Tooth whitening the human eye and brain. Teeth are composed of a surface enamel layer, which is whiter and semitransparent, and an underlying dentin layer, which is darker and less transparent. These are calcified, hard tissues comparable to bone. The natural shade of teeth is best considered as such; an off-white, bone-color rather than pure white. Public opinion of what is normal tooth shade tends to be distorted. Portrayals of cosmetically enhanced teeth are common in the media. In one report, the most common tooth shade in the general population ranged from A1 to A3 on the VITA classical A1-D4 shade guide.
Human tooth forces. As teeth flex under pressure, the arrangement of teeth touching each other, known as occlusion, causes tension on one side of the tooth and compression on the other side of the tooth. This is believed to cause V-shaped depressions on the side under tension and C-shaped depressions on the side under compression. When tooth destruction occurs at the roots of teeth, the process is referred to as internal resorption, when caused by cells within the pulp, or external resorption, when caused by cells in the periodontal ligament. Discoloration of teeth may result from bacteria stains, tobacco, tea, coffee, foods