Unicompartmental knee replacement (UKR) was first tried in the 1970s. The early results weren't too good. Since then surgical tools and techniques have improved. Surgeons are using this approach more often now.
One problem that occurs is over correction of the joint. Some surgeons release a ligament on the medial (inside) edge of the knee when replacing the joint on that side. This transfers force to the other (outer) surface of the joint. The joint gets damaged and shows a loss of joint space.
In some cases there was already the start of arthritic damage to the side that wasn't replaced. Time and activity continue to take their toll on the remaining joint until the person develops painful symptoms.
This problem doesn't happen a lot but with the increasing number of UKRs being done, we may see the condition more often.