Proximal open-wedge tibia osteotomy is an accepted treatment option for younger patients with unilateral osteoarthritis of knee. 70 such operations are performed in the investigators Orthopaedic Divisions each year. The surgery involves cutting through the upper end of the tibia and opens the bone using specialized instruments. The tibia is lengthened on the medial side with the purpose of correcting the mechanical axis of the knee joint. The surgery is associated with many pains in the early postoperative phase. Until now, pain treatment has been controlled with either epidural block, or regional block via femoral catheter. Both treatments may involve risks such as loss of strength and sensibility disturbance, which often prolongs the early recovery and mobilization. Moreover, pain treatment administered via femoral catheter is often not sufficient as a part of the area behind the knee joint is not covered.