Funding for this research was provided by:
Natural Science Foundation of Shandong Province (ZR2013HQ016)
Key Research and Development Project of Shandong Province (2015GGH318011)
Article History
Received: 8 August 2018
Revised: 8 November 2018
Accepted: 2 December 2018
First Online: 6 January 2019
Compliance with ethical standards
: This retrospective study was approved and supported by the ethical committee of Shandong University, Shandong province, China.
: The authors declare that they have no conflict of interest.