What's special about the kneecap.

Although it is a bone, the kneecap (patella) is actually contained right within the tendon of the quads muscle. This is very unusual in the body - to find a bone inside a tendon (a 'sesamoid' bone) - and what it means in reality is that any alteration of the direction of forces in the whole quads muscle/tendon/bone complex has to affect the patella.

The patella bone lies within the substance of the tendon of the quadriceps muscle and is in contact with the femur bone behind it.

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 yourself get of the knees when sitting down.