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Article History

Received: 9 May 2017

Revised: 21 July 2017

Accepted: 26 July 2017

First Online: 5 March 2018

Compliance with ethical standards

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: All analyses were based on previous published studies, all procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. For this retrospective study formal consent is not required.

: All the authors declare no conflict of interest.