Liu, K
Shi, Y
Guo, X
Wang, S
Ouyang, Y
Hao, M
Liu, D
Qiao, L
Li, N
Zheng, J
Chen, D
Article History
Received: 2 October 2013
Revised: 25 May 2014
Accepted: 29 May 2014
First Online: 17 July 2014
Competing interests
: The authors declare no conflict of interest.