This image shows the three long bones of the knee - femur, tibia and fibula - and their relationship to one another.
The knee is bent and you are looking at it from the front, so the inside of the knee joint is widely exposed.
Note that the small fibula bone does not directly participate in the articulation (joint) - it does have some ligamentous attachments to important structures, and is generally somewhat neglected in knee evaluation.
From another perspective the fibula often really helpful in demonstrating which is the outer (lateral) side of the knee when you are looking at X-rays.
The condyles and the notch
The femur (thighbone) has two rounded knuckles at its lower end, and these are called 'condyles'. Between the condyles is a notch, within which are contained the cruciate ligaments, knee ligament stabilisers of the knee. Normally these would be concealed by the kneecap (patella) and its tendon, but we have removed these for the moment so that you can see the inside.
Sandwiched between the rounded condyles of the femur and the flattened top (plateau) of the tibia are the menisci (singular=meniscus) - shock absorbers that fill the space and further allow smooth movement of the joint.
In this more oblique illustration we have included the patella, and removed the cruciate ligaments, so that you can appreciate how the patella snugs within a groove in the femur bone. Again, the surface of the patella is covered in shiny white cartilage to lower friction between it and the femur. This contact area between the femur and the patella is known as the patellofemoral joint.
The patella and extensor mechanism
The patella is tethered below to the tibia via its patellar tendon, so the gliding within the groove is really caused by the knee bending and straightening.