The objective of this study is to detect the effect of trans-cranial direct current stimulation on postoperative pain and opioid consumption in total knee arthro-plasty patients. Fifty patients undergoing unilateral TKA will be randomly assigned to receive 4 sessions, total of 80 minutes of real (n=25) or sham tDCS (n=25). Stimulation will be given on 4 consecutive postoperative days, group 1 will receive real stimulation with the active electrode over the leg representation of the motor strip (cortical motor area of the leg) and the reference electrode over the arm, group 2 will receive Sham tDCS. Pain level will be evaluated according to visual analogue scale (VAS) and LANSS pain scale before (base line) after 1st, 2nd, 3rd and 4th sessions.