1 Department of Neurosurgery, Russian People's Friendship University, Moscow, Russia.
2 Division of Spine Surgery, Central Clinical Hospital of the Russian Academy of Sciences, Moscow, Russia.
3 Charité - Universitätsmedizin Berlin, Corporate member of Freie Universität Berlin, Humboldt-Universität zu Berlin, and Berlin Institute of Health, Berlin, Germany. ibrahim.efe@charite.de.
4 Centre for Surgery, Klinik Hirslanden, Zurich, Switzerland. ibrahim.efe@charite.de.