Teeth

C

Deciduous and Permanent Teeth

In humans, one set of 20 teeth is produced for use during early jaw development; these are called the deciduous teeth or, more popularly, milk or baby teeth. A second set of 32 larger permanent teeth replaces the deciduous teeth as the jaw matures. As a result of the growth and enlargement of the jaw, the roots of the deciduous teeth separate, allowing space for the larger permanent teeth to develop between them. The pressure of developing permanent teeth causes the tissues of the jaw to resorb, or suck up, the roots of the corresponding deciduous teeth, leaving only the crowns. As each permanent tooth erupts, it dislodges the crown of the deciduous tooth.

D

Types of Teeth

The crowns of the permanent teeth are of three general types: the incisors, the cuspids, and the molars. The front teeth, or incisors, are spade shaped to facilitate the cutting of food. In each quarter of the mouth is a central and lateral incisor. Behind the incisors are the three cuspid, or ripping, teeth. The first cuspid, or “canine”, situated directly at the back of the lateral incisor, has a single, pointed cusp. The two teeth at the back of the cuspid are called the bicuspids, as each has two cusps. Behind the biscuspids are the first, second, and third molars, which have a relatively flat chewing surface, permitting the grinding or milling of the food. In general, food is cut with the front incisal teeth, reduced in size by the cuspids and bicuspids, and ground to a digestible size by the molars. Human teeth are still evolving; dental scientists expect that third molars, or wisdom teeth, will eventually disappear as the human jawbone continues to shrink and as refined foods eliminate the need for extra molars.

E

Tooth Alignment

The sequence of tooth eruption in the upper and lower jaws occurs in an ordered pattern. Occasional irregularities in the sequence of eruption may cause faulty alignment of the teeth. In some instances, the deciduous tooth is not shed, or the succeeding permanent tooth may be lacking. In other cases the permanent tooth may be impacted in the bone and incapable of eruption; supernumerary, or extra, teeth may also be present. Misalignment, or malocclusion, of the teeth also may occur after eruption. Because the position of a tooth in the jaw is not static, the loss of a tooth may permit the adjoining teeth to tilt towards the empty space, and the corresponding tooth in the opposite jaw may continue to erupt into the space.

Such a shift in position is possible because the tooth is attached to the jaw by the short elastic fibres of the periodontal membrane. The teeth are subjected to a wide range of jaw movements, which are permitted by the ball-and-hinge joints in the human jaw. Normally, each tooth is afforded protection by the adjoining and opposite teeth, which serve to equalize the forces of jaw motion and prevent shifts in position. When severe malocclusion occurs, the teeth should be brought into position by a dental specialist known as an orthodontist (see Dentistry).

F

Tooth Decay

Teeth are extremely susceptible to a process of decay (dental caries). The ever-present oral acidogenic bacteria react with the carbohydrates in the mouth to form acids capable of dissolving the enamel. The breakdown of the enamel permits other bacteria to penetrate the dentine. Decay in a tooth eventually produces a cavity, or hole, in the structure. Extension of the decay causes infection of the tissue within the pulpal chamber, resulting eventually in necrosis or abscess formation, which, if unabated, may affect the jawbone. The decay process is accompanied by the formation of putrefactive gases. If the opening into the pulp is obstructed, severe pain ensues as the pressure of the gases increases. In many instances, the tooth can be treated by root-canal therapy, which removes the infected material from the root canal. In severe cases, the tooth is extracted.

Because teeth, unlike most other organs, are not self-reparative, early dental treatment is required to prevent serious complications. The removal of decayed material in the tooth and its replacement with an inert filling material restores the tooth. Fillings may be made of gold, silver amalgam, porcelain, synthetic cement, or plastic. Sometimes damaged or diseased teeth are capped, that is, given a new crown or a cover of appropriate material.

Proper oral hygiene and periodic dental examination help to prevent dental disease. A well-balanced diet with a minimum of carbohydrates may minimize dental infections. Brushing the teeth after meals to remove food residues helps to reduce decay. Teeth should be brushed in the direction of their growth to avoid gum irritation.

G

Recent Developments

Researchers demonstrated in 1949 that a 2 per cent solution of sodium fluoride applied directly on the surfaces of the teeth reduced dental decay by 40 per cent. Experiments indicated that the addition of 1 part of fluoride to 1 million parts of drinking water can reduce dental decay by as much as 65 per cent. Although strongly opposed by various groups, fluoridation has proved an effective deterrent to the development of caries in children's teeth since that time. False teeth are now commonly implanted.