Kneecap fractures. Coloured X-ray of the kneecap (patella bone) of a 35 year old man, as viewed from above when the knee is bent, showing hairline fractures. The fractures are visible on the outer (upper) surface. The kneecap is a disc of bone that covers the front of the knee joint. The knee, the largest joint in the body, is formed where the thigh bone (femur, seen at bottom) articulates with the shin bone (not visible). Multiple hair- line fractures such as these may be due to a sport's injury.