The root canal system has complex morphology. Isthmus is observed in the maxillary premolar, molar mesio-buccal root and mandibular molar mesial root in cases involving two root canals. It is observed that 76% and 92.5% of cases with maxillary molar mesio-buccal root and 55.5, 57.8, 64.4% and 83% of cases with mandibular molar mesial root and classified into five classes. Isthmus is observed not only from the cervical third to the apical third but also in the cervical third or apical third alone, and in 31.32% to 63.91% of cases, it is seen in the apical third of the mandibular molar. The middle mesial (MM) canal can also be found in the mandibular molar mesial root, meeting the mesio-buccal root canal or mesio-lingual root canal. An isthmus can become a site of bacteria colonization and is difficult to clean mechanically. Inadequate treatment of isthmus is considered a source of failure of root canal treatment particularly when isthmus is located at the root canal apex. In addition, an isthmus can become packed with debris and thereby become an infection source during root canal preparation, making isthmus obturation difficult. Due to difficulties with mechanical management, such as via syringe irrigation, Ni-Ti file for irrigation, sonically activated irrigation, ultrasonically activated irrigation, negative pressure irrigation, laser-activated irrigation, manual dynamic irrigation or ultrasonic wave aspiration device, attempts to clean the isthmus chemically have been reported. However, no method yet developed has proven wholly sufficient to clean the isthmus.

 Isthmus is not rare and is considered one of the reasons for failure of root canal treatment. The development of more efficient irrigation techniques and effective and safe methods for mechanically preparing an isthmus is awaited.

 Purpose: Oxyapatite (Ca₁₀ (PO₄)₆O: OAP) contains reactive oxygen radicals and acts as an antimicrobial material because it oxidizes and resolves adsorbed materials such as organic matter and bacillus, and also functions as a bone substitute material. It is reported that photocatalytic activity of oxyapatite is observed when a part of the calcium ions in the crystalline lattice are substituted with lantern ions. The purpose of this study was to examine a method of synthesizing lanthanum-doped OAP (Ca₈La₂ (PO₄)₆O₂: La-OAP) and to investigate the photocatalytic activity of the prepared lanthanum-doped OAP.

 Methods: La-OAP was synthesized by the dry method. La-OAP was prepared by heat treating mixtures of La₂O₃ and Ca₂P₂O₇ in a multi-atmosphere furnace at three different temperatures (1,000, 1,200 and 1,400°C) for 4 hours in an oxygen flow or a nitrogen flow. The synthetic La-OAP was characterized by using XRD. The photocatalytic activity of La-OAP was evaluated by degradation of methylene blue (MB) under visible light (irradiation for 10-60 minutes). The antibacterial ability of La-OAP was evaluated by a halo test. The halo test was performed using filter test pieces (5 mmφ) containing the synthetic La-OAP powder. After the filter test piece was placed on the agar medium containing Escherichia coli or Streptococcus mutans, light irradiation was performed by a halogen lamp from the upper and lower sides of the test piece for 30 minutes. The inhibition zone which arose around the test piece was investigated after incubation (37°C, 12 hours).

 Results: XRD analyses confirmed that the heat-treated precipitate was La-OAP. The amount of La-OAP varied according to heating temperature regardless of the type of gas atmosphere. La-OAP samples synthesized by the dry method at 1,000, 1,200 and 1,400°C in oxygen flow completely resolved MB with visible light irradiation for 30-60 minutes. After the irradiation the MB solution became transparent. Inhibition zones were observed around the filter test pieces against both E. coli and S. mutans.

 Conclusion: Lantern-oxyapatite (Ca₈La₂ (PO₄)₆O₂) synthesized by the dry method showed a high photocatalytic activity under visible light and might work as an antimicrobial material.

 Purpose: This study examined the influence of cleaning agents on the bonding performance of a resin luting cement to saliva-contaminated core foundation resin by means of shear bond strength (SBS) and surface free energy (SFE) measurements.

 Methods: The agents used for removing saliva contamination were: Katana Cleaner (KC, Kuraray Noritake Dental), Multi Etchant (ME, Yamakin), and Ultra-Etch (UE, Ultradent Products). Panavia V5 (Kuraray Noritake Dental) was used as a resin luting cement. Core foundation resin surfaces were ground with SiC papers, ending at #320 grit. To create saliva-contaminated resin surfaces, 10 μl of human saliva was applied to the ground surface. Uncontaminated resin surfaces were prepared as a control group. The saliva-contaminated specimens were divided into four groups, treated with one of the cleaning agents (KC, ME, or UE) or without any treatment (SC group). A stainless-steel metal ring was placed over the bonding area, and the luting agent was condensed into the ring and light irradiated. The assembled specimens were stored in 37°C water for 24 h. Half of the specimens in each group were subjected to thermal cycling (TC) for 10,000 times. SBS tests were conducted on each group at the crosshead speed of 1.0 mm/min. For SFE determination, specimens were prepared as for the SBS tests. The contact angles of three test liquids that have known SFE parameters were measured using a contact angle meter. Scanning electronic microscopic observation was also performed.

 Results: After 24 h storage, although the SC group showed a significantly lower SBS value than the control group, the cleaning agent treated groups showed higher SBS values than the SC group. After TC, the cleaning agent treated groups showed significantly lower SBS values than the control group, but significantly higher SBS values than the SC group. Although SC showed significantly lower SFE values than KC and ME, the SFE value after cleaning agent treatment depended on the material.

 Conclusion: After saliva contamination of core foundation resin surfaces, treatment with the cleaning agents might be helpful for enhancing the bonding performance of resin luting agents. The SFE values of cleaning agent treated surfaces were different due to their cleaning mechanisms.

 Purpose: It is important to elucidate the relationship between periodontal disease and rheumatoid arthritis (RA). Thus, we investigated the effect of oral infection with Porphyromonas gingivalis on the exacerbation of RA by histopathological and imaging analysis using an RA mouse model.

 Methods: In this study, 11-week-old DBA/1J mice were used as RA model mice for collagen-induced arthritis. In the experimental group, 0.1 ml of P. gingivalis (1×10⁹ CFU/ml) suspended in 2.5% carboxymethylcellulose (CMC) was administered directly into the oral cavity of mice every other day. In the control group, 0.1 ml of 2.5% CMC was administered in the same manner. At day 45 after oral administration, mice in both groups were sacrificed, and the left lower limbs and spine were collected and analyzed. Soft X-ray images of the left lower limbs were used to evaluate the development of RA symptoms. The periarticular bone tissue of the spine was evaluated by micro-computed tomography (micro-CT) or histological morphological observation by safranin O/fast green staining. In spinal joints, immunohistochemical staining using matrix metalloproteinase-13 (MMP-13) antibody was performed to detect the type II collagen present in the cartilage.

 Results: Soft X-ray showed ankylosis, bone obscuration, and a rough appearance in the experimental group. Micro-CT analysis showed a significant decrease in the interspinous distance in the experimental group. Histological analysis showed a marked decrease in type II collagen in the cartilage endplates. Furthermore, the number of MMP-13-positive cells detected in the cartilage endplates and fibrous rings was 1.6 times higher in the experimental group than in the control group.

 Conclusion: Oral infection with P. gingivalis promotes tissue destruction in the spinal joints of the mouse model of collagen-induced arthritis, suggesting that periodontal disease caused by P. gingivalis may have a systemic effect on the progression of symptoms in patients with rheumatoid arthritis.

 Purpose: Since 2018, the Asahi University School of Dentistry has been holding hands-on training using a microscope (“microscopy training”) for fifth-year students undergoing clinical training at the Department of Endodontics. The introduction of group learning into microscopy training has enabled students to focus as smoothly as instructors. However, the Covid-19 pandemic that began in 2020 resulted in a number of unavoidable lifestyle changes. In the university educational setting, countermeasures had to be introduced to enable lectures to be held for large audiences and to hold practical sessions, and microscopy training for clinical training was no exception. In 2020, the previous group learning format of microscopy training was replaced by individual practical learning at a time specified by the instructor. We compared the effect of this change in method on learning and report the results here.

 Method: Microscopy training was provided to 42 students who applied to participate. Handouts summarizing the structure and the method of using the microscope were distributed 1 week before the training, and advance study was encouraged. At the beginning of the training session, the time taken to focus at maximum magnification on the orifice of the palatal root canal of a maxillary left first premolar fitted to a model jawbone was measured. The participants then engaged in individual learning with the microscope while referring to the handouts. Next, the instructor provided feedback and performed a demonstration, after which further time was allotted for individual learning. The focus adjustment time was re-measured at the end of the training session, and statistical analysis was conducted using a Mann-Whitney U test with p<0.05 regarded as significant.

 Results and Discussion: There was no significant difference in the time required to focus at the beginning of the training session between the 2020 and 2019 students, which involved group learning. However, the focus adjustment time at the end of the individual learning in 2020 was significantly longer than that of the group learning implemented in 2019. Although the results of this study did show a significant difference in focus adjustment time, we do not conclude that individual study is inferior. Instead, the results appear to suggest that the effectiveness of individual study is not maximized when it simply replaces group learning, and that further creative adjustments to learning methods are required to fit the context.

 Purpose: Few studies have investigated the 2017 classification of periodontitis and its effects on the prognosis of periodontal disease in Japanese people. The aim of this study was to assess the association of the new periodontitis classification with changes in the non-incidence rate of periodontitis-related tooth loss during maintenance.

 Methods: A total of 339 patients treated for periodontitis with a minimum follow-up period of 5 years at a dental clinic in Osaka were retrospectively categorized according to the new 2017 classification of periodontitis at baseline. The association of the new classification of periodontitis with the non-incidence rate of patients with tooth loss due to periodontitis during maintenance was explored. A Kaplan-Meier survival analysis and log-rank test were performed to show the event-free curves of periodontitis-related tooth loss over time. A multivariate Cox regression analysis for the dependent variable was used.

 Results: At baseline, patients with a mean age of 52.0±11.3 years with a mean number of teeth of 26.3±3.6 were followed for a mean period of 98.8±28.2 months. Overall, 12 patients were classified as stage Ⅰ, 77 as stage Ⅱ, 186 as stage Ⅲ, and 64 patients as stage Ⅳ. Five patients were classified as grade A, 177 as grade B, and 157 as grade C. Patients were classified as localized in 164 cases and generalized in 175 cases. Tooth loss due to periodontitis was not found in Stage Ⅰ or Ⅱ or grade A patients during maintenance. The 8-year event-free survival rates for tooth loss were 80.8% and 37.8% for stage Ⅲ and stage Ⅳ patients, respectively, 96.8% and 56.9% for grade B and grade C patients, respectively, and 89.6% and 67.5% for localized and generalized patients, respectively. Statistically significant differences were observed among stages and grades, and between extents. The multilevel Cox regression analysis demonstrated that the new periodontitis classification was associated with the event-free rate for periodontitis-related tooth loss.

 Conclusion: The new periodontitis classification has been shown to be a useful predictive tool for patients without periodontitis-related tooth loss.