OSTEOTOMY literally means “cutting of the bone.” In a knee osteotomy, either the tibia or femur is cut and reshaped to relieve pressure on the knee joint. Knee osteotomy is used when you have early-stage osteoarthritis that has damaged just one side of the knee joint. By shifting your weight off of the damaged side of the joint, an osteotomy can relieve pain and significantly improve function in your arthritic knee.

Knee osteotomy has the goals to transfer weight from the arthritic part of the knee to a healthier area, to correct knee axis malalignment and prolong the life span of the knee joint. High tibial osteotomy (HTO) has been established as an effective treatment of the varus osteoarthritic knee to decrease the stress on the load-bearing cartilage in the medial compartment. By preserving knee anatomy, a successful osteotomy may delay the need for a joint replacement for several years. Another advantage is that there are no restrictions on daily activities and sports participation after an osteotomy. 

Medial Opening wedge High tibial osteotomy for varus knee joint video link

https://www.youtube.com/watch?v=GE1O1lqv800