We divide the knee into three parts or compartments.
 

1. The inside of the knee  between the femur and tibia (the medial part of the femoro-tibial joint)

2. The outside of the knee between the femur and tibia (the lateral part of the femoro-tibial joint)

3. Between the knee cap (patella) and the femur (the patello-femoral joint)
 

Each one of these compartments can be replaced separately with a partial (or unicompartmental) replacement or all three can be replaced with a total knee replacement.