Arthritis of the tibio-femoral joint (tibia and femur bones within the knee joint) of a knee frequently involves only the one aspect (most commonly the inner - or medial - aspect). This is called 'uni-compartmental' arthritis. It is often a consequence of the previous removal of the meniscus (shock absorber) on that side.
Unicompartmental arthritis is more commonly medial, with the legs bowing outwards (varus deformity). Less common is collapse on the lateral sidewhen the bowing is the other way causing knock-knees (valgus deformity).
Osteotomy attempts to transfer the forces of the body weight more onto the normal, non-arthritic side, by actually changing the angulation of either the tibia or the femur bone. This is done by cutting a wege into the bone and prising it open on the collapsed side ('opening wedge') osteotomy, or by cutting out a wedge and collapsing the gap on the other side ('closing wedge') osteotomy. Opening wedge is performed more frequently in the knee than closing wedge.
The same procedure may also be used for congenital angular deformity in children, where the deformity is not caused by an arthritic process.