2.5 Assessment Pain is a complex, subjective and multidimensional phenomenon that is difficult to measure by unidimensional pain scores only. Apart from the visual analog scale (VAS), the intake of pain medication is thought to be a valid tool of measuring pain relief. Adding analgesic drug intake as an outcome parameter could provide a more realistic assessment of long-term benefits of MCS. Four outcome variables were examined: 1) the amount of pain relief, measured by the mean difference between VAS score pre-operatively and the VAS score during the follow-up (1 month, 6 months, 1 year, and 3 years after implantation of the MCS electrodes); 2) the change in the drug regimen of all patients per day; 3) adverse events (infection, bleeding, hardware removal, temporary seizures, and battery dysfunction); and 4) the correlation between stimulation parameters and the pain relief per patient. Pain relief was divided into three categories. A good pain relief, level 1, was defined as a VAS reduction of 70-100%. Reduction of pain according to a VAS scores change between 40% and 69% was defined as satisfactory (level 2), while a minimal pain relief was defined as a reduction of ≤ 40% on the VAS scores. An effective pain relief was defined as ≥ 40% reduction of pain (levels 1 and 2). The use of medication was monitored using the electronic patient record during follow-up. The medication quantification scale (MQS) was used in order to quantify medication use and was calculated for each drug by multiplying the dosage levels by their respective detriment weight. The dosage levels (0-6) were based on the recommended daily dosage range as described by Masters Steedman et al. These scores are summed to provide a quantitative index of total drug intake suitable for statistical analysis. The occurrence of complications was documented as well. Apart from biological complications (eg. bleeding, infection), the removal of the hardware due to a minimal effect was evaluated as well. To determine whether there was a correlation between the used stimulation parameters and the pain relief, the used stimulation parameters (intensities [V], pulse widths [µs], and frequencies [Hz]) were reviewed. Interference of pain with quality of life (QoL) was measured before and after (> 1 year) MCS with use of the Quality of Life Index (QLI), based on the Dutch version of the McGill pain questionnaire (MPQ-DLV).