Text and Data Mining valid from 2019-01-14
Received: 21 March 2018
Accepted: 3 January 2019
First Online: 14 January 2019
Compliance with ethical standards
: The authors confirm that this article has no conflict of interest.
: All procedures performed in studies involving human participants were in accordance with the ethical standards of Qingdao Municipal Hospital and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.
: A written informed consent was obtained from each participant or the legal guardian.