Objective By investigating the retention and fatigue performance of polyetheretherketone(PEEK)clasps with increased proportions of clasp arm engaging the undercut,an innovative method to improve PEEK clasps'performance was proposed and verified.Methods Three groups of PEEK clasps(n=10/group)with the terminal 1/3,2/3 and whole of their retentive arms engaging the 0.75 mm undercut were designed and fabricated respectively along with a group of cobalt-chrome clasps(n=10)with their terminal 1/3 of retentive arms engaging the 0.25 mm undercut.Retentive forces were recorded initially and every 1 500 cycles for a total of 15 000 in-sertion and removal fatigue cycles.Optical scanning and software analysis were applied to evaluate the deformation of the clasps by root mean square(RMS)after fatigue test.The inner surface morphology of clasps'arms was observed by scanning electron microscopy.Re-sults PEEK clasps with the whole of their retentive arms engaging the undercut exhibited highest mean retentive force(9.24±1.78)Namong other PEKK clasps,which was slightly lower than that of cobalt-chrome clasps(11.88±2.05)N.The retentive force of each group turned out to be reduced after fatigue cycles and cobalt-chrome clasps showed the greatest reduction(38.38%).The RMS of the PEEK clasps groups were of no statistical difference(P=0.111)and were lower than that of cobalt-chrome clasps(105.47±10.82)μm.Evidence of surface abrasion was observed on all groups of clasps especially on the section that engaged the undercut.Conclusion Increasing the proportion of PEEK clasp arm engaging the undercut effectively improved the retentive force and satisfied clinical re-quirements after fatigue cycles.PEEK clasps exhibited greater retention stability than cobalt-chrome clasps.It is feasible to improve the performance of PEEK clasps by increasing the proportion of clasp arm engaging the undercut.

Objective:To analyze the characteristics of COVID-19 case spectrum and spread intensity in different provinces in China except Hubei province.Methods:The daily incidence data and case information of COVID-19 were collected from the official websites of provincial and municipal health commissions. The morbidity rate, severity rate, case-fatality rate, and spread ratio of COVID-19 were calculated.Results:As of 20 March, 2020, a total of 12 941 cases of COVID-19 had been conformed, including 116 deaths, and the average morbidity rate, severity rate and case-fatality rate were 0.97/100 000, 13.5 % and 0.90 %, respectively. The morbidity rates in Zhejiang (2.12/100 000), Jiangxi (2.01/100 000) and Beijing (1.93/100 000) ranked top three. The characteristics of COVID-19 case spectrum varied from province to province. The first three provinces (autonomous region, municipality) with high severity rates were Tianjin (45.6 %), Xinjiang (35.5 %) and Heilongjiang (29.5 %). The case-fatality rate was highest in Xinjiang (3.95 %), followed by Hainan (3.57 %) and Heilongjiang (2.70 %). The average spread ratio was 0.98 and the spread intensity varied from province to province. Tibet had the lowest spread ratio (0), followed by Qinghai (0.20) and Guangdong (0.23). Conclusion:The intervention measures were effective in preventing the spread of COVID-19 and improved treatment effect in China. However, there were significant differences among different regions in severity, case-fatality rate and spread ratio.