Double-blinded randomized control study investigating intrathecal morphine as an additive to spinal anesthesia in patients undergoing elective total knee arthroplasty. The investigators aim to include a total of 120 patients, randomized to two equal groups. Participants in the intervention group are administered intrathecal 0,2mg morphine and 12,5mg bupivacaine for spinal anaesthesia. Participants in the placebo group are administered 12.5mg bupivacaine and saline to match the volume of intervention goup. Primary end points are opioid consumption and possible adverse effects.

Inclusion Criteria:

• elective primary total knee arthroplasty
• ASA I-III
• written consent

Exclusion Criteria:

• rearthroplasty
• ASA IV-V
• inadequate spoken finnish for reliable pain assessment
• Dementia or otherwise impaired cognition
• contraindication for any medication or substance used in survey protocol
• weight <50kg or BMI ≥35 kg/m2
• preoperative SpO2 less than 93%
• clinical suspicion that subject can not use PCA adequately
• history of substance abuse or current excessive use of alcohol
• preoperative use of either pregabalin, gabapentin or strong opiates