The main bones of the knee joint.
Why do anatomy illustrations often miss things out?
The anatomy of any body part is layered and complex. Illustrations generally simplify things by focusing on one aspect of the anatomy, such as the bones as in this illustration above.
One can see how the bones fit together, but the illustration may leave off the ligaments, tendons and muscles that make the bones functional. So you need to look separately at different illustrations of all the structures to fully understand the functional anatomy.
Doctors have the benefit of having dissected cadavers during their studies - which are, of course three dimensional.
The knee is really three 'joints'
The knee joint is really three separate joints or 'articulations' enclosed within a capsule.
There are two 'tibiofemoral' joints (contact surfaces of tibia and femur), and one 'patellofemoral' joint (contact surface of patella and femur).
The thinner fibula bone at the side of the tibia does not form part of the knee joint itself.
What keeps the knee bones together?
There are ligaments inside and outside the capsule that hold the knee in proper alignment
Amongst most important ligaments are the cruciate ligament (inside the capsule) and the collateral ligaments (outside the capsule).