A door hinge is similar to a knuckle. It's joint can open and close at least 180 degrees. If it is bent the wrong way or twisted it can break.
A knee is similar to a knuckle. It is composed of the same organic material and is a type of joint. The knee bends and straightens at 180 degrees just like a knuckle does. The knee and knuckle can also be used to bear weight, but not too much or for very long. As a person gets older, both the knees and knuckles can develop arthritis and become inflamed.
