The knee is divided into three compartments – the medial compartment (the inside portion of the knee), the lateral compartment (the outside portion of the knee), and the patello femoral compartment (the front of the knee between the kneecap and the thighbone). Patients with osteoarthritis that is limited to just one part of the knee may be candidates for a unicompartmental knee replacement. This type of surgery is not suitable for all patients and your physician will determine if this procedure is an option for you. With this surgery, only the affected portion of the knee is replaced with a prosthesis.