Received: 29 June 2018
Accepted: 28 November 2018
First Online: 22 December 2018
Ethics approval and consent to participate
: The Ethics Committee of the Federal State of Hamburg deliberated upon this research project (File PV5817). According to the Ethics Committee, retrospective database-based studies do not require an ethic committee approval, whenever the data was acquired, saved and treated anonymously. This applies to the present study. According to the same Ethics Committee, the need for written or verbal consent is unnecessary in studies with the characteristics of the present one.The database used for this research is not publicly available, it is property of the Schoen Clinic Group, whose access is regulated by the rules of procedure of its in-house Science Office.
: Not applicable.
: The authors declare that they have no competing interests.
: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.