific effect of violet light irradiation remains largely unknown. In this study, we analyzed the effect of violet light-emitting diode (LED) irradiation on interdental plaque microbiota. Interdental plaque was collected from 12 human subjects, exposed to violet LED irradiation, and cultured in a specialized growth medium. Next-generation sequencing of the 16S ribosomal RNA genes revealed that α-diversity decreased, whereas ß-diversity exhibited a continuous change with violet LED irradiation doses. In addition, we identified several operational taxonomic units that exhibited significant shifts during violet LED irradiation. Specifically, violet LED irradiation led to a significant reduction in the relative abundance of Fusobacterium species, but a significant increase in several species of oral bacteria, such as Veillonella and Campylobacter. Our study provides an overview of oral plaque microbiota changes under violet LED irradiation, and highlights the potential of this method for adjusting the balance of the oral microbiome without inducing antibiotic resistance.

Clinical effectiveness of direct resin composite restorations bonded using one-step or two-step self-etch adhesive systems: A three-year multicenter study.

The purpose of this multicenter clinical study was to compare the mid-term clinical effectiveness of direct resin composite restorations using one-step or two-step self-etch adhesives (1-SEAs or 2-SEAs). In total, 352 restorations of class I-V cavities and non-carious cervical lesions in vital teeth (1-SEAs; 52 cases, 2-SEAs; 300 cases) were placed at nine university hospitals and evaluated according to the modified USPHS criteria at baseline, and after 1, 2, and 3 years. The recall rates were 86.6% (1 year), 80.1% (2 years), and 62.2% (3 years). Two restorations failed due to fracture during the follow-up, and there was no significant difference in survival rates between 1-SEAs (97.6%) and 2-SEAs (99.4%). However, 2-SEAs exhibited significantly lower occurrences of discoloration, marginal discoloration, fracture, and plaque retention. Moreover, the subjects reported a significantly lower postoperative hypersensitivity and higher overall satisfaction at all evaluation periods if 2-SEAs were used.

Gas Permeability of Mold during Freezing Process Alters the Pore Distribution of Gelatin Sponge and Its Bone-Forming Ability.

Freeze-drying, also known as lyophilization, is widely used in the preparation of porous biomaterials. Nevertheless, limited information is known regarding the effect of gas permeability on molds to obtain porous materials. We demonstrated that the different levels of gas permeability of molds remarkably altered the pore distribution of prepared gelatin sponges and distinct bone formation at critical-sized bone defects of the rat calvaria. Three types of molds were prepared: silicon tube (ST), which has high gas permeability; ST covered with polyvinylidene chloride (PVDC) film, which has low gas permeability, at the lateral side (STPL); and ST covered with PVDC at both the lateral and bottom sides (STPLB). The cross sections or curved surfaces of the sponges were evaluated using scanning electron microscopy and quantitative image analysis. The gelatin sponge prepared using ST mold demonstrated wider pore size and spatial distribution and larger average pore diameter (149.2 µm) compared with that prepared using STPL and STPLB. The sponges using ST demonstrated significantly poor bone formation and bone mineral density after 3 weeks. The results suggest that the gas permeability of molds critically alters the pore size and spatial pore distribution of prepared sponges during the freeze-drying process, which probably causes distinct bone formation.

Nitric Oxide Donor Modulates a Multispecies Oral Bacterial Community-An In Vitro Study.

The deterioration of human oral microbiota is known to not only cause oral diseases but also to affect systemic health. Various environmental factors are thought to influence the disruption and restoration of the oral ecosystem. In this study, we focused on the effect of nitric oxide (NO) produced by denitrification and NO synthase enzymes on dental plaque microbiota. Interdental plaques collected from 10 subjects were exposed to NO donor sodium nitroprusside (SNP) and then cultured in a specialized growth medium. Depending on the concentration of exposed SNP, a decrease in α-diversity and a continuous change in ß-diversity in the dental plaque community were shown by sequencing bacterial 16S rRNA genes. We also identified eight operational taxonomic units that were significantly altered by NO exposure. Among them, the exposure of NO donors to Fusobacterium nucleatum cells showed a decrease in survival rate consistent with the results of microbiota analysis. Meanwhile, in addition to NO tolerance, an increase in the tetrazolium salt-reducing activity of Campylobacter concisus cells was confirmed by exposure to SNP. This study provides an overview of how oral plaque microbiota shifts with exposure to NO and may contribute to the development of a method for adjusting the balance of the oral microbiome.

Application of hydroxyapatite nanoparticle-assembled powder using basic fibroblast growth factor as a pulp-capping agent.

We have previously fabricated hydroxyapatite (HAP) nanoparticle-assembled powder (nano-HAP) plates and granules by assembling low-crystallinity HAP nanoparticles without template/binder molecules or high-temperature/pressure treatments. In this study, we combined the nano-HAP with fibroblast growth factor (FGF) 2, which promotes odontoblast differentiation, and used this as a pulpcapping agent for dentin defects created in rat molars. The tissue response was then radiologically and histologically assessed at 1 and 2 weeks after capping, to assess the biocompatibility and ability of this material to promote hard tissue formation. The application of nano-HAP/FGF2 induced the invasion of dental pulp cells and vessels, and was consistently found to stimulate formation of a dentinal bridge containing numerous dentinal tubules. We thus succeeded in treating the pulp exposure by using a physiological approach to promote tissue regeneration. Further investigations should be performed to explain exactly how the nano-HAP/FGF2 combination contributes to calcified tissue formation.

Selective removal of carious human dentin using a nanosecond pulsed laser operating at a wavelength of 5.85 µm.

Less invasive methods for treating dental caries are strongly desired. However, conventional dental lasers do not always selectively remove caries or ensure good bonding to the composite resin. According to our previous study, demineralized dentin might be removed by a nanosecond pulsed laser operating at wavelengths of around 5.8 µm . The present study investigated the irradiation effect of the light on carious human dentin classified into "remove," "not remove," and "unclear" categories. Under 5.85-µm laser pulses, at average power densities of 30 W/cm² and irradiation time of 2 s, the ablation depth of "remove" and "not remove," and also the ablation depth of "unclear" and "not remove," were significantly different (p<0.01 ). The ablation depth was correlated with both Vickers hardness and Ca content. Thus, a nanosecond pulsed laser operating at 5.85 µm proved an effective less-invasive caries treatment.

In vitro study on selective removal of bovine demineralized dentin using nanosecond pulsed laser at wavelengths around 5.8 µm for realizing less invasive treatment of dental caries.

In the treatment of dental caries, less invasive methods are strongly required. However, conventional dental lasers cannot always achieve selective removal of caries or good bonding with a composite resin. Based on the optical absorption characteristics of dentin, wavelengths around 6 µm are promising in this regard. Our previous study indicated the possibility of selective removal of demineralized dentin using a nanosecond pulsed laser at wavelengths around 6 µm. In the present study, the optimal laser irradiation conditions were investigated for achieving selective removal of demineralized dentin. Bovine dentin was used, and its laser ablation characteristics were evaluated. The results indicated that demineralized dentin could be selectively removed, without causing cracking or damage to sound dentin, at laser wavelengths of 5.75 and 5.80 µm and average power densities of 30-40 W/cm(2). These optimal laser irradiation conditions also realized higher bonding strength with a composite resin than was possible using an Er:YAG laser. The use of nanosecond pulses allowed the thermal confinement condition to be satisfied, leading to a reduction in tissue damage, including degradation of dental pulp vitality. Thus, a nanosecond pulsed laser at 5.8 µm was found to be effective for less invasive caries treatment.

XPS analysis of the dentin irradiated by Er: YAG laser.

The aim of this study was to investigate the effect of Er:YAG laser irradiation on human dentin surface using X-ray photoelectron spectroscopy (XPS). 10 human dentin disks were prepared from extracted human molars for XPS analysis. These specimens were divided into two groups of five: a control group and group that were irradiated by an Er:YAG laser beam (100 mJ, 1Hz). All specimens were analyzed by XPS over a wide scanning range and narrow scanning ranges. The Ca/P ratio was calculated from the XPS results. In the results, the binding energies of Ca, P, and N in the laser-irradiated group were higher than those in the control group. The Ca/P ratio of the Er:YAG laser irradiated group (1.24+/-0.05) was significantly lower than that of the control group (1.52+/-0.16). This study showed that Er:YAG laser irradiation decreased Ca/P ratio and denatured the collagen of human dentin.

The effect of antibacterial monomer MDPB on the growth of organisms associated with root caries.

MDPB, 12-methacryloyloxydodecylpyridinium bromide, was tested for its ability to inhibit the growth of organisms associated with active root caries lesions and to modify the growth characteristics of these organisms at sub-MICs. MICs and MBCs of MDPB for independent isolates (n=5) of the following taxa: Streptococcus mutans, Streptococcus oralis, Streptococcus salivarius, Actinomyces naeslundii, Actinomyces israelii, Actinomyces gerensceriae, Actinomyces odontolyticus, Lactobacillus spp., and Candida albicans were determined, and the effects at sub-MIC on microbial growth kinetics were assessed. All isolates were sensitive to inhibition by MDPB. The median MICs and MBCs of MDPB for these organisms were in the range of 3.13 to 25.0 microg/ml and 6.25 to 50.0 microg/ml, respectively. As for the influence of pH, inhibition was sensitive to acidic pH. Even at sub-MICs, the growth of all strains, measured as cell yield and doubling time, was significantly reduced. Based on the results of this study, MDPB exhibited the potential to inhibit the growth of microbiota associated with active root caries lesions.