"Bones appear white on an x ray as they stop the x ray beam. Anything that lets the beam through (air, cartilage, skin) shows up as black.
Our joints are covered with a layer of super-smooth cartilage that allows the joints to move without causing pain. This cartilage does not show up on x-ray so the bones should appear as though they are not touching.
Anywhere that sees the bones closer together than usual implies that the cartilage has been worn down - this is a sign of arthritis. If the bones appear to be touching, it signifies severe arthritis as all of the smooth covering has been worn away."