Tapentadol (CG5503)

Inclusion Criteria: * Patients diagnosed with osteoarthritis of the knee based on the American College of Rheumatology (ACR) criteria and functional capacity class of I-III * patients taking analgesic medications for at least 3 months prior to screening and/or dissatisfied with their current therapy * Patients requiring opioid treatment must be taking daily doses of opioid-based analgesic, equivalent to \<160 mg of oral morphine * baseline score of greater than or equal to 5 on an 11-point numerical rating scale, calculated as the average pain intensity during the last 3 days prior to randomization. Exclusion Criteria: * History of alcohol and/or drug abuse in Investigator's judgement * history of significant liver insufficiency * chronic hepatitis B or C, or HIV, presence of active hepatitis B or C within the past 3 months * life-long history of seizure disorder or epilepsy * history of malignancy within past 2 years, with exception of basal cell carcinoma that has been successfully treated * uncontrolled hypertension * patients with severely impaired renal function * patients with moderate to severly impaired hepatic function or with laboratory values reflecting inadequate hepatic function