The area between the kneecap and the femur

i believe the answer would be a bursa

a bursa cushions the area between the kneecap and the femur.

inside this capsule are pads that cushion the joint. one of those pads is a bursa, a sac filled with a gelatinous liquid that prevent friction between bones in the joint. when the fluid becomes irritated or infected, it is known as bursitis, a condition that can make joint movement painful and stiff in the morning.