A partial knee replacement (or unicompartmental knee replacement) replaces one region (compartment) of the knee with a metal and plastic implant.
Tibiofemoral partial knee replacement, replacing one compartment of the joint.
Hemicap patellofemoral replacement from the front.
Hemicap patellofemoral replacement from the side
Knee replacement and the concept of compartments
Where the bones articulate in the knee is called a 'compartment' - the two articulations of tibia and femur are called the medial and lateral compartments, while the articulation of patella and femur are called the patellofemoral compartment. Replacing the joint of any of these compartments is called a 'partial' or 'uni-compartmental' knee replacement.
Unicompartmental knee arthroplasty. Rodríguez-Merchán EC and Gómez-Cardero P. EFORT Open Rev. 2018 Jun; 3(6): 363–373.