Treatment: Each dose of study drug will be administered as one intraarticular (IA)injection carried out in each single knee joint that will be randomly assigned to blind treatment with 0.5 ml of etanercept (E) (12.5mg) or placebo (P) (NaCl). The IA injection will be administered once every two weeks for an eight week period. After 2 weeks time the patients will be crossed-over and each arthritic knee joint will receive E (12.5 mg) for four times or P for three times, respectively, administered every two weeks for an eight week period. All the IA injections will be administered after synovial fluid aspiration by the same operator. The study, patients with KJS flare up after IA injection will be asked to enter the open-label IA extension treatment with IA-E injection once every two weeks, for eight weeks.