The kneecap, or patella, exists embedded within the tendon of the quads muscle that makes up the bulk of one's lap, and its excursion is contained within the groove of the underlying femur.
Side view of the knee bones - femur above, tibia below, and patella (kneecap)
The shiny cartilage undersurface of the kneecap pokes through the quads tendon to make contact with the cartilage of the femur bone, but its highest point is firmly attached to the quads tendon above it and its lowest point to the bit of the quads tendon below it ('the patellar tendon').
Because the patellar tendon is firmly tethered to the tibia bone, during knee bending and straightening the patella is pulled along by the movement of the tibia, changing its relationship to the femur bone, and pressed closely against it by tension in the quads muscles.
Illustration showing the relationship of the patella to the femur during bending and straightening of the knee
These sketches are from the side, but if you rotated the joint and looked at this junction from a different angle, you would appreciate that the femur has two rounded ends ('condyles') with a groove between them, and it is in this groove that femur and patella make contact. This is called the 'trochlear' groove.
The knee joint, pictured obliquely, showing the trochlear groove
As the joint bends, the patella makes contact in the deeper part of the groove between the rounded ends of the femur, and as the joint is straightened it makes contact in the shallower part of the groove above the rounded ends.
What is easy to appreciate is that a good deep groove and a congruent shape under the patella would help to maintain this relationship. Were the groove to be abnormal, or the underside of the patella, then stability might not be so good as the knee bends and straightens.
The knee from the front, when bent to 90 degrees
Now, let’s rotate the joint a little more, so that we are looking at it from the front, and remove the tendon bits and the tibia bone below - just focusing on the relationship between femur and patella at the top of the groove.
This first illustration demonstrates a healthy relationship. The walls of the groove are deep, with the lateral (or outer) wall a little higher, and the sides of the patella snug nicely into this groove.
However in some people - sometimes in their one knee and sometimes in both their knees - the groove is not normal. It may be shallow - even flat - or concave rather than convex, and in extreme cases the one side may look like a ‘bump’ while the other side is under-developed and resembles a ‘cliff edge’. In each of these, corresponding structural anomalies may also be evident on the undersurface of the kneecap.
Trochlear 'bump' and 'cliff'
The importance of such abnormalities is that the kneecap may be unstable, and may sublux (pop over to one side) or dislocate (come right out the groove).
The relationship of the kneecap - or patella - to the underlying bone is very important.
Every step that a person takes transmits forces through the kneecap. Anything affecting the normal function of the kneecap will knock on to affect gait, kneecap stability and kneecap comfort.
These following three videos (involving knee surgeon Lars Blønd) give more information about the symptoms that might occur when there are problems with the relationship of the kneecap to the femur - patellar instability, maltracking, subluxation (J-sign):
Patellar Instability - a webinar
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