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.