4 Dental Anatomy and Occlusion
This section is a review of those basic courses in dental anatomy and occlusion important for a foundation in the study and practice of dentistry. The coverage of these subjects is not exhaustive but should reflect what fundamental information has already been covered early in the dental curriculum. Insofar as possible, the material is based on an evidence-based paradigm that is consistent with the highest level of external evidence, recognizing that the application of such evidence has to be based on appropriate clinical expertise as well as meeting the needs of individual patients. The test questions should be viewed as a learning experience. The basis for answers may be found as needed in the attached references.
Dental anatomy requires special terminology and nomenclature for communication, for learning, and for clinical and insurance record keeping in the dental office. The term dental anatomy, as used here, is an inclusive term for considering the morphological features of the human primary and permanent dentitions, including their pulp cavities and root canals; pertinent information about the development, calcification, and eruption/emergence of the teeth; and the clinical relevance of these odontological features to their function and esthetics.
There are tables in this section that provide information on the dimensions and chronologies of the teeth and, in context, referral to frequency variations in eruption sequences, number of roots and root canals, and variations in tooth size and form. It should be recognized that collections of samples that make up these data come largely from Euro-American populations, sometimes limited because of the problem of obtaining data from other populations, access to in utero material, and statistical methods of incorporating diverse data into a common table derived from various sources. For example, prevalence data on a major anatomical variant of the two-rooted mandibular (i.e., one with an additional distolingual and third root) suggest real differences between specific populations as to the prevalence of this variant of the mandibular first molar, which is of special interest for endodontic therapy. Differences in data on tooth eruption and tooth dimensions may not appear to be significant; however, differences in data found in various textbooks and journals need to be addressed. An example of such differences may be seen in the answers to some of the questions. Sexual, racial, and individual variations in dentofacial patterns reinforce the need to carefully consider interceptive extraction or space-regaining therapy for each patient because of the unpredictability of crowding behavior during the transition from mixed to permanent dentition. The effect of racial origin should be considered when using dental sclerosis as a means of age determination in forensic cases.
It is not unusual in the literature to find that more than one term is used to describe a particular dentition (e.g., primary or deciduous dentition, and permanent or succedaneous dentition, recognizing that the term deciduous can mean “not permanent, transitory,” and the term permanent does not indicate necessarily that all the teeth that succeed the primary dentition will not be lost because of disease or injury. In context, both terms for each dentition should present no problem; however, consistent use of a term is important and should reflect the requirements or traditions of a particular journal, dental association, or specialty.
The term occlusion can be defined simply as the contact relationship of the teeth in function and parafunction. The contacting interface between the teeth of opposing dental arches may be considered from the standpoint of a static, functional, or parafunctional morphological tooth contact relationship. It should be kept in mind that such relationships reflect a number of factors concerned with the development and stability of occlusion.
It also must be recognized that changes do occur in terminology; however, they occur in the literature over time, and the reader must be aware of some of the past as well as newer terms that may conceptually relate somewhat differentially to the same aspect of occlusion (e.g., working side contacts [older term] versus laterotrusive contacts [newer term]); therefore, the literature may exhibit several terms that reflect various conceptual views of occlusal relationships. For example, in the Glossary of Dental Terms for the National Board Dental Exam, centric relation is defined conceptually as a position of the mandible with the condyles in a specified location that is independent of tooth contact; however, also included is a synonym for centric relation—retruded contact position. The term retruded has related historically to a concept of centric relation and occlusal contact in which the condyles were thought to be in a most retruded position (Figure 4-1, A) compared to the position of the condyles that is now currently held (Figure 4-1, B).
Figure 4-1 A, Most retruded position of the condyles (bilaterally). B, Condyles (bilaterally) in most superior-anterior position, resting on the posterior slopes of the articular eminences with the discs properly interposed.
(From Ash MM, Ramfjord SP: Occlusion. Philadelphia: WB Saunders, 1995.)
The number and denomination of mammalian teeth are expressed in formulas that reflect the differences between human dentitions and nonhuman dentitions. The denomination of each tooth may be represented by the first letter in its name (I for Incisor, C for Canine, P for Premolar, and M for Molar). A notation expressing the number of such teeth in the upper and lower jaws follows these denominations. The formulae include one side only. The dental formula (10 teeth on one side) for the primary/deciduous dentition in humans is indicated as:
Similarly, a dental formula for the permanent/succedaneous human dentition reflects the addition of two maxillary and mandibular premolars, and the addition of one maxillary and one mandibular third molar. The dental formula (16 teeth on one side) for the human permanent dentition is indicated as:
The dental practitioner is expected to be able to differentiate the human dentition from the more common nonhuman dentitions. On occasion it is necessary to communicate with veterinarians and, in some instances, with attorneys dealing with bite wounds and the identification of human and animal remains in forensic matters. The formula (21 teeth on one side) for a dog (collie) is indicated as:
It is not unusual for dentists to be interested in some of the morphological traits used in anthropological studies (e.g., Carabelli’s trait, peg-shaped incisors, enamel extensions, and shoveling). Because of keyboard limitations, notations in anthropological tables are often limited to di1, di2, dc, dm1, and dm2 for the deciduous dentition, and to I1, I2, C, P1, P2, M1, M2, and M3 for the succedaneous dentition.
Terms of orientation in dental anatomy generally relate to indicate place, direction, and extent, including such terms as mesial, distal, facial, buccal, lingual, and anterior/posterior as shown in Figure 4-2. Abbreviations related to tooth orientation include: B, buccal; D, distal; DB, distobuccal; M, mesial; MB, mesiobuccal; L, lingual; DL, distolingual; ML, mesiolingual; MM, mesiomarginal; CR, cusp ridge. Abbreviations related to permanent tooth identification in tables include: CI, central incisor; LI, lateral incisor; C, canine; P1, P2, first and second premolars; M1, M2, M3, first, second, and third molars. All the teeth can be identified by one or more numbering systems. The universal system to be considered later is used in this text except where otherwise indicated.
A labiolingual section of a maxillary central incisor (Figure 4-3) shows several of the landmarks relative to the tooth and periodontium.
Anatomical landmarks include such terms as crown, root, apex, incisal edge, cervical line, pulp chamber, pulp horn, pulp (root) canal, fissure, cusp, apex, and bifurcation (furcation) as indicated in Figure 4-4, A, B.
The primary and permanent teeth are divided for discussion purposes into the crown and root, which is a division marked on the tooth surface by the cervical line. This line is the junction between the enamel covering the crown of the tooth and the cementum covering the root. It is referred to as the cementoenamel junction (CEJ). It may occur in several forms: (a) the enamel overlapping the cementum; (b) an end-to-end approximating junction; (c) the absence of connecting enamel and cementum so that the dentin is an external part of the surface of the root; and (d) an overlapping of the enamel by the cementum. These different junctions have clinical significance in the presence of disease (e.g., gingivitis, recession of the gingiva with exposure of the CEJ, depth of gingival crevice and level of attachment of the supporting periodontal fibers [Figure 4-3]; cervical sensitivity, caries, and erosion; and placement of margins of dental restorations).
Figure 4-5 A, Periodontal probe with 3-mm increments for measuring depth of gingival crevice or level of periodontal attachment. B, The distance from the free gingival margin (FGM) to the tip of probe at the level of attachment (LA) indicates the depth of the pathologically deepened crevice of 6 mm and a loss of attachment of 3+ mm relative to the cementoenamel junction (CEJ). C, Enamel projection into the bifurcation of a mandibular molar.
(From Ash MM, Nelson SJ: Wheeler’s Dental Anatomy, Physiology and Occlusion [8th edition]. Philadelphia: WB Saunders, 2003.)
It is necessary to know how morphological features relate to contact relations in functional jaw movements and in jaw closure (e.g., the occlusal contact relation of the mesial buccal cusp of the right mandibular first molar with the central fossa of the maxillary right first molar when the mandible is closed into the maximal intercuspal [clenching] position). These and other morphological features, as well as the positions of the teeth, may be related to the development of occlusal stability and oral motor behavior as the occlusion develops.
Practitioners need to be able to designate easily (using an acceptable tooth identification system) which tooth or teeth are being considered for diagnosis and treatment, and to be able to indicate the identity of a tooth or teeth in dental and insurance records. Thus, in the dental office the dentist, the hygienist, the dental assistant, and the front office assistant have to be knowledgeable about the tooth identification system used in their office and systems used elsewhere for purposes of referral and for reading pertinent professional literature.
In 1968 the American Dental Association recommended the universal numbering system; however, because it does not have universal usage, there are calls to change it. Even so, the notation of one letter for each tooth in the primary dentition and one number for each tooth in the permanent dentition has made its use favored in the United States. An overview of the primary dentition and its universal numbering is shown in Figure 4-7. A simplified scheme to show the letter notations for the 20 primary teeth in four quadrants follows:
As shown, the right maxillary central incisor is indicated with the letter E. Similarly, the left mandibular left central incisor is indicated with the letter O. There is no provision for supernumerary teeth. An overview of the permanent dentition and its universal numbering is shown in Figure 4-8. A simplified notation scheme for the permanent dentition follows:
A single number is used to represent each tooth in the permanent dentition (normally 32 teeth). For example, the right maxillary first molar is indicated in the universal system with the number 3 and the left mandibular first molar as 19. There is no provision for numbering supernumerary teeth, nor specific indications for missing teeth that have been replaced with restorative treatment. However, there have been suggestions for such additions.
Credit for a symbolic system for tooth numbering is given to two dentists and can be referred to as the Zsigmondy/Palmer notation system. However, in the United States the system is usually indicated as the Palmer system. In this symbolic system the arches are divided into four quadrants, as shown in the following depiction of the entire primary dentition. The Palmer notation for the primary dentition follows:
For a single tooth, such as the primary maxillary right central incisor, the designation is |. For the maxillary left central incisor, the notation is given as |. This numbering system presents difficulties when an appropriate font is not available for keyboard recording of these notations.
The Palmer notation for the permanent dentition divides the arches into four quadrants with eight or more teeth in each quadrant. With a complement of 32 teeth, the entire dentition would appear as follows:
The notation for the right permanent maxillary central incisor is |. The notation for the right permanent maxillary first molar would be |. The Palmer notation system is used frequently in the orthodontic literature.
The Fédération Dentaire Internationale (FDI) recommends a two-digit system for both the primary and permanent dentitions. This system has been adopted by the World Health Organization (WHO) and is accepted by other organizations and in research and public health journals. The FDI system of notation for the primary dentition follows:
Numeral 5 indicates the right maxillary quadrant; number 6, the left maxillary quadrant; number 7, the left mandibular quadrant; and number 8, the right mandibular quadrant. Teeth for each quadrant are numbered from 1 to 5, beginning with the central incisors. Hence, the right and left maxillary central incisors of the primary dentition would be numbered 51, and 61, respectively, and the left and right mandibular central incisors as 71 and 81, respectively. The FDI system for the permanent dentition follows:
In the permanent dentition, the first digit indicates the quadrant and the second digit the tooth in that quadrant. Quadrant indication numbers are: the right maxillary quadrant is 1, the left maxillary quadrant is 2, the left mandibular quadrant is 3, and the right mandibular quadrant is 4. The teeth in each quadrant are numbered from 1 to 8. Hence, the right maxillary central incisor is indicated by the double digit 11 (pronounced one-one, not eleven); the left maxillary central incisor as 21; the left mandibular central incisor as 31; and the right mandibular central incisor as 41. For several reasons it is important that practitioners know the FDI system, including its use in the international literature.
Knowledge of the development of the teeth and their emergence in the oral cavity is applicable to clinical practice as well as to other areas of interest such as anthropology, demography, forensics, and paleontology, which will be referred to only briefly here. Although the bibliography can be used to broaden the scope of reader interest, only those areas considered to be basic for the practice of dentistry will be reviewed here.
The interactive mechanisms of patterning, morphogenesis, and cytodifferentiation during organogenesis have been covered in Oral Embryology. Tooth development involves interactions between epithelium and mesenchyme, with the formation of a bud-like epithelial structure that becomes convoluted into a cap and bell stage. Subsequently, epithelial and mesenchymal cells such as dental papilla differentiate into enamel-secreting ameloblasts and dentin-secreting odontoblasts. These stages of early tooth development are well-defined histologically; however the formation of different shapes of teeth (morphogenesis) and their correct position in the jaws (patterning) have only recently demonstrated the importance of molecular genetics and signaling pathways in tooth morphogenesis.
These intercellular signaling networks are composed of proteins, including ligands, receptors, and transcription factors. The outcome of these intricate mechanisms during the development of the teeth generally leads to the right shape of teeth in the right place; however, morphological variability and dental malformations do occur, which may be of significant clinical importance.
The timing of chronological events in the development of the dentitions has been historically difficult to ascertain because of the lack of adequate documentation of the sources of information. Tables of dental chronologies reflect an abbreviated version of a long history of accumulated successive compilations and revisions of chronologies of the primary teeth, which is also true for the permanent dentition. It is recognized that such chronologies have some deficiencies in population sampling and collection methods, and that incorporating revised data based on making critical choices from available sources is not without methodological errors. Recent partial chronologies of dental development reflect the use of statistical methods that provide three different types of formation data: age of attainment chronologies based on tooth emergence, age of prediction chronologies based on being in a stage of development, and maturity assessment scales used to assess whether a subject of known age is ahead of or behind when compared with a reference population. These types of chronologies are used when more precise information about a particular aspect of dental development is needed for research and surgical procedures.
Dental age is usually based on the formation or emergence of the teeth, as well as simply counting the number of teeth, the presence of permanent teeth, and the amount of root resorption of the primary teeth. Thus, looking into the mouth and noting which teeth are present is a simple way to approximate the age of young children and adolescents. The dentition may be considered to be the best physiological indicator of chronological age in juveniles.
The term eruption has been defined historically as the emergence of the tooth into the oral cavity; however, it has recently been considered to mean continuous tooth movement from the dental tooth bud to occlusal contact. Thus, the term emergence is thought to be more specific for the emergence of the teeth through the alveolar gingiva.
The primary dentition (Figure 4-9) is considered to be clinically complete when the second primary molars are in occlusion at about the mean age of 29 months, keeping in mind that the completion of the roots of the canines occurs at about 3.25 years of age. The emergence of the permanent first molars signals the start of the mixed dentition period, which is considered to have been completed when all the primary teeth are lost and only the succeeding permanent (succedaneous) teeth are present.
Figure 4-9 Primary dentition in a 5-year-old child.
(From Ash MM, Ramjford SP: Occlusion. Philadelphia: WB Saunders, 1995.)
The chronology of the primary dentition is summarized in Table 4-1. The universal numbering system for the primary teeth is used, as well as general indications of primary incisors, canine, and molars i1, i2, C, m1, and m2. The dimensions for the primary dentition are given in Table 4-2.
A detailed coverage of the individual morphology of the primary teeth is beyond the scope of this review, and the reader is invited to consider the material in the bibliography. However, it is anticipated that the present coverage of those aspects of morphology for identification and function of the primary dentition will provide a reminder of the important details that have already been considered in the dental curriculum.
Schematic views of the primary dentition are shown in Figure 4-10. Illustrated are the labial and facial surfaces as well as the occlusal incisal views of the incisors and canines. Occlusal views of first and second primary molars are provided in Figure 4-11.
Figure 4-10 Schematic illustration of primary teeth. Primary right molars (buccal aspect): B, maxillary first molar; A, maxillary second molar; S, mandibular first molar; T, mandibular second molar. Primary right maxillary teeth, (facial/labial aspect); C, maxillary canine; D, maxillary lateral incisor; E, maxillary central incisor. Primary right mandibular teeth; P, mandibular central incisor; Q, mandibular lateral incisor; R, mandibular canine. Primary teeth between the anterior teeth depict the incisal aspect of the central incisors, lateral incisors, and canines. Primary left molars (mesial aspect); I, maxillary first molar; J, maxillary second molar; K, mandibular second molar; L, mandibular first molar.
Figure 4-11 Primary molars. Occlusal views. A, Maxillary first molar: CDG, BDG, DDG, DTF, central, buccal, and distal developmental grooves. B, Maxillary second molar: CP, central pit; CDG, BDG, LDG, DDG: central, buccal, lingual, and distal developmental grooves. C, Mandibular first molar: BDG, CDG, LDG: buccal, central, and lingual developmental grooves. D, Mandibular second molar: CP, central pit; CDG, LDG, MBDG, DBDG: central, lingual, mesiobuccal, and distobuccal developmental grooves.
The permanent dentition of 32 teeth is completed by 18 to 25 years of age, as indicated in Table 4-3. Such a table suggests the complexity of bringing together all the biological mechanisms at the right time and place to provide the appropriate relationships between tooth form and jaw movements, tooth form, and supporting structures of the teeth, and the alignment of the teeth and their contact relationships with adjacent teeth and opposing teeth in opposing arches, all in such a way as to stabilize the occlusion and protect the supporting structures of the teeth. The dimensions for the permanent dentition are provided in Table 4-4.
A schematic representation of the permanent dentition is shown in Figure 4-12. The teeth of each side of the arches are indicated with their universal system numbers.
The relationship between the roots of the maxillary teeth and the sinus is important in root canal therapy, oral surgery, and sinus lift procedures for implants. The first and second maxillary molars are generally of particular interest, especially with alveolar extension of the maxillary sinus, because of the possibility of perforating the sinus membrane during tooth removal and during placement of implants. Perforation with root canal instrumentation is also a risk to be evaluated closely.
The position of the mandibular molars, especially the third molar, is important because of the proximity of the roots and imbedded third molars to the inferior dental (mandibular) canal (IDC) and the inferior alveolar nerve (IAN). Radiographic indicators of risk of exposure of IAN after mandibular third molar extraction includes darkening of roots, interruption of the white lines of the canal, diversion of the canal, and narrowing of the tooth root.
The development of occlusion involves three time frames (i.e., time of emergence and contacting of the primary teeth, period of the mixed dentition with emergence and contacting of permanent teeth, and the time when the rest of the permanent teeth emerge and make occlusal contact). The factors that determine the size of the teeth and dimensions of the jaws and provide room for succedaneous teeth relate to the orderly transition from the primary dentition through the mixed dentition to the permanent dentition and its completion. For example, the chances for crowding in the permanent dentition based on the available spaces between the primary teeth (>6 mm) would be none; however, for 3 to 5 mm of spacing in the primary dentition, the chances of crowding are 1 in 5. If the primary teeth are crowded, there is a 1:1 chance of crowding of the permanent dentition. Among other factors, the availability of interdental spaces in the primary dentition is dependent on tooth size and dimension of the arches. The chance is small for crowding of the permanent dentition in the patient seen in Figure 4-13.
(From Ash MM, Nelson SJ: Wheeler’s Dental Anatomy, Physiology, and Occlusion [8th edition]. Philadelphia: WB Saunders, 2003.)
The primary teeth should be in normal alignment and occlusion should occur shortly after the age of 2 years, with all the roots fully formed by the time the child is 3 years old. With the growth of the jaws, the anterior teeth separate, beginning between 4 and 5 years of age. The primary occlusion is also supported and made more efficient by the emergence of the first permanent molars (sometimes referred to as 6-year molars) immediately distal to the primary second molars, as illustrated in the development of occlusion shown in Figure 4-14, A, B. The sequence of eruption (in months) is depicted in Figure 4-14, C. The completion of the primary occlusion and developing permanent dentition is shown in Figure 4-14, D.
Figure 4-14 Development of primary occlusion. A, Birth to mixed dentition. B, Mixed dentition with first permanent molar in position. C, Mean age of emergence (in months) primary teeth. D, Anterior-lateral view of mixed dentition with first permanent molars in position and developing permanent dentition (empty crypt is a preparation artifact).
A (From Schour L, Massler M: Studies in tooth development: the growth pattern of human teeth. Part II. J Am Dent Assoc 27:1918, 1940.) D (From Ash MM, Ramfjord SP: Occlusion. Philadelphia: WB Saunders, 1995.)
The development of the permanent occlusion (Figure 4-15, A) begins with the emergence and contacts of the first permanent molars at about 6 years of age and, except for the third molar, is concluded at about 15 to 18 years of age. The emergence of the mandibular incisors (Figure 4-15, B) follows the 6-year molars at 6 to 7 years of age. The most favorable sequence of eruption/emergence of the permanent dentition for a normal occlusion is shown in Figure 4-15, C. The third molars emerge and come into occlusal contact later, usually by 25 years of age (Figure 4-15, D).
Figure 4-15 Development of the permanent occlusion. A, Schematic representation of developing occlusion. B, Emergence of the mandibular central incisors. C, Sequence of eruption of the permanent teeth. D, Permanent dentition of 28 teeth at about 14 to 16 years of age.
(From Schour L, Massler M: The development of the human dentition. J Am Dent Assoc 28:1153, 1941.)
When the teeth are in ideal alignment so that proximal contacts and marginal ridges are in proper position (Figure 4-16), there is less of a chance of impaction of food into the interproximal areas, resulting in loss of periodontal attachment.
Room for the development, eruption, and emergence of the permanent teeth during the mixed dentition period is influenced by the forward rotation of the maxillomandibular complex. An important part in the development of the occlusion of the permanent dentition is the premolar segment where the erupting premolars are significantly smaller in the mesiodistal diameter than the primary molars, which they replace (i.e., the mesiodistal diameters of the mandibular primary molars are greater than the mesiodistal diameter of the replacing premolars). This gain in space between the primary and permanent dentition in the dental arch is referred to as the leeway space. It has importance for the alignment of the mandibular incisors and for mandibular molar movement to correct for the end-to-end molar relationship in the mixed dentition period into a normal molar relationship in the permanent dentition (e.g., mesiobuccal cusp of the maxillary first molar occluding in the mesiobuccal developmental groove of the mandibular first molar, as shown in Figure 4-17, A, which is an angle class I molar relationship).
The arch form and width of the primary dentition is established generally for both the primary and permanent dentitions by the age of 9 months. What does change is the increase in anterior-posterior dimensions of the jaws, which is necessary for the incorporation of the molars into the occlusion. The supporting alveolar bone and basal bone determine the shape of the dental arches.
There is a general relationship between the size of the teeth and the size of the dental arches; however, when a discrepancy is evident between the aggregate mesiodistal diameters of the crowns of the teeth and the size of the bony supporting arches, crowding or protrusion can occur. Arch width and perimeter dimensions can relate to differences between crowded and uncrowded dentitions.
In cases of restorative dentistry, the dimensions of the replacement teeth must be related to the size of the existing teeth and arch dimensions. For example, for a maxillary arch length of 128 mm and a mandibular arch length of 126 mm, the sum of the mesiodistal diameter of all the mandibular teeth would have to be 126 mm, and the sum of the mesiodistal diameter of all the maxillary teeth would have to be 128 mm. Arch dimensions and tooth size vary considerably, and there is no template for any patient. The aesthetics of tooth form, size, and color are important considerations in restoring teeth.
The restorative dentist must preserve comfort and health throughout the functional (chewing) and parafunctional range of the patient’s mandibular movements. Concepts of occlusion that incorporate principles of mandibular movement require a basic understanding of occlusal relations and potential influences upon dental anatomy as a foundation. This section will introduce occlusal contact relations that occur in maximum intercuspation of the teeth (centric positions) and the basic relations associated with mandibular movement (eccentric positions).
In the generally accepted definition of normal occlusion, each mandibular tooth is positioned lingual to the maxillary counterpart. It may also be helpful to note that the mandibular teeth are positioned about one half of a tooth anterior to the maxillary counterpart, excluding the central incisors (Figure 4-18). In this position, occlusal contacts may follow two primary forms. (Figure 4-19). It should be noted that the following descriptions relate to so-called idealized contact relations—a concept that can be used as a basis for discussion, as well as for developing occlusal contact relations in restorative dentistry and orthodontics. Individuals may often show variations from the patterns as presented without having occlusal dysfunction.
Again, from Figure 4-19, A, the contact relations occurring with the maxillary lingual cusps (maxillary supporting cusps) follow a similar embrasure contact pattern, with the exception of the following two cusps: