Many people develop unicompartmental arthritis (i.e., arthritis that only affects one side of the joint). Usually the medial compartment (sie closest to the other knee) is damaged but some patients do experience lateral unicompartmental arthritis (side away from the other knee).
In either case, joint replacement is an option as you discovered. But, depending on certain factors, there are some other potential options. For example, unicompartmental arthroplasty can be done. In this procedure, just the side of the joint that is painful, damaged, and arthritic is replaced. Of course, this is a type of joint replacement and you may not want that.
Another possibility is a procedure called an osteotomy. In this operation, the surgeon removes a wedge- or pie-shaped piece of bone from one side of the tibia/i (lower leg bone). The purpose of the osteotomy is to correct the malalignment and take pressure off the medial compartment. There are two ways to do this surgery. Both remove bone from the upper tibia near the knee. The medical term for this type of osteotomy is high tibial osteotomy (HTO).
The first way to do the high tibial osteotomy is called a medial opening wedge tibial osteotomy. Bone is removed from the medial side of the tibia, shifting the weight off the medial compartment and more toward the midline. The two edges of remaining bone are held open with a metal plate or special device called a fixator.
The second method is a lateral closing wedge osteotomy. In this type of osteotomy, bone is taken from the lateral side of the tibia (side away from the other knee). The two edges of the bone are then allowed to shift closer together. The effect is the same as the opening wedge osteotomy: to take pressure off the damaged medial compartment.
Osteotomy is offered to patients who have malalignment of the joint contributing to the arthritic problem in the first place. Replacing the painful, arthritic joint without balancing the weight-bearing load will just result in the same problem later.