Your question is a good one. The three parts of a full knee joint implant include: (1) a polished metal shell that fits over the lower end of the femur (thigh bone), (2) a plastic and metal platform that covers the top of the tibia (lower leg bone), and (3) a plastic piece that goes on the back of the kneecap to help it glide smoothly when the knee bends and straightens.
It is possible to have only half of a joint replaced. This is called a hemiarthroplasty. Most of the hemiarthroplasties are done in the hip or shoulder. The knee is different and must be replaced completely. This is because of the ligaments that criss-cross inside the knee.
It is possible to have the femoral and tibial parts replaced without the patellar piece. If the back of the patella is smooth and tracks well up and down over the joint, then it can be left alone. The process of fitting a plastic piece onthe back of the kneecap is called patellar resurfacing.