2.6 Statistical analysis IBM SPSS Statistics version 22 was used for statistical analyses of the retrieved data (IBM Corp. Released 2013. IBM SPSS Statistics for Windows, Version 22.0. Armonk, NY: IBM Corp.). To analyze the differences in pain relief and MQS scores and QoL-indices per subgroup, the Mann-Whitney U test was used. In order to correlate the applied stimulation parameters, the Spearman rank correlation coefficient was conducted. Values are represented as mean ± standard deviation (minimum- maximum). Alterations in MQS scores and QoL-indices before and after MCS were calculated with use of the Wilcoxon signed-rank test. Statistical tests were two sided and with a significance level of P < 0.05.