• Change From Baseline of the Average Pain Intensity Based on an 11-point Numerical Rating Scale(NRS) Over the Last Week of the Maintenance Period at Week 12. [ Time Frame: Baseline and 12 weeks (Primary endpoint is the average pain intensity score during the last week of the maintenance period). ] [ Designated as safety issue: No ]
  For this twice daily pain assessment, the subjects were to indicate the level of pain experienced over the previous 12 hours on an 11-point Numerical Rating Scale (NRS) where a score of 0 indicated "no pain" and a score of 10 indicated "pain as bad as you can imagine".
  
  

• Change From Baseline in Western Ontario McMaster Questionnaire (WOMAC) Assessing Pain, Disability and Joint Stiffness of the Knee Over the Last Week of the Maintenance Period at Week 12 [ Time Frame: Baseline and 12 week endpoint ] [ Designated as safety issue: No ]
  Change from baseline to Week 12 of WOMAC Global Score: WOMAC is measure with a Likert ordinal scale from 0-4 with lower scores indicating lower levels of symptoms or physical disability
  
  
• Change From Baseline in Sleep Latency Time in Hours Over the Last Week of the Maintenance Period at Week 12. [ Time Frame: Baseline and 12 week endpoint ] [ Designated as safety issue: No ]
  A Sleep Questionniare addressed the following question: "How long after bedtime/lights out did you fall asleep last night (hours)?" 12 week endpoint-mean changes from baseline at endpoint for sleep latency. Decrease in time(hours) indicates improvement.
  
  
• Percentage of Patients Who Reported Very Much Improved or Much Improved From Baseline in Patient Global Impression of Change Over the Last Week of the Maintenance Period at Week 12 [ Time Frame: Baseline and 12 week endpoint ] [ Designated as safety issue: No ]
  Ordinal measure indicating change from start of treatment (on a scale of 7 = Very much worse to 1 = Very much improved)
  
  
• Distribution of Time to Treatment Discontinuation Due to Lack of Efficacy [ Time Frame: Baseline to 12 weeks ] [ Designated as safety issue: No ]
  The median time to treatment discontinuation due to lack of efficacy from baseline to endpoint
  
  
• Change From Baseline in EuroQol-5 (EQ-5D) Health Status Index to Week 12 [ Time Frame: Baseline and 12 week endpoint ] [ Designated as safety issue: No ]
  Change from baseline to end point in EuroQol-5 (EQ-5D) Dimension Questionnaire. A higher score indicates an improvement in health in the Health Status Index. The EQ-5D is a five dimensional health state classification. Each dimension is assessed on a 3-point ordinal scale (1=no problems, 2=some problems, 3=extreme problems). The responses to the five EQ-5D dimensions were scored using a utility-weighted algorithm to derive an EQ-5D health status index score between 0 to 1, with 1.00 indicating "full health" and 0 representing dead
  
  
• Change From Baseline in Responder Analysis 50% Improvement to Week 12 [ Time Frame: Baseline and Week 12 ] [ Designated as safety issue: No ]
  Defined by the percentage of subjects achieving at least 50% improvement from baseline in the primary endpoint based on the 11-point NRS at week 12. For this twice daily pain assessment, the subjects were to indicate the level of pain experienced over the previous 12 hours on an 11-point Numerical Rating Scale (NRS) where a score of 0 indicated "no pain" and a score of 10 indicated "pain as bad as you can imagine".
  
  

The primary objective of this randomized (study medication assigned to patients by chance), double-blind (neither patient nor investigator knows the study medication) , phase III, placebo and active controlled trial is to evaluate the efficacy and safety of orally administered tapentadol (CG5503) Extended Release (ER) (base) at doses of 100-250 mg twice daily in patients with moderate to severe chronic pain from osteoarthritis (OA) of the knee. The study is being conducted for registration and approval of tapentadol (CG5503) in the US and outside the US. The trial will consist of five periods: screening (to assess eligibility) , washout (3-7 days with determination of a baseline pain intensity), titration (of dose over 3 weeks to the optimal individual level) , maintenance (investigational drug intake for 12 weeks with adjustments allowed), and follow-up (2 weeks post treatment discontinuation). The study hypothesis is that the study drug will be more effective than placebo in reducing patients pain intensity. The Secondary objectives include the collection of pharmacokinetic (related to how the body uses the drug) information for dose verification. The trial objectives will be assessed by comparing the baseline pain level to the level of week 12 of the maintenance phase. This will be done by looking at the patient's pain diary information. Titrate tapentadol (CG5503) ER (extended release) 50mg to patient's optimal dose ranging between 100mg and 250mg twice a day; Oxycodone CR (controlled release) 10mg to 50mg twice a day; Placebo (no active ingredients). All doses of trial treatment will be taken orally with approximately 120 mL of water with or without food for a maximum timeframe of 15 weeks.