Why are teeth seen in humans

Structure and function of the teeth

Adults' teeth consist of 16 teeth each in the upper and lower jaw, making a total of 32 teeth. Four different types of teeth are distinguished based on their function and position in the dentition. At the very front there are four per jaw Incisorsthat hold onto and bite off the food. They have a chisel or shovel-like shape and have a sharp cutting edge. Usually the upper incisors protrude about 1–2 mm above the lower incisors and partially cover them. The incisors close on both sides Canines at. The canines are only vaguely reminiscent of the pronounced canines and tusks in the animal kingdom, but in humans they are particularly robust and more firmly rooted in the jaw than their neighboring teeth. This is followed by two on each side frontMolars (Premolars), which further crush the food, and three Molars (posterior molars, molars) that grind plant-based foods such as fruits or grains. While the front molars in the lower jaw have only one and the molars two roots, the corresponding teeth in the upper jaw sometimes have two, three or more roots.

The rearmost teeth (the third molars) are the so-called Wisdom teeth (Eighth), which often only break through in adulthood. They often have to be pulled when there is not enough space on the jaw and they threaten to shift the other teeth. Unfortunately, wisdom teeth in particular often have hook-shaped or overgrown roots, which often make pulling them a bloody and difficult procedure.

There are large differences in the shape, size, and color of teeth, but these are normal and not cause for concern.

Tooth formula and tooth surfaces

So that dentists, oral surgeons and everyone else involved in dental treatment always know exactly which tooth is meant, each tooth is assigned a unique two-digit number. After Tooth formula (Dentition formula) the upper and lower jaw are divided into two halves and numbered counterclockwise from top right (1) to bottom right (4). In the second place is the number of the tooth counted from the center. For example, the lower left canine tooth is called 33 or the wisdom tooth is referred to as an eighth because it is the eighth tooth of the respective half of the jaw.

The different sides and surfaces of a tooth also have technical names that indicate the exact position of holes or fillings on the tooth. A distinction is made between mesial (front, towards the middle), occlusal (on the chewing surface itself) and distal (rear) of the tooth surfaces. In incisors and canines that do not have a chewing surface, the incisal designates the cutting edge. The dentist calls the outside of the teeth (towards the cheek) vestibular, the inside (towards the tongue) lingual, and in the upper jaw also palatal (towards the palate).

If during a dental examination there is talk of a defect in "45 mod", then the second molar has a fairly large hole on the right half of the lower jaw - one that extends from the front over the chewing surface to the back (mesial-occlusal distal, abbreviated to mod).

Structure of the teeth

With healthy teeth, only a small part of the tooth (dens) is visible from the outside: Only the tooth crown (corona dentis) protrudes from the gums. Below the gums is the usually much longer tooth root (radix dentis), whereby teeth can have one, two or more roots depending on the shape of the tooth. The transition between the tooth crown and the root is called the tooth neck (Collum dentis), it is covered by the gums.

The visible part of the tooth is protected from the outside by the enamelum. Tooth enamel is the hardest material in the human body, it consists of 95% minerals such as calcium and phosphate. The enamel gives the teeth their characteristic milky-white shine. Many small crystals can be seen under the microscope - Hydroxyapatite, a very hard mineral that gives tooth enamel its hardness. Hydroxyapatite is extremely robust - only acids can harm it, which is why acid-forming bacteria are the key caries pathogens.

Once the teeth are developed, neither nerves nor blood vessels remain in the tooth enamel - so the body can no longer form new tooth enamel. This explains, on the one hand, why we do not feel any damage to the tooth enamel, and on the other hand, why holes in the tooth enamel have to be "repaired" with other materials such as fillers.

In the area of ​​the root, a thin layer of dental cement (cementum) protects the tooth. Enclosed in tooth enamel and cement, the dentin makes up the largest part of the tooth. Its structure resembles bone material and is therefore still quite hard, but significantly softer than the enamel. In turn, the dentin encloses the sensitive dental nerve (pulp, tooth pulp). The term dental nerve is anatomically imprecise as it actually refers to the cavity of the dental nerve (pulp cavity) through which the dental nerve and its branches run together with fine blood vessels - but in fact the dental nerve is the crucial part of the pulp cavity. Its extensions run through the dentin and reach the edge of the tooth enamel, so stimuli such as hot and cold, sweet or sour are passed on to the tooth nerve.

Dentin-forming cells, the odontoblasts, are located at the boundary between the pulp cavity and the dentin. They produce new dentine for life (tertiary dentine) and thus reduce the size of the cavity for the tooth pulp over the years. This automatically reduces the sensitivity of teeth to pain with age. Even when the tooth nerve is irritated (e.g. by penetrating caries), they produce dentin to keep the stimulus away from the nerve (secondary dentine).

Teeth-supporting apparatus

The teeth are not fused with the bone, but are anchored in the jawbone with the help of the tooth holding apparatus (tooth bed, periodontium). Both jaw bones have recesses for the tooth roots, depending on the number of roots there are one, two or more bony tooth compartments (alveoli) per tooth. Nerves and blood vessels lead in and out of the tooth through the tip of the root.

In healthy teeth, the periodontium (periodontium) surrounds the root area and acts as a buffer between the tooth root and the tooth socket. The teeth are elastically suspended in their tooth sockets with tooth retention fibers (Sharpey's fibers). When stressed by chewing or grinding teeth, these spiral-shaped fibers are stretched and then contract again. This mobility prevents the tooth from breaking when we strike it or bite something hard. In addition, this construction ensures that the forces acting on the tooth are passed on as a pull to the jaw and stimulate bone formation there. On the other hand, the jawbones react sensitively to pressure, e.g. B. Where teeth are missing, the pressure on the jaw can lead to bone loss.

The gums (gingiva) seal off the tooth holding apparatus from the oral cavity. The gum is a special part of the oral mucosa that surrounds the jaw and part of the tooth and is connected to it by the epithelial attachment. This connection closes the body surface at this point from bacteria and other microorganisms.

Jawbone and joint

The upper jaw (maxilla) and lower jaw (mandible) each hold 16 teeth that form a natural dental arch. The part of the jaw in which the teeth sit in their sockets is called the tooth process (alveolar process). The upper jaw is firmly fused with the other bones of the skull, the lower jaw, on the other hand, can move in almost all directions in the temporomandibular joint. This mobility enables many functions such as eating, drinking, speaking or facial expressions - without this mobility the teeth would be like a mill wheel that cannot turn.

The lower jaw is shaped like a horseshoe and has an ascending side part (branch) at the rear end on both sides. This side part ends in two processes towards the temporomandibular joint: in the front (towards the mouth) is the muscle process to which parts of the masticatory muscles attach, and further back the articular process with the articular head. The joint head slides back and forth in the temporomandibular joint socket, in which a movable cartilage disc (disc) sits. The temporomandibular joint is the most frequently moving joint in the human body.

Authors

Dr. med. dent. Gisbert Hennessen, Thilo Machotta, Dr. med. Arne Schäffler in: Gesundheit heute, edited by Dr. med. Arne Schäffler. Trias, Stuttgart, 3rd edition (2014). Revision and update: Dr. med. Sonja Kempinski | last changed on at 11:09