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.