3. Human dentition

Chapter 3. Human dentition

tooth morphology and occlusion

Tooth morphology18
Dental notation 18
Differences between teeth of the deciduous and permanent dentitions 19
Incisors 19
Canines 21
Premolars 21
Molars 22
Pulp morphology24
The occlusion of the permanent teeth24
Centric occlusal position 24
Malocclusions 25
Self-assessment: questions 27
Self-assessment: answers 33
Overview

The human dentition, structurally and functionally, is characteristic of an omnivorous mammal. Indeed, the heterodonty includes all basic tooth forms in the dentition (i.e. incisors, canines and molars). The defnition is diphyodontic (having two generations or series of teeth — there being a primary or deciduous dentition that is succeeded by a secondary or permanent dentition). Occlusion of the dentition refers to the way in which the teeth bite together. Its main feature relates to the fact that, in normal (or anatomical) centric occlusion, a maxillary (upper) tooth occludes with its opposite tooth in the mandible plus the tooth located distally. Thus, the maxillary first (central) incisor occludes with the mandibular first (central) and second (lateral) incisors.
Learning objectives

You must:
• be able to identify precisely a tooth from either the permanent or the deciduous dentition (excepting the variable permanent third molar teeth)
• be able to describe the typical pulp morphologies for such teeth
• be able to describe the relationships of permanent teeth within the dental arches
• be able to describe the characteristics of normal (anatomical) centric occlusal position
• be able to categorize malocclusions in terms of Angle’s classification and the incisor relationship classification.

Tooth morphology

Humans have two generations of teeth: the deciduous (or primary) dentition and the permanent (or secondary) dentition. This is termed diphyodonty. In both dentitions, there are three basic tooth forms: incisiform, caniniform and molariform.
• Incisiform teeth (incisors, I in dental notation) are cutting teeth, with thin, blade-like crowns.
• Caniniform teeth (canines, C) are piercing/tearing teeth, having a single cone-shaped cusp on their crowns.
• Molariform teeth (molars, M, and premolars, PM) are grinding teeth, possessing more than onecusp on an otherwise flattened biting surface. Premolars are bicuspid teeth; they are unique to the permanent dentition and replace the deciduous molars.

Table 3.1 gives definitions of terms used for the descriptions of tooth form.

Table 3.1 Terms used for the description of tooth form
Term Definition
Crown Clinical crown — that portion of a tooth visible in the oral cavity
Anatomical crown — that portion of a tooth covered with enamel
Root Clinical root — that portion of a tooth lying within the alveolus
Anatomical root — that portion of a tooth covered by cementum
Cervical margin The junction of the anatomical crown and the anatomical root
Occlusal surface The biting surface of a posterior tooth (molar or premolar)
Cusp A pronounced elevation on the occlusal surface of a tooth
Incisal margin The cutting edge of anterior teeth, analogous to the occlusal surface of the posterior teeth
Tubercle A small elevation on the crown
Cingulum A bulbous convexity near the cervical region of a tooth
Ridge A linear elevation on the surface of a tooth
Marginal ridge A ridge at the mesial or distal edge of the occlusal surface of posterior teeth. Some anterior teeth have equivalent ridges
Fissure A long cleft between cusps or ridges
Fossa A rounded depression in a surface of a tooth
Buccal Towards or adjacent to the cheek. The term buccal surface is reserved for that surface of a premolar or molar which is positioned immediately adjacent to the cheek
Labial Towards or adjacent to the lips. The term labial surface is reserved for that surface of an incisor or canine which is positioned immediately adjacent to the lips
Palatal Towards or adjacent to the palate. The term palatal surface is reserved for that surface of a maxillary tooth which is positioned immediately adjacent to the palate
Lingual Towards or adjacent to the tongue. The term lingual surface is reserved for that surface of a mandibular tooth which lies immediately adjacent to the tongue
Mesial Towards the median. The mesial surface is that surface which faces towards the median line following the curve of the dental arch
Distal Away from the median. The distal surface is that surface which faces away from the median line following the curve of the dental arch

Dental notation

The types and numbers of teeth in any mammalian dentition can be expressed using dental formulae. The formula for the deciduous human dentition is DI 2/2 DC 1/1 DM 2/2 = 20, and for the permanent dentition I 2/2 C 1/1 PM 2/2 M 3/3 = 32, where the numbers following each letter refer to the number of teeth of each type in the upper and lower jaws on one side only. In both the permanent and deciduous dentitions, the incisors may be distinguished according to their relationship to the midline. Thus, the incisor nearest the midline is the first (or central) incisor and the more laterally positioned incisor the second (or lateral) incisor. The permanent premolars and the permanent and deciduous molars can also be distinguished according to their mesiodistal relationships. The molar most mesially positioned is designated the first molar, the one behind it being the second molar. In the permanent dentition, the tooth most distally positioned is the third molar. The mesial premolar is the first premolar, the premolar behind it being the second premolar.

Differences between teeth of the deciduous and permanent dentitions

The crowns of deciduous and permanent teeth are distinguished essentially by:
• size
• the greater constancy of shape of the deciduous teeth
• the crowns of deciduous teeth appearing bulbous and often with pronounced labial or buccal cingula
• the cervical margins of deciduous teeth being more sharply demarcated and pronounced
• the cusps of newly erupted deciduous teeth being more pointed
• the opacity, whiteness and thinner covering of the enamel of deciduous teeth
• the enamel of deciduous teeth being more permeable, softer and more easily worn
• the lack of neonatal lines in deciduous teeth.

The roots of deciduous and permanent teeth are distinguished essentially by:

• being shorter and less robust in deciduous teeth
• the fact that the roots of the deciduous incisors and canines are longer in proportion to the crown than those of their permanent counterparts
• the roots of the deciduous molars being more widely divergent.

The pulps of deciduous and permanent teeth are distinguished essentially by:

• the fact that the pulp chambers of deciduous teeth are proportionally larger in relation to the crown
• the pulp horns in deciduous teeth being more prominent
• the root canals of deciduous teeth being extremely fine.

Incisors

Maxillary first (central) permanent incisor

The maxillary first (central) permanent incisor is the widest mesiodistally of all the permanent incisors and canines, the crown being almost as wide as it is long. Like all incisors, it is basically wedge- or chisel-shaped and has a single conical root. From the incisal view, the crown and incisal margin are centrally positioned over the root of the tooth. The incisal margin may be grooved by two troughs, the labial lobe grooves, which correspond to the divisions between three developmental lobes (or mammelons) seen on newly erupted incisors. The mammelons are lost by attrition soon after eruption. From the labial view, the crown length can be seen to be almost as great as the root length. The convex labial surface may be marked by two faint grooves that run vertically towards the cervical margin and which are extensions of the labial lobe grooves. The mesial surface is straight and approximately at right angles to the incisal margin. The disto-incisal angle, however, is more rounded and the distal outline more convex. Viewed palatally, the crown has a slightly shovel-shaped appearance and is bordered by mesial and distal marginal ridges. Near the cervical margin lies a prominent cingulum. The sinuous cervical margin is concave towards the crown on the palatal and labial surfaces, and convex towards the crown on the mesial and distal surfaces, the curvature on the mesial surface being the most pronounced of any tooth in the dentition. The single root of the first incisor tapers towards the apex and is conical in cross-section.

Maxillary second (lateral) permanent incisor

The maxillary second (lateral) permanent incisor is one of the most variable teeth in the dentition, although generally it is morphologically a diminutive form of the maxillary first incisor. The crown is much narrower and shorter than that of the first incisor, though the crown:root length ratio is considerably decreased. From the incisal aspect, the crown has a more rounded outline than the adjacent first incisor. Viewed labially, the mesio-incisal and disto-incisal angles, and the mesial and distal crown margins are more rounded than those of the first incisor. From the palatal aspect, lying in front of the cingulum is a pit (foramen caecum) that may extend some way into the root. A common morphological variation is the so-called ‘peg-shaped’ lateral incisor. The course of the cervical margin and the shape of the root are similar to those of the first incisor.

Mandibular first (central) permanent incisor

The mandibular incisors have the smallest mesiodistal dimensions of any teeth in the permanent dentition. They can be distinguished from the maxillary incisors by:
• the marked lingual inclination of the crowns over the roots
• the mesiodistal compression of their roots
• the poor development of the marginal ridges and cingula.

The mandibular first (central) permanent incisor has a bilaterally symmetrical triangular shape. In the newly erupted tooth, three mammelons are usually present. The incisal margin is at right angles to a line bisecting the tooth labiolingually. The mesio-incisal and disto-incisal angles are sharp and the mesial and distal surfaces are approximately at right angles to the incisal margin. The profiles of the mesial and distal surfaces appear very similar, being convex in their incisal thirds and relatively flattened in the middle and cervical thirds. The lingual cingulum and mesial and distal marginal ridges appear less distinct than those of the maxillary incisors. The cervical margins on the labial and lingual surfaces show their maximum convexities midway between the mesial and distal borders of the root. The cervical margin on the distal surface is less curved than that on the mesial surface. The root is narrow and conical, though flattened mesiodistally, and is frequently grooved on the mesial and distal surfaces (the distal groove being more marked).

Mandibular second (lateral) permanent incisor

The mandibular second (lateral) permanent incisor closely resembles the mandibular first incisor. However, it is slightly wider mesiodistally and is more asymmetric in shape. The distal surface diverges at a greater angle from the long axis of the tooth, giving it a fan-shaped appearance, and the disto-incisal angle is more acute and rounded. Another distinguishing characteristic is the angulation of the incisal margin relative to the labiolingual axis of the root; in the first incisor, the incisal margin forms a right angle with the labiolingual axis, whereas that of the second incisor is ‘twisted’ distally in a lingual direction.

Maxillary first (central) deciduous incisor

The maxillary first (central) deciduous incisor is similar morphologically to the corresponding permanent tooth. However, because the width of the crown of the deciduous incisor nearly equals the length it appears plumper than its permanent successor. Unlike the permanent teeth, no mammelons are seen on the incisal margin. The labial surface is unmarked by grooves, lobes or depressions. The mesio-incisal angle is sharp and acute, while the disto-incisal angle is more rounded and obtuse. On the palatal surface, the cingulum is a very prominent bulge. Unlike those of its permanent successor, the marginal ridges are poorly defined and the concavity of the palatal surface is shallow. As with all deciduous teeth, the cervical margins are more pronounced, but less sinuous, than those of their permanent successors. The fully formed root is conical in shape, tapering apically to a rather blunt apex. Compared with the corresponding permanent tooth, the root is longer in proportion to the crown.

Maxillary second (lateral) deciduous incisor

The maxillary second (lateral) deciduous incisor is similar in shape to the maxillary first deciduous incisor, though smaller. One obvious difference is the more acute mesio-incisal angle and the more rounded disto-incisal angle. The palatal surface is more concave and the marginal ridges more pronounced. Viewed incisally, the crown appears almost circular (in contrast to the first incisor, which appears diamond-shaped). The palatal cingulum is generally lower than that of the first deciduous incisor.

Mandibular first (central) deciduous incisor

The mandibular first (central) deciduous incisor is morphologically similar to its permanent successor. However, it is much shorter, has a low labial cingulum, and the marginal ridges are poorly defined. The mesio-incisal and disto-incisal angles are sharp right angles and the incisal margin is straight in the horizontal plane. The single root is more rounded than that of the corresponding permanent tooth and, when complete, tapers and tends to incline distally.

Mandibular second (lateral) deciduous incisor

The mandibular second (lateral) deciduous incisor is a bulbous tooth that resembles its permanent successor. It is wider than the mandibular first deciduous incisor and is asymmetric. The mesio-incisal angle is more obtuse and rounder than that of the mandibular first deciduous incisor, and the incisal margin slopes downwards distally. Unlike the permanent tooth, the root is rounded.

Canines

Canines are the only teeth in the dentition with a single cusp. Morphologically, they can be considered transitional between incisors and premolars.

Maxillary permanent canine

The maxillary permanent canine is a stout tooth with a well-developed cingulum and the longest root of any tooth. Viewed from its incisal aspect, it appears asymmetric such that the distal portion of the crown is much wider than the mesial portion. Prominent longitudinal ridges pass from the cusp tip down both the labial and palatal surfaces. The incisal part of the crown occupies at least one-third of the crown height. From this view, the mesial arm of the incisal margin is shorter than the distal arm, and the disto-incisal angle is more rounded than the mesio-incisal angle. The profiles of the mesial and distal surfaces converge markedly towards the cervix of the tooth. The mesial profile is slightly convex; the distal profile is markedly convex. The mesial surface of the crown forms a straight line with the root; the distal surface meets the root at an obtuse angle. The palatal surface shows distinct mesial and distal marginal ridges and a well-defined cingulum. The longitudinal ridge from the tip of the cusp meets the cingulum and is separated from the marginal ridges on either side by distinct grooves or fossae. Viewed mesially or distally, the distinctive feature is the stout character of the crown and the great width of the cervical third of both the crown and root. The cervical margin of this tooth follows a course similar to that of the incisors but the curves are less pronounced. The root is the largest, and stoutest, in the dentition and is triangular in cross-section. The mesial and distal surfaces of the root are often grooved longitudinally.

Mandibular permanent canine

The mandibular permanent canine is similar to the maxillary canine, but smaller, more slender and more symmetrical. The cusp is generally less well developed. From the incisal aspect, there are no distinct longitudinal ridges from the tip of the cusp on to the labial and lingual surfaces. Viewed labially, the incisal margin occupies only one-fifth of the crown height and the cusp is less pointed. The crown is narrower mesiodistally than that of the maxillary canine so it appears longer, narrower and more slender. The mesial and distal profiles tend to be parallel or only slightly convergent towards the cervix. The labial and mesial surfaces are clearly defined, being inclined acutely to each other, whereas the labial surface merges gradually into the distal surface. The lingual surface is flatter than the corresponding palatal surface of the maxillary permanent canine, and the cingulum, marginal ridges and fossae are indistinct. The mesial and distal surfaces are longer than those of the maxillary canine. The cervical margin of this tooth follows a course similar to that of the incisors. The root is normally single, though occasionally it may bifurcate. In cross-section, the root is oval, being flattened mesially and distally. The root is grooved longitudinally on both its mesial and distal surfaces.

Maxillary deciduous canine

The maxillary deciduous canine has a fang-like appearance and is similar morphologically to its permanent successor, though more bulbous. It is generally symmetrical. Bulging of the tooth gives the crown a diamond-shaped appearance when viewed labially or palatally, with the crown margins overhanging the root profiles. The width of the crown is greater than its length. On the labial surface, there is a low cingulum cervically, from which runs a longitudinal ridge up to the tip of the cusp. A similar longitudinal ridge also runs on the palatal surface. This ridge extends from the cusp apex to the palatal cingulum and divides the palatal surface into two shallow pits. The marginal ridges on the palatal surface are low and indistinct. The root is long compared with the crown height and is triangular in cross-section.

Mandibular deciduous canine

The mandibular deciduous canine is more slender than the maxillary deciduous canine. The crown is asymmetrical and the cusp tip displaced mesially. Consequently, the mesial arm is shorter and more vertical than the distal arm. On the labial surface, there is a low cingulum. On the lingual surface, the cingulum and marginal ridges are less pronounced than the corresponding structures on the palatal surface of the maxillary deciduous canine. The longitudinal ridges on both the labial and lingual surfaces are poorly developed. The width of the crown is less than the length. The root is single and tends to be triangular in cross-section.

Premolars

Premolars are sometimes referred to as ‘bicuspids’, having a buccal and a palatal (or lingual) cusp.

Maxillary first premolar

The maxillary first premolar has an ovoid crown with distinct marginal ridges. The buccal and palatal cusps are separated by a central occlusal fissure running mesiodistally. The occlusal fissure crosses the mesial marginal ridge on to the mesial surface (the canine groove). Viewed buccally, the mesial slope of the buccal cusp is generally longer than the distal slope. Viewed palatally, the palatal cusp is lower than the buccal cusp and its tip lies more mesially. From the mesial aspect, the mesial surface is marked by a distinct concavity, the canine fossa. The cervical margin follows essentially a level course around the crown. There are usually two roots, a buccal and a palatal root.

Mandibular first premolar

The mandibular premolars differ from the maxillary premolars in that occlusally the crowns appear rounder and the cusps are of unequal size, the buccal cusp being the most prominent.
The mandibular first premolar has a dominant buccal cusp and a very small lingual cusp that appears not unlike a cingulum. The buccal and lingual cusps are connected by a ridge that divides the poorly developed mesiodistal occlusal fissure into mesial and distal fossae. The mesial fossa is generally smaller than the distal fossa. A canine groove often extends from the mesial fossa over the mesial marginal ridge on to the mesiolingual surface of the crown. The mandibular first premolar differs from other premolars in that the occlusal plane does not lie perpendicular to the long axis of the tooth but is included lingually. The cervical line follows an almost level course around the tooth. The root is single and is grooved longitudinally both mesially and distally, the mesial groove being more prominent.

Mandibular second premolar

The lingual cusp of the mandibular second premolar is better developed than that of the first premolar, although it is not as large as the buccal cusp. The occlusal outline is round or square. The mesiodistal occlusal fissure between the cusps is well defined. However, like the first premolar, the fissure ends in mesial and distal fossae, the distal fossa being the larger. Unlike the first premolar, a ridge does not usually join the apices of the cusps. The lingual cusp is usually subdivided into mesiolinguaI and distolingual cusps, the mesiolingual cusp being wider and higher than the distolingual. From the mesial and distal aspects, the occlusal surface appears horizontal to the long axis of the tooth, unlike the mandibular first premolar. The crown is wider buccolingually than that of the first premolar and the buccal cusp does not incline as far over the root. The mesial marginal ridge is higher than the distal marginal ridge. The cervical margin follows a level course around the tooth. The root is single and its apex may be curved distally.

Molars

Molars present the largest occlusal surfaces of all teeth. They have 3–5 major cusps (although the maxillary first deciduous molar has only two). Molars are the only teeth that have more than one buccal cusp. Generally, the lower molars have two roots while the upper have three. The permanent molars do not have deciduous predecessors.

Maxillary first permanent molar

The maxillary first permanent molar is usually the largest molar and the crown is rhombic in outline, the mesiopalatal and distobuccal angles being obtuse. It has four major cusps (mesiobuccal, mesiopalatal, distobuccal and distopalatal) separated by an irregular H-shaped occlusal fissure. The mesiopalatal cusp is the largest, the buccal cusps being smaller and of approximately equal size. An accessory cusplet of variable size is seen in 60% of first molars on the palatal surface of the mesiopalatal cusp (the tubercle of Carabelli). The distopalatal cusp is generally the smallest cusp of the tooth. A buccal groove extends from the occlusal table, passing between the cusps to end about halfway up the buccal surface. Viewed palatally, the disproportion in size between the mesiopalatal and distopalatal cusps is evident. A palatal groove extends from the occlusal surface, between the palatal cusps, to terminate approximately halfway up the palatal surface. The mesial marginal ridge is more prominent than the distal ridge and may have distinct tubercles. The cervical margin follows an even contour around the tooth. There are three roots, two buccal and one palatal, arising from a common root stalk. The palatal root is the longest and stoutest. The buccal roots are more slender and are flattened mesiodistally; the mesiobuccal root is usually the larger and wider of the two.

Maxillary second permanent molar

The maxillary second permanent molar closely resembles the maxillary first permanent molar but is reduced in size and has different cusp relationships. Viewed occlusally, the rhomboid form is more pronounced and the oblique ridge is smaller. The distopalatal cusp is considerably reduced. The occlusal fissure pattern is more variable and supplemental grooves are more numerous. A tubercle of Carabelli is not usually found on the mesiopalatal cusp and the tubercles on the mesial marginal ridge are less numerous or less pronounced. Like the first molar, the second molar has two buccal roots and one palatal. They are shorter and less divergent than those of the first molar and may be partly fused.

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Jan 5, 2015 | Posted by in General Dentistry | Comments Off on 3. Human dentition
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