Geometric constructions can be used to describe the mandibular dental arch form. These include the Bonwill circle with its tangents, the Pound line, and the centers of the ridges.
Because teeth develop according to the law of form and function, the rows of teeth are defined in terms of their form and position in relation to each other as a functional dentition. Therefore, the forms should be interpreted as indications of their particular functions; ie, the form is the material expression of function.
Eruption refers to when the developing tooth breaks through the epithelial covering of the
alveolar process in an occlusal direction. The development and the phase of eruption of the primary and permanent dentitions are linked to the growth in size of the skull and the jaws and hence to a person's general physical development. A distinction is made between the primary dentition and permanent dentition. The teeth of the permanent dentition comprise 20 successional and 12 accessional teeth; the latter originate from the same dental lamina as the 20 primary teeth and are the 12 molars of the permanent dentition.
The teeth of the primary dentition are known as deciduous (dentes decidui) or primary teeth as well as the mixed or temporary dentition, and they form the masticatory apparatus up to the sixth year of life. The primary dentition is made up of 20 teeth: eight incisors, four canines, and eight primary molars. The forms of these teeth match those of the permanent dentition, but there are no premolars (Fig 4-62).
The mandibular first molar (Fig 4-60) usually has a five-cusp crown with two lingual and three buccal prominences, which gives the tooth a long, rectangular outline. The roots are highly characteristic: there are only two roots, one anterior (mesial) and one posterior (distal). These roots are flattened and contain pronounced longitudinal depressions. The posterior root contains one canal; the anterior root contains two canals.
The molars (dentes molares) are ideal for the complete crushing and grinding of solid foods. The occlusal surfaces are capable of withstanding the greatest masticatory pressure. In the maxilla, the lingual cusps have an occlusion-fixing function and are referred to as crushing cusps. The buccal cusps have cutting functions and are referred to as shearing cusps. In the mandible, the crushing cusps are buccal and the shearing cusps are lingual.
The main difference between the mandibular and maxillary premolars is that the mandibular ones have an almost circular crown outline. Furthermore, they are always single-rooted. Unlike the maxillary premolars, the mandibular first and second premolars differ considerably from each other.
Premolars (dentes praemolaris: prae = in front of / before, mola = mill[stone]; dentes bicuspidati: bi = two, cuspis = eminence or cusp, bicuspid tooth).
Occlusal surfaces
Before the individual teeth are described in detail, features common to the posterior teeth are examined.
Dens caninus means dog's tooth (canis = dog); dens angularis from angulus = angle or corner; dens cuspidatus = single-cusp tooth from cuspis = cusp, protuberance.