Received: 1 November 2017
Accepted: 1 February 2018
First Online: 17 February 2018
: Department of Neurosurgery, The First Affiliated Hospital of Soochow University.
: The protocol of this study was carried out according to the principles of the Declaration of Helsinki and approved by the Medical Ethics Committee of First Affiliated Hospital of Soochow University. As this article is a retrospective study, ethics committees have been granted exemption of patient informed consent.
: No applicable.
: The authors declare that they have no competing interests.