• The primary efficacy outcome is the change in arthritis pain intensity score
  
• averages over the 12 weeks of the study.
  

• The secondary outcomes include change from baseline in the daily arthritis
  
• pain intensity VAS score from patient diaries; WOMAC OA Index pain subscale
  
• scores for pain and physical function, patient's global assessment, patient's
  
• sleep assessment
  

Immediate release (IR) tramadol has demonstrated efficacy in several pain conditions including: obstetrical, gynecological, orthopedic, abdominal, and oral surgery. The short elimination half-life of IR tramadol requires every 4-6 hour dosing to maintain optimal levels of analgesia in chronic pain. The study medication in this study is a once-daily, extended-release (ER) tramadol formulation. This is a 12-week, multi-center, double-blind, randomized, dose-titration, parallel-group, placebo-controlled study designed to evaluate the analgesic effectiveness and safety of once daily tramadol HCl ER with placebo in the treatment of patients with OA of the knee. Patients with OA Functional Class I-III of the knee will be eligible for participation, if appropriate criteria are met. After a 2-7 day washout period, during which the use of all analgesic medications used for chronic pain management will be discontinued, patients experiencing moderate to severe pain (pain intensity >= 40 mm on a 100 mm visual analog scale) in the index knee and who meet all other study criteria will enter a 2-week, double-blind, titration period. During this period, patients will be randomly assigned to treatment with tramadol HCl ER or placebo, once daily (QD). The initial dose will be tramadol HCl ER 100 mg or matching placebo QD. On Day 4 and for the remainder of the week, patients will be titrated to 200 mg or matching placebo QD, based on tolerability. Beginning at Visit 3, a minimum dose of 200 mg or matching placebo QD are to be maintained for the remainder of the 12-week study. Patients unable to tolerate at least 200 mg or matching placebo QD will be discontinued from the study. After Week 1 (Visit 3), further increases to 300 mg or 400 mg or matching placebo QD were permitted, depending on the adequacy of pain relief and tolerability of side effects. Efficacy and safety evaluations will be collected at study visits occurring at Weeks 1, 2, 4, 8, and 12 or at early termination. Study medication will be discontinued at Week 12 and patients will return after 1 week for a post-treatment visit (Week 13).