ABSTRACT
Objective:To analyze the epidemiological characteristics of a family and workplace clustering of COVID-19， identify the source of infection and the transmission chain， and provide evidence for prevention and control of COVID-19 pandemic. Method:Field epidemiological method was used to conduct the investigation of confirmed cases and close contacts in this cluster. Data were analyzed with descriptive method. Real-time fluorescent quantitative PCR （RT-PCR） was used to detect the novel coronavirus nucleic acid in the collected respiratory tract samples. Results:A total of 18 epidemiological related cases were collected including 16 confirmed cases and 2 cases of asymptomatic infections. The involved places included 1 beauty clinic workplace and 3 families. Seven cases were males and 11 cases were females， with the minimum， maximum and median age of 3， 65 and 32 years old， respectively. Among them， the employees attack rate was 9.80% （10/102）， the family attack rate was 7.70% （5/78）， and the customer attack rate was 0.58% （1/173）. Positive nucleic acid test result in the respiratory tract sample of asymptomatic infection lasted for more than 2 months. Conclusions:The cause of this clustered COVID-19 epidemic is that the workplace environment is relatively closed with clustering crowds， and the source of imported infection is not discovered in time， which lead to a point-source outbreak and spread through family close contacts and clustering.
ABSTRACT
OBJECTIVE@#To explore the effects from the thread shapes of custom-made root-analogue implant (RAI) on distributions of von Mises stress around the peri-implant bone.@*METHODS@#Five one-stage RAI three-dimensional finite element (FE) models with different thread shapes (V-shaped design, square design, buttress design, reverse buttress design and none thread design) and congruent bone were created through reverse engineering technology. The data of the five models were imported into the FE analysis software to calculate. A force of 100 N was applied parallelly and of 45° to the implant axis respectively. Analysis was performed to evaluate the von Mises stress distributions at the peri-implant regions with the help of the Ansys 16 software.@*RESULTS@#The von Mises stresses distributed mostly at the implant cervical regions and the tip ends of the threads on the cortical bone under oblique loading, while on the cancellous bone, the stresses concentrated mostly on the implant lateral cervical regions, the tip ends of the threads and the apical regions. When under vertical loading, the von Mises stresses distributed mostly at the implant cervical regions on the cortical bone while at the tip ends of the threads and the lateral apical regions on the cancellous bone. The von Mises stresses were better distributed on the thread groups under both kinds of loadings compared with no thread design. But there was no obvious difference among the different thread groups. The concentrations of the von Mises stresses on the cancellous bone in the thread groups were mostly at the tip ends of the threads while less in the apical area. The von Mises stresses were better distributed on the cancellous bone on the other three thread designs than on square design.@*CONCLUSION@#Thread designs are advocated for the reason that adding thread designs to the RAI standard design will have a positive effect on stress distributions at the peri-implant regions and it will reduce the concentrations of von Mises stresses on the cortical bone. From the standpoint of the stress distribution, V-shaped design, buttress design and reverse buttress design are more suitable for RAI than square design. There is no difference of the distributions of the von Mises stresses in the RAI between different thread designs.