The patellofemoral joint is the area of contact of the undersurface of the patella (kneecap) with the femur bone.
The patellofemoral joint is considered to be a 'compartment', because disease may affect just this one part of the knee articulation. The other two compartments are the two tibiofemoral compartments.
Clinical features of symptomatic patellofemoral joint osteoarthritis. Peat G, Duncan RC, Wood LRJ, Thomas E and Muller S. Arthritis Res Ther. 2012; 14(2): R63.