Total Knee Replacement The end of your thigh bone (femur) can be compared to a rocking chair. It has two distinct surfaces called condyles, which rest on the shin bone (tibia), forming two ‘compartments’, the inner (or medial) and the outer (or lateral compartment). A third compartment is in the front of the knee behind the knee cap (patello-femoral compartment). The bones in each compartment are covered by a thin layer of cartilage which prevents the bones from touching each other.