1. Department of Anaesthesiology and Pain Management; Rijnstate Hospital; Arnhem the Netherlands
2. Department of Anaesthesiology and Pain Medicine; Maastricht University Medical Centre; Maastricht the Netherlands
3. Department of Clinical Epidemiology and Medical Technology; Maastricht University Medical Centre; Maastricht the Netherlands
4. Department of Orthopaedic Surgery; Maastricht University Medical Centre; Maastricht the Netherlands
5. Department of Neurosurgery; Maastricht University Medical Centre; Maastricht the Netherlands
6. Department of Neurosurgery; Atrium Medisch Centrum Heerlen; Heerlen the Netherlands