ABSTRACT
Objective@#To understand the status of teaching environmental health in primary and secondary schools and explore its association with myopia, so as to provide a basis for precision myopia prevention and control.@*Methods@#From September to October 2020, a stratified cluster sampling method was used in 16 districts of Shanghai. After matching the teaching environment testing classrooms with the classrooms where the students were surveyed, totally 470 classrooms and 14 624 students were monitored. Trained professional technicians used laser range finder, luxmeter to measure parameters about teaching environmental health. Students were subject vision testing and questionnaires. Multilevel models was used to analyze the association between teaching environment and screening myopia.@*Results@#The qualified rates of each indicators of teaching environmental health were as follows: window to floor area ratio (93.0%), classroom area per capita (82.1%), uniformity ratio of illuminance of blackboard ( 79.7% ), blackboard size (68.7%), daylight factor (67.0%), blackboard hanging height (66.9%), average illuminance on desk (66.4%), uniformity ratio of illuminance of desk (63.6%), reflectance of blackboard (46.8%), average illuminance on blackboard (41.4%), reflectance of wall (33.2%), desk and chair allocation compliance rate (19.8%). The results of two level Logistic model analysis showed that unqualified average illuminance on blackboard was positively associated with screening myopia ( OR =1.24, 95% CI = 1.05 -1.47, P <0.05).@*Conclusion@#Teaching environmental health of primary and secondary schools in Shanghai is not optimistic, teaching environment especially the desk and chair allocation compliance rate need to be improved. The average illumination on blackboard is positively correlated with screening myopia, suggesting that myopia prevention and control can focus on improving the illuminance on blackboard.
ABSTRACT
Objective @#To analyze the effect of different cavosurface angles on the stress distribution of ClassⅠ cavity composite resin filling of molars through the three-dimensional finite element method and to provide references for the preparation of ClassⅠ cavities.@*Methods@#Three-dimensional finite element models of ClassⅠ composite resin filling of mandibular first molars with three different cavosurface angles (group A: 90°, group B: 120°, group C: 135°) were established. Polymerization shrinkage of composites was simulated with a thermal expansion approach. The mechanical behavior of the restored models in terms of stress and displacement distributions under the combined effects of polymerization shrinkage and occlusal load (600 N) was analyzed.@*Results@# For ClassⅠ cavities with the same cavity size, the total stress of the restoration model and the maximum stress of the enamel in group A were less than those in groups B and C after cavity composite resin restoration with three cavity cavosurface angles (in which the width of the enamel bevel was 1 mm in groups B and C). The maximum stress of the dentin and adhesive was similar in the three groups, the maximum stress of the composite in group C was the largest, and the maximum stress of the composite in group B was the smallest. In terms of stress distribution, the maximum stress in each restoration model was mainly concentrated in the enamel at the cavosurface, near the enamel-dentin interface and at the edge of the restoration material.@*Conclusion@#From the point of reducing the stress of residual tooth tissue, the preparation of 90° angle without enamel bevel is an ideal method for cavity preparation when composite resin is used to fill ClassⅠ cavities of molars.
ABSTRACT
@#Three-dimensional finite elemennt method (3-D FEM) is an approach to simulate the intraoral environment by fabricating virtual model in computer software, to provide various options of restorations according to the requirements of study, and finally to give relevant mechanic data for clinical reference by stress analysis. 3-D FEM has been applied in stomatological research more and more widely in recent years. This review has summarized the applications of 3-D FEM in various kinds of dental restorations.