<qa>  What is the purpose of this study?The purpose of this study is to compare the effectiveness of tramadol hydrochloride 37.5 milligram mg plus acetaminophen 325 mg maintenance with that of non-steroidal anti-inflammatory drugs NSAIDs maintenance in participants with knee osteoarthritis a progressive and degenerative joint disease, in which the joints become painful and stiff whose pain was relieved after the add-on treatment of tramadol hydrochloride to NSAIDs. 