Knee prosthesis infection is a severe complication. The use of a tourniquet during surgery impairs the efficacy of the antibiotic prophylaxis. We hypothesize that the antibiotic administration before tourniquet release decrease the infection rate. Methods: patients who undergo a total knee arthroplasty will be randomized to receive one of the following regimens of antibiotic prophylaxis:Standard: cefuroxime 1.5 g i.v. 10 min before tourniquet + placebo 10 min before tourniquet release + cefuroxime 1'5 g i.v. 6h after closing surgical wound.Experimental: cefuroxime 1.5 g i.v. 10 min before tourniquet + cefuroxime 1.5 g i.v. 10 min before tourniquet release + cefuroxime 1'5 g i.v. 6h after closing surgical wound.