Depot-opioids for Pre- and Postoperative Pain Relief After Primary Knee Arthroplasty.Tapentadol vs Oxycodone vs Placebo

Inclusion Criteria: * scheduled for knee arthroplasty at St. Olavs University Hospital * consent in participation in the study Exclusion Criteria: * Contraindications for any of the study drugs * Lactose intolerance * Known hypersensitivity against any of the additives * Chronic Obstructive Pulmonary Disease (COPD) grade 3 or 4 * Paralytic ileus * Known alcohol or medical addiction/abuse * History of asthma, urticaria or allergic reaction caused by acetyl salicylic acid or other NSAIDs * Peptic ulcer * Hemophilia * Gastrointestinal bleeding * Cerebrovascular bleeding * Inflammatory bowel disease (ulcerous colitis, Crohn disease) * Concomitant use of the following drugs: ACE-inhibitors, AT2-blockers, SSRI, Anti-psychotic drugs, MAO-inhibitors, Atazanavir and Nelfinavir (medicine used for HIV-infection) * Known kidney failure (creatinin level above reference value) * Known heart failure (NYHA III-IV) * Pregnancy * Women in fertile age with risk of pregnancy * Nursing women * operated under general anesthesia without use of spinal anesthesia * using opioids on a regular basis except users of codeine or tramadol, or known former abuse of opioids * Cognitive failure or other factors which make follow up impossible (for example language difficulties) * No cell phone or internet connection at home (making follow up difficult)