This randomised controlled trial aims to investigate the efficacy of cabbage leaf cataplasms in the treatment of primary symptomatic osteoarthritis of the knee. 81 patients will be randomised into one of 3 groups and apply either cabbage leaf cataplasm or diclofenac gel daily for 4 weeks or receive only usual care. Efficacy will be measured using questionnaire on pain, disability, quality of life and pressure pain sensitivity.