Knee bent to show the condyles
As the knee is bent, the two rounded ends (condyles) of the femur (thighbone) are exposed revealing the notch between them (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. For the purposes of this illustration, the patella and its tendon have been omitted to allow you to see into the knee.
The knee surgeon performs many of the standard surgical procedures with the knee bent, creating the portals along the side of the patellar tendon for the instruments to enter this cavity.
The patella and its underlying groove
The patella is a special bone because it is embedded in the tendon of the quadriceps muscle, and is restrained by the underlying groove of the femur bone (trochlear groove), along which it glides as the knee bends and straightens.
The lower part of the tendon is called the patellar tendon and is tethered below to the tibia bone. The upper part of the tendon is the quadriceps tendon and becomes the quadriceps muscle which is tethered to the femur.
As one rounds the femur from top to bottom, the shallow trochlear groove becomes the deep intercondylar notch, where one can see the two cruciate ligaments tethering femur to tibia, and the two menisci sandwiched between the bones as a shock absorber.
The knee is not a simple hinge - there are several bony points of contact. 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.
Identifying the bones 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.
Note that 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.
