Partial knee replacement, also known as unicompartmental knee arthroplasty (UKA), is a surgical procedure used to treat isolated areas of severe knee joint degeneration or damage. A native knee joint has three compartments: the medial compartment (inner side of the knee), the lateral compartment (outer side of the knee), and the patellofemoral compartment (behind the knee cap). If one of the three compartments is significantly worn out due to arthritis but the other two are very well preserved, partial knee replacement can be considered. Partial knee replacement involves replacing just one compartment out of the three. The components are made of alloys of cobalt and chromium for the femur and tibia, and a special plastic in the middle.