Unicompartmental Knee Replacement Information

The knee is the largest joint in the body and consists of the lower end of the thigh bone (femur), which rotates on the upper end of the shin bone (tibia), along with the knee cap (patella), which slides in a groove on the end of the femur. Large ligaments attach to the femur and tibia to provide stability. The long thigh muscles give the knee strength.

The joint surfaces where these three bones touch are covered with articular cartilage, a smooth substance that cushions the bones and enables them to move easily and smoothly.

All remaining surfaces of the knee are covered by a thin, smooth tissue liner called the synovial membrane. This membrane releases a specialised fluid that lubricates the knee and reduces friction to nearly zero.

In a healthy knee all of these components work in harmony. But disease or injury can disrupt this, resulting in pain, muscle weakness and reduced function.

In an arthritic knee the components no longer function at their optimum level which restricts movement and interferes with day to day activities.


In an arthritic knee the following is often the result:

  • The cartilage lining is thinner than normal or completely absent. The degree of cartilage damage and inflammation varies with the type and stage of arthritis
  • The capsule of the arthritic knee is swollen
  • The joint space is narrowed and irregular in outline; this can be seen in an X-ray image
  • Bone spurs or excessive bone can also build up around the edges of the joint


The combinations of these factors make the arthritic knee stiff and limit activities due to pain or fatigue.

A typical knee replacement replaces the ends of the femur (thigh bone) and tibia (shin bone) with a plastic prosthesis. The patella (knee cap) is usually replaced as well. In a Unicompartmental Knee Replacement only part of the knee joint is replaced. This is done through a smaller incision than would normally be used for a Total Knee Replacement. 

Unicompartmental Knee Replacement surgery is an option for patients with osteoarthritis that is limited to just one part of the knee.

The surgery is performed through a smaller incision than that used in a Total Knee Replacement which means the surgery is not as traumatic to the knee and allows for a quicker recovery time. 


The knee is divided into three major compartments: the medial compartment (the inside part of the knee), the lateral compartment (the outside part) and the patellofemoral compartment (the front of the knee between the kneecap and thighbone).

In a Unicompartmental Knee Replacement only the damaged compartment is replaced with metal and plastic. The remaining healthy cartilage and bone in the rest of the knee is left untouched.