Bone is a unit of the skeleton that is largely composed of calcium. Any disease that alters the calcium balance in the body affects the bones in some way. Of interest in the oral scenario are the two main bones that make up the middle and the lower third of the face, and are responsible for the shape of the face and its characteristic features to a large extent. One is the upper jawbone called the maxilla and the other is the lower jaw called the mandible. Some diseases of the bones arise as a result of some problem in later life, and some diseases are developmental in nature.