Knee compartments
The knee is considered to have three compartments - the medial compartment, the lateral compartment and the patellofemoral compartment. Arthritis and other conditions may affect only one of the compartments.
The medial compartment is that part of the knee where the femur articulates with the tibia - but referring only to the inner or medial side.
The small fibula bone always identifies the lateral side of the knee. The red circle is the medial compartment.

The knee is considered to have three compartments - the medial compartment, the lateral compartment and the patellofemoral compartment. Arthritis and other conditions may affect only one of the compartments.