This is a quick overview of the main structures constituting the knee joint.
Bony articulation of the knee
Three long bones make up the bony articulation (joint) of the knee - femur, tibia and fibula - and their relationship to one another. A fourth bone, the patella or kneecap, is contained within the quads tendon and is not really part of the articulation. In some people the patella is mirrored at the back of the knee by the fabella - a tiny bony also contained within a tendon.
Note that the skinny fibula bone does not directly participate in the articulation - it does have some ligamentous attachments to important structures and contributes to stability, but is generally somewhat neglected in knee evaluation. However it is really helpful in confirming on an X-ray which is the outer (lateral) side of the knee because the fibula is always on the lateral side.
In this image, the knee is bent and you are looking at it from the front, so the inside of the knee joint is widely exposed to show the soft tissue structures within.
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.
The condyles and the flattened top of the tibia are covered in shiny white joint cartilage which lowers friction when the knee bends and straightens.
The main ligaments
The main ligaments stabilising the knee from the inside are the two cruciate ligaments. At the front of the intercondylar notch is the anterior cruciate ligament, and behind it lies the posterior cruciate ligament. The former prevents excessive forward movement of the tibia in relation to the femur. The latter prevents excessive backward movement of the tibia in relation to the femur.
At the sides of the joint, on the outside of the enclosing capsule, lie the two collateral ligaments. The one on the outer aspect of the knee is the lateral collateral (also called the fibular collateral) ligament, and on the inner aspect is the medial collateral ligament.
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.
Note that in this illustration we have not shown the upper relationships of the patella to the quads tendon and quadriceps muscle, which are covered in another section.