Unless you are medical, it may not be easy to appreciate which way round a bone is in an illustration. So we have tried to make this easier.
Knee bones from front (bent knee)
The notch between the condyles is also revealed (intercondylar notch) as well as the cruciate ligaments which tether femur and tibia together. When the knee is straight, the condyles roll over and these structures are concealed.
This is the reason for the knee surgeon to do many of the surgical procedures with the knee bent. Normally the patella (kneecap) and its tendon is in the way, and the keyhole surgeon creates the portals for the instruments to the side of the patellar structures. For the purposes of this first illustration, the patella and its tendon have been removed to allow you to see into the knee.
Inside the intercondylar notch are the two cruciate ligaments, which appear to cross over one another. The one in the front is called the anterior cruciate ligament and the one behind is called the posterior cruciate ligament.
The two crescentic shock absorbers - the menisci - are situated between the femur and the tibia. They are seated on the flattened top of the tibia (tibial plateau). Although it is not shown here, they have ligamentous attachments to both bones via the capsule - the waterproof casing of the knee. The medial meniscus is the one most commonly injured because it is less mobile that the lateral meniscus, but injury to the lateral meniscus may be especially problematic as its anatomy is more complex.
The patella normally glides in the groove (trochlear groove) between the two condyles as the knee bends and straightens. It is tethered below to the tibia bone via its tendon (patellar tendon). Above, and not shown here, the patella forms part of the quads muscle group (quadriceps) and is attached to the muscle via the quadriceps tendon.
This very simplified way of illustrating the joint by stripping away details is merely to make the primary relationships clear, and more sophistication will be added to the drawings as you go through this anatomy section.
Knee bones from front (straight knee)
The knee bones at the joint look different depending upon whether the knee is straight or bent.
When the knee is straight the interior anatomy is concealed from view.
The rounded condyles of the femur bone roll over onto the rear of the flat surface of the tibia, so one cannot see into the notch between the condyles. The white cartilage (gristle) of the femur also largely disappears from view. The patella also obscures the entrance into the joint cavity.
To gain access to the joint with the knee straight knee the surgeon may cut alongside the patella and its tendons, and pull them over to the one side. The keyhole surgeon, in comparison, inserts his instruments at the side of these structures, and looks into the joint from behind the patella, but he has to bend the knee in order to properly reveal the internal anatomy.
You can see that the patella now lies in the shallow part of the groove in the femur. If the knee was bent, it would engage instead in the deeper part of the groove. So dislocation of the patella is more common when the patella is in the shallow part of the groove, or if the patella has developed in an abnormally high position.
The 'extensor mechanism'
When the quads (quadriceps) muscles contract the leg is straightened or 'extended'. This system of quads, quads tendon, patella and patellar tendon are together referred to as the extensor mechanism.
Identifying the structures on X-ray
The white arrow is pointing to the outline of the patella, which is visible as a whiter circle 2-3 cm above the joint line.
The joint space between the bones appears as a black gap - actually it is not a gap but is filled with the menisci, which are not visible under x-ray. Compare the X-ray and the illustration to appreciate this. The joint space on X-ray should be about the same on both sides - if there is a difference (as in this film where one side is more closed than the other) then the assumption is made that there is some destruction of the meniscus on that side.
You can also see a thin bone on the outer (lateral) aspect of the tibia. This is the fibula. It is always useful to identify the fibula on an X-ray or drawing when you are trying to work out if you are looking at a right knee or a left one.
Knee bones from the side
From the side one can appreciate the incongruity of the rounded lower end of the femur and the flattened upper end of the tibia.
The two menisci help to reduce this incongruity and absorb shock between the bones.
PF and TF joints
You can see also that there are several joints - bony points of contact - not just one. These joints are marked on the small X-ray.
The contact surface between femur and tibia is called the tibio-femoral joint. Because the femur has two rounded ends, there is a medial and a lateral tibio-femoral contact area.
The area of contact between the undersurface of the patella and the groove of the femur is called the patellofemoral joint.
The position of the patella relative to the groove of the femur changes as the knee is bent. In the straight knee the patella lies above the groove and can be freely wiggled from side to side. As the knee is bent the patella engages with the groove of the femur and it should not be possible to wiggle it from side to side.
When you look at the X-ray again, you will see that the patella bone seems to be 'floating' - but it is just that the tendons above and below it do not show on X-ray. Neither do the two shock-absorber menisci, which appear just as a dark gap on X-ray between the two bigger bones. [We also omitted from this little series the quads muscle - normally attached to the top of the patella - just to better illustrate our point].
Knee bones from above
To fully evaluate the kneecap and its relationship to the femur below it, one needs to look at the bones from above.
It is easy to relate to this view, as it is the view you get of your own knees when sitting down.
When the knee is bent, the undersurface of the patella (kneecap) lies snugly like this in the trochlear groove. The patella should be central in the groove, and not favouring one or other side, and it should not be tilted. The sides of the patella and the walls of the groove should be almost parallel. The apparent gap that one sees on X-rays from this angle (Merchant views) is not really a gap but the space is filled with the white joint cartilage that covers both of the bony surfaces where they are in contact.
Note that you cannot see the rounded condyles of the femur in this view, nor can you see the tibia, as this angle looks down on the 'lap' and the condyles they are on the under-surface of the femur, while the tibia is at right angles to this view.
This view is also one that the surgeon can explore during arthroscopy, where he can get a superb opportunity to see how the patella glides in the trochlear groove. Many surgeons, unfortunately, miss this opportunity because they do not put the camera into this 'suprapatellar' region, and thus they fail to fully appreciate the anatomy here. This is also the best view to appreciate something called a 'plica', and problems with a plica may also be missed.