The main objective of the study is to evaluate the effectiveness, tolerability, and safety of tapentadol hydrochloride prolonged release (PR) in participants suffering from severe chronic pain due to osteoarthritis of the knee who are taking WHO Step III analgesics and show lack of tolerability. This is a clinical effectiveness trial designed to establish a link between anticipated clinical outcomes and the clinical practice by means of selected measures of clinical and subject-reported outcome. The trial will compare the effectiveness of previous analgesic treatment (WHO Step III) with that of tapentadol hydrochloride PR treatment during defined periods of evaluation.