After evaluation by the investigator during a control visit or via on site-patient registry, potential participants will be asked to perform an inclusion visit (V1). During this visit, the investigator will assess the eligibility of the patients. Eligible patients will sign an informed consent. The investigator will make a general evaluation (demography, medical history, OA history, OA medication history, concomitant medications including "rescue treatments") and a pain evaluation. Synovial fluid will be collected in case of knee swelling.