Teeth: Form, formation, and function
In Latin, the word teeth is dentes (dens = tooth; dentis = of the tooth). Dental means relating to the tooth. In Greek, tooth is odous (odontes = teeth).
Teeth are hard structures in the oral cavity that, as modified parts of the dermoskeleton, form the dentition. Each tooth is comprised of a basal bony mass, a dentinal crown, and an inner pulp cavity. Food is held, incised, ground, and chewed by the teeth. Figure 1-13 presents comparisons of the forms and arrangements of teeth in different animals. Primitive rootless teeth are found in fish, amphibians, and reptiles and are the basic form of tooth. They can be slightly pointed, conical teeth, which can be angular or serrated (in sharks) and are shaped into so-called pavement teeth in fish and fangs in venomous snakes. These teeth of the same form (haplodont) vary only in size and thus create a dentition with just one shape of tooth, ie, a haplodont dentition.
The dentitions of mammals consist of teeth of different shapes (heterodont). The names of the different teeth are incisors, canines, premolars, and molars. Depending on the animal species, molars can take various different forms (see Fig 1-13d): bunodont (with cone-shaped cusps), hyp-sodont (with a high cylindric crown), brachyo-dont (with a shallow crown), lophodont (with a broad crown and yoke-like enamel crests), or selenodont (masticatory surfaces with crescentshaped enamel crests).
The teeth are anchored in the jaw by fibrous structures (acrodontia; in Greek, acros = highest) in sharks, bony fish, snakes, and some lizards. The teeth of amphibians and reptiles are firmly joined to the underlying bone mass on the inside of the jaw (pleurodontia; in Greek, pleura = side). In mammals and crocodiles, the root of a tooth lies in a cavity (alveolus dentalis) of the jawbone and is firmly and flexibly anchored to the bone with collagenous fibers and a connective tissue periodontium (thecodontia; in Greek, theke = container).
With a few exceptions, the teeth are shed several times or, in the case of mammals, once. This is known as exfoliation of the teeth. The rather smaller teeth of the first dentition are known as primary or deciduous teeth (dentes decidui). A distinction is made between vertical and horizontal exfoliation.
In vertical exfoliation, the old or erupted tooth is replaced by a new tooth from below (that is, from within the jawbone), with the root of the old tooth being resorbed. In horizontal exfoliation, the teeth located in the posterior region of the jaw gradually migrate anteriorly as the anterior teeth are worn out and shed (eg, in elephants, manatees, and kangaroos).
The size of the teeth is hereditary and differs between individuals. Size is inherited independently of the jaw size, which is also hereditary. This is why relatively small teeth can develop in a large jaw with gaps between the teeth, and relatively large teeth can develop in a small jaw with resultant crowding.
Following are the functions of human teeth:
- Cutting (biting off) food (by the incisors)
- Preparing food for swallowing (by teeth with grinding surfaces)
- Transmitting masticatory forces via the attachment apparatus to the jawbone
- Protecting the marginal periodontium during chewing
- Allowing unimpeded sliding contacts during mandibular movements
- Detecting foreign bodies in food and thereby performing a protective function (by proprioception)
- Allowing self-cleaning
- Enhancing esthetics
- Orienting the tongue when making sounds (phonation)
- Biomechanical transfer of forces to the other teeth