Christoph E Heyde1, P Gahr2, D Haschtmann3, S K Tschöke4. 1. Spine Division, Department of Orthopaedic Surgery, Trauma and Plastic Surgery, University Hospital Leipzig, Liebigstrasse 22, 04103, Leipzig, Germany. Christoph-Eckhard.Heyde@medizin.uni-leipzig.de. 2. Department of Orthopaedic and Trauma Surgery, Charité, Campus Benjamin Franklin, Hindenburgdamm 30, 12200, Berlin, Germany. 3. Spine Center, Schulthess Clinic, Lengghalde 2, 8008, Zurich, Switzerland. 4. Department of Spine Surgery, Klinikum Dortmund gGmbH, Beurhausstrasse 40, 44137, Dortmund, Germany.