An osteotomy is a controlled break of a bone and the bone is moved into a new position before being fixed to ensure the bone heals in the new position. Osteotomies are realignment procedures that are performed mostly for arthritis which affects one side of the joint only. It is the operation of choice for patients who are on the young side for a joint replacement or are of the correct age but are still working in a heavy job. Osteotomies are also used to correct deformities following injuries.
The most common osteotomies are for arthritis. If the arthritis is on the outer side of the knee and the deformity is a knock-knee one, then the osteotomy is performed in the end of the thigh bone. If the patient is bow-legged and the arthritis is on the inner side of the knee, the osteotomy is performed in the upper end of the shin bone. The correction can be achieved by removing a piece of bone or by opening the bone out and filling it with one graft. A third option is to make the break and slowly move the ends apart using an external fixator (frame). All three techniques have pros and cons which you need to discuss with your surgeon.
Another, less common, osteotomy that may be performed is a trochleoplasty where an abnormal bump in the top of the groove in which the patella runs is removed without losing the overlying cartilage.