2.1 Study protocol In 2003, an observational study protocol was set up in the university medical centers of Nijmegen and Groningen, the Netherlands, in order to study the effects of MCS in patients that suffered from chronic neuropathic pain. Patients were included between 2005 and 2013 when they suffered from chronic intractable pain and reported high levels of pain (VAS > 5, measured three times daily during four days) and when the chronic neuropathic pain had showed to be intractable. Furthermore, on radiographic imaging techniques performed less than three years before inclusion for the implantation of MCS should show a possible conflict that might contribute to the pain. A multidisciplinary approach by the anesthesiologist-pain specialists, neurosurgeons, and clinical psychologists was chosen for the selection of patients. Patients with severe, current psychological problems (e.g., depression, high anxiety) or substance-abuse were excluded. Other exclusion criteria were the use of therapeutic anticoagulants, cognitive and/or psychiatric disorders in the medical history, nociceptive pain, an expected life expectancy less than 3 years due to other diseases (e.g., cancer), contra- indications for general anesthesia (e.g., severe cardio-pulmonal diseases), convulsive disorders and the presence of other neuromodulation systems. All patients underwent preoperative somatosensory-evoked potential (SSEP) measurement to determine the integrity of the somatosensory system in order to facilitate intra-operative neurophysiological monitoring. A MRI-scan was used to determine any anatomical contra-indications (brain atrophy, pathological structures) for the operative procedure. All patients presented in this study had a follow up of three years.