A Study Comparing the Effectiveness and Safety of Extended Release Tramadol Versus Placebo for the Treatment of Osteoarthritis (OA) of the Knee

Inclusion Criteria: * Patients with American College of Rheumatology (ACR) Functional Class I-III OA of the knee; involvement of at least one knee joint that warrants treatment with NSAIDs, acetaminophen, or opioid analgesics for at least 75 of the 90 days preceding the screening visit; patients with radiographic evidence of osteoarthritis within the last 6 months; patients with a pain intensity in index joint \> = 40 mm on the visual analog scale (VAS) at the baseline visit; patients who are able to discontinue NSAIDs, COX-2 selective inhibitors, and other analgesics during the washout period and throughout double-blind study. Exclusion Criteria: * Patients with a medical condition, other than OA, uncontrolled with treatment or any clinically significant condition that, in the investigator's opinion, precludes study participation or interferes with the assessment of chronic pain and other OA symptoms; patients with a diagnosis of inflammatory arthritis, gout, pseudo-gout or Paget's disease, that, in the investigator's opinion, interferes with the assessment of pain and other symptoms of OA; patients with a diagnosis of chronic pain syndrome, patients with an ACR or a clinical diagnosis of fibromyalgia; patients with a clinically significant form of joint disease or prior joint replacement surgery at the index joint; patients with an anticipated need for surgery or other invasive procedure in the index joint.