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.

knee bones from the front

knee when standing

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.

You can see also how the patella is an integral part of the muscle of the quads tendon (in this top example we are only showing the central part of the quads muscle - for simplicity).

The tendon above the patella is called the quads tendon and below the patella it is called the patellar tendon (sometimes called the patellar ligament).


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.


extensor mechanism of the knee

You can see from this next illustration that the four heads of the quads muscle are intimately involved with a network of fibrous tissue on the sides of the patella (retinaculum) and the whole works pretty much as a unit.  If any part of this extensor mechanism breaks, for example -

  • fractured patella
  • rupture of quads muscle or tendon
  • rupture of patellar tendon
  • avulsion or 'pulling off' of the quads tendon or patellar tendon from their attachment to the patella
  • fracture of tibial tubercle (where the patellar tendon attaches to the tibia)
  • avulsion or 'pulling off' of the patellar tendon from its attachment to the tibia

then the knee will not be able to fully actively extend.


X-ray of front of knee

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.