The main reason to have knee replacement surgery is to ease pain and disability caused by arthritis or other joint problems, while preserving movement. Knee replacement may be either total or partial/unicompartmental. In total knee replacement (on the left), the entire knee joint is replaced. In partial, or unicompartmental knee replacement (on the right), the surgeon only replaces the damaged part of the knee.
Image courtesy of the American Academy of Orthopaedic Surgeons (AAOS).