Study on the Safety and Effectiveness of Switching Between Two Forms of Tapentadol in Patients With Chronic Low Back Pain

21 day Open Label: an adjustable dose of Tapentadol IR 50-100mg orally every 4-6 hours to maximum total daily dose (TDD) dose of 500 mg during open label period

During 2 double blind periods: Tapentadol ER 100, 150, 200 or 250 mg tablets twice daily in the first intervention period of double-blind phase and Tapentadol IR in the second or Tapentadol IR in first intervention period of double-blind phase and Tapentadol ER in second

Following open label period is 2 double blind periods: Tapentadol IR in first intervention period of double-blind phase and Tapentadol ER 100, 150, 200 or 250 mg tablets twice daily in second or Tapentadol ER in first intervention period of double-blind phase and Tapentadol IR in second