Typical permanent teeth.
(See color plates.)
Covering the root of the tooth and holding it in place in its alveolus (socket) in the jaw is a fibrous connective tissue called the periodontium. Its many strong fibers are embedded in the cementum and also the wall of the tooth socket. The periodontium not only helps hold the tooth in place but also acts to cushion it against the pressure caused by biting and chewing.
There are 20 primary teeth, called also deciduous teeth, baby teeth, or milk teeth, which are eventually replaced by 32 permanent teeth, evenly divided between the upper and lower jaws.
Teeth have different shapes because they have different functions. The incisors, in the front of the mouth, are shaped like a cone with a sharp flattened end, and cut the food. There are eight deciduous and permanent incisors, four upper and four lower. The canines (or cuspids ) are at the corners of the mouth and are shaped like simple cones; they tear and shred food. There are four permanent canines; the two in the upper jaw are popularly known as the “eye teeth.” The premolars (or bicuspids ) are next behind the cuspids and consist of two cones, or cusps, fused together; they tear, crush, and grind the food. There are eight permanent premolars. The molars are in the back of the mouth; they have between three and five cusps each, and their function is to crush and grind food. There are 12 permanent molars in all, three on each side of both the upper and lower jaw. The hindmost molar in each of these groups, and the last one to emerge, is popularly known as the wisdom tooth.
Development and Eruption. Both the primary teeth and the permanent teeth begin to develop before birth. Because of this, it is vitally important that expectant mothers receive foods that will supply the calcium, phosphorus, and vitamins necessary for healthy teeth. The primary teeth begin to form about the sixth week of prenatal life, with calcification beginning about the sixteenth week. A considerable part of the crowns of these teeth is formed by the time the child is born. Eruption, or cutting of teeth, is slower in some children than others, but the primary teeth generally begin to appear when the infant is between 6 and 9
months of age, and the process is completed by the age of 2 to 2½ years.
When the child is about 6, the first permanent molar comes in just behind the second molar of the primary teeth. About the same time, shedding of primary teeth begins. The permanent teeth form in the jaw even before the primary ones have erupted, with the incisors and the canines beginning to calcify during the first 6 months of life. Calcification of the others takes place shortly after. As the adult teeth calcify, the roots of the primary ones gradually disappear, or resorb, and are completely gone by the time the permanent teeth are ready to appear. Occasionally a primary tooth root does not resorb, and as a result the permanent tooth comes in outside its proper position. When this happens, it is necessary to remove the primary tooth and root.
The first teeth to be shed, about the sixth year, are the central incisors. The permanent incisors erupt shortly afterward. The lateral incisors are lost and replaced during the seventh to ninth years, and the canines in the ninth to twelfth years. The first premolars generally appear between the ages of 10 and 12, the second molars between 11 and 13, and the third molars, or wisdom teeth, between 17 and 22. It is not uncommon for the third molars to fail to erupt.
Occasionally there is a partial or total lack of either the primary or the permanent teeth ( anodontia ). In some cases this is inherited, and in others it may be related to endocrine gland disturbances.
Tooth Decay and Its Prevention. Tooth decay, or dental caries. is the most common disease in the United States. It begins on the outside of the teeth in the enamel as bacteria and refined carbohydrates adhere to the tooth surface to form plaque. The action of the bacteria on starchy and sugary foods produces lactic acid. which is believed to dissolve the enamel. Once there is a break in the enamel ( demineralization ), the decaying process moves on into the dentin and then to the pulp, attacking the nerves and causing toothache. For methods of treatment and prevention, see dental caries .
accessional teeth the permanent molars, so called because they have no primary tooth predecessors in the dental arch. See also succedaneous teeth .