When surgeons began doing knee arthroplasties, or replacements, the surgeries were done by replacing the whole knee. While quite successful, surgeons and researchers learned that replacing the whole knee wasn't always necessary since it wasn't always the whole knee that was damaged.
surgery for a partial knee replacement, also called a unicompartmental arthroplasty is not as invasive as a total replacement, and usually requires a smaller incision and shorter recovery time. Therefore, if it is possible to do a partial replacement, this would be preferable in many cases.
Usually, the most ideal patients for a partial replacement are those who are not obese, whose cartilage (tissue attaching muscle to bone) are intact, and who have damage limited to one area of the knee.