OBJECTIVEThis study aims to identify the crack tip stress intensity factor of the propagation process, crack propagation path, and the changes in the shape of the crack tip by the finite element method.

METHODSThe finite element model of dentino-enamel junction was established with ANSYS software, and the length of the initial crack in the single edge was set to 0.1 mm. The lower end of the sample was fixed. The tensile load of 1 MPa with frequency of 5 Hz was applied to the upper end. The stress intensity factor, deflection angle, and changes in the shape of the crack tip in the crack propagation were calculated by ANSYS.

RESULTSThe stress intensity factor suddenly and continuously decreased in dentino-enamel junction as the crack extended. A large skewed angle appeared, and the stress on crack tip was reduced.

CONCLUSIONThe dentino-enamel junction on human teeth may resist crack propagation through stress reduction.

Objective To study the antagonistic effects of magnesium-selenium-fluorine preparation on dental fluorosis of mice and its mechanism,and to provide a foundational basis for prevention and control of dental fluorosis.Methods Eighty male SPF ICR mice,were divided into 8 groups according to body weight by random number table method:control group,magnesium group,selenium group,magnesium-selenium group,fluoride group,magnesium-fluorine group,selenium-fluorine group and magnesium-selenium-fluorine group.The control group,magnesium group,selenium group and magnesium-selenium group drank double steamed water,and the other four groups drank 50 mg/L F-double steamed water.The control group and fluoride group fed conventionally.Magnesium group and magnesium-fluorine group fed conventionally by adding MgSO4·7H2O 162.5 mg/kg.Selenium group andselenium-fluorine group fed conventionally by adding Na2SeO3 ·5H2O 2.0 mg/kg.Magnesium-selenium group and magnesium-selenium-fluorine group fed conventionally by adding MgSO4·7H2O 162.5 mg/kg and Na2SeO3·5H2O 2.0mg/kg.Then incisor specimens were obtained after the mice were put into death after treatment for 42 days.The expression of amelogenin was observed with immunohistochemical staining.And gray value is expressed as a result,the greater the gray value,the less protein expression.Results The results of light microscope showed that ameloblasts in the fluoride group showed disarrangement and even had vacuolar.The change of ameloblasts in the magnesium-selenium-fluorine group had no significant difference with control group.The expressions of amelogenin in enamel matrix of fluoride group (131.03 ± 11.14) was significantly higher than those of others (control group:143.44 ± 2.52,magnesium group:143.73 ± 12.43,selenium group:148.89 ± 2.85,magnesium-selenium group:148.38 ± 7.58,magnesium-fluorine group:145.90 ± 7.00,selenium-fluorine group:148.70 ± 4.90,and magnesiumselenium-fluorine group:151.89 4± 4.59,all P ＜ 0.05).The expression of amelogenin in ameloblast of fluoride group (165.49 ± 5.66) was significantly lower than those of control group,magnesium group,magnesium-fluorine group and selenium group (151.35 ± 2.52,149.27 ± 11.13,146.21 ± 4.84 and 150.39 ± 6.65,all P ＜ 0.05).The expression of amelogenin in ameloblast of selenium-fluorine group (165.46 ± 5.81) was significantly lower than those of magnesium group,magnesium-fluorine group and selenium group (all P ＜ 0.05).The results of factorial analysis showed that magnesium and selenium affected the expression of amelogenin in enamel matrix (F =4.195,15.009,all P ＜ 0.05),the interaction between fluoride and magnesium had an effect on the expression of amelogenin in enamel matrix (F =4.402,P ＜ 0.05).Interaction of fluoride,magnesium and selenium had no effect on the expression of amelogenin in enamel matrix (F =1.561,P ＞ 0.05).The fluoride,magnesium and selenium affected the expression of amelogenin in ameloblast (F =18.463,9.372,4.741,all P ＜ 0.05),the interactions between fluoride and magnesium,magnesium and selenium had effects on the expression of amelogenin in ameloblast (F =10.351,5.919,all P ＜ 0.05).Interaction of fluoride,magnesium and selenium had no effect on the expression of amelogenin in ameloblast (F =1.460,P ＞ 0.05).Conclusions There is an antagonistic effect of magnesium on the expression of enamel protein in fluorosis mice.However,it can not be considered that there is an effect of selenium on the expression of enamel protein in fluorosis mice.

Objective To study the tooth germ development of mouse molar in fluorosis.Methods We established an acute fluorosis model and used HE-staining after preparing specimens of different stage of mouse first molar developing tooth germ.Results The developing molars of the control fetuses were at the bell stage of odontogenesis,whereas those of the experimental population were at cap stage.The ameloblasts became shorter and lost polarity.The arrangement of ameloblasts fell into disorder at the bell of differeiational stage and secretory stage.Conclusion The developing molars of the experimental fetuses were retarded.High-dose fluoride has a strong effect in the ameloblasts at the bell of differetiational stage and secretory stage.

Objective To elucidate the relationship between f luoride and cell apoptosis of ameloblasts and to further study the mechanism of dental fluorosis. Methods By establishing an experimental m odel of fluoride intoxication in pregnant C57BL/6N, we observed apoptosis in the fetal secretory ameloblasts in mice by means of transmission electron microscop e and TUNEL. Results High fluoride exposure in pregnant mic e could cause subameloblastic cysts in first lower molar of fetal mice. Programm ed cell death (PCD) occurred in the disturbed ameloblasts above cysts. Conclusion Intoxicat ion mechanism of fluoride on ameloblasts may occur by means of PCD.

From the view of medical ethics,the importance of the communication between intern and patient are illustrated.The author holds that the intern should develop the communication consciousness,realize the patient's expectation,and satisfy the patient's desire with humanism consciousness and role-switch consciousness.

To improve the clinical teaching work, the objective structured clinical examination (OSCE) was first introduced into the periodontal clinical assessment held by Departments of Periodon-titis and Mucosal Disease. With the OSCE exam station and item pool established, standardized pa-tients tralned, the examination form reformed, the knowledge, skill, and attitude of the examinee were assessed comprehensively. Compared with traditional exam,the OSCE evaluated the examinee's skills on clinical admissions, history collection, treatment plan development better. Most examinee have got satisfactory result (pass rate: 86.4 %, 19/22; good rate: 36.4%, 8/22; excellent rate: 9.1%, 2/22). The feedback of the following questionnalre also showed that most of the candidates thought the OSCE helpful to promoting the learning of theoretical knowledge (87.5%, 14/16) and good for converting the learned knowledge and skills into occupation ability (87.5%, 14/16).

The dental operative microscope has been widely employed in the field of dentistry, particularly in endodontics and operative dentistry, resulting in significant advancements in the effectiveness of root canal therapy, endodontic surgery, and dental restoration. However, the improper use of this microscope continues to be common in clinical settings, primarily due to operators' insufficient understanding and proficiency in both the features and established operating procedures of this equipment. In October 2019, Professor Jingping Liang, Vice Chairman of the Society of Cariology and Endodontology, Chinese Stomatological Association, organized a consensus meeting with Chinese experts in endodontics and operative dentistry. The objective of this meeting was to establish a standard operation procedure for the dental operative microscope. Subsequently, a consensus was reached and officially issued. Over the span of about four years, the content of this consensus has been further developed and improved through practical experience.
Humans ; Dentistry, Operative ; Consensus ; Endodontics ; Root Canal Therapy ; Dental Care