A total knee replacement or knee arthroplasty is a full or partial resurfacing of the closest bones to the knee joint. This is the end of the femur and top of the tibia (shin) bone. The surgery applies a metal shell to the femur, a metal plate to the shin bone, and a plastic “spacer” between these two surfaces. Occasionally they will replace the surface behind the knee cap, however, this will depend on the nature and severity of the diagnosis. This surgery is most common with advanced arthritis (Osteoarthritis/Rheumatoid), trauma, or reduced function with pain. Symptoms leading up to this may include pain, achiness, and joint stiffness.
As can be seen in the image, the surgery aims to replace the areas where the meniscus (the natural cushion between the bones) has worn down. Without this rubbery cushion, it is harder for the bones to glide smoothly through movement (such as bending or straightening) and also with weight-bearing.