The definition of the term occlusion in many dictionaries only states that it is the act of closure or being closed, but some dental dictionaries (e.g., Zwemer1) go well beyond this simple definition to include a static, morphological tooth contact relationship. It may be defined also as the contact relationship of the teeth in function or parafunction.2 However, the term refers not only to contact at an occlusal interface, but also to all those factors concerned with the development and stability of the masticatory system and with the use of the teeth in oral motor behavior. Because brief definitions of occlusion are too limited to be useful as a basis for dental practice, more complete explanations evolve into concepts or paradigms of occlusion that reflect prevailing interests and clinical convenience. Thus a modern concept of occlusion must include the idea of an integrated system of functional units involving teeth, joints, and muscles of the head and neck. The solutions to problems such as bruxism, orthodontic relapse, denture instability, and peri-odontal trauma require concepts of occlusion that extend well beyond static arrangements of teeth, occlusal contacts, and jaw position.
Concepts of Occlusion
Concepts of occlusion vary with almost every specialty of dentistry. Common to some are definitions based on a static view of the dentition in which descriptions of the occlusion emphasize the fit of particular parts of individual maxillary teeth with specified parts of mandibular teeth. Until recently, only a few concepts of occlusion have included functional criteria, and because the dentofacial complex is highly mobile, ideas of occlusal stability and homeostasis are often misunderstood and seldom mentioned as a part of a concept of occlusion.
In the past, ideas regarding occlusion were often based on complete dentures. Because of the problems of instability of denture bases, the concept of "balanced occlusion" was developed to consider bilateral contacts in all functional excursions to prevent tipping of the denture bases. Although some clinicians have advocated such concepts in the past for use with regard to the natural dentition, acceptance was limited and unsupported by research evidence. Even so, some of the concepts related to condylar guidance, cusp height.These concepts stress to varying degrees static and/or functional characteristics of an occlusion as being theoretical practical goals for diagnosis and treatment of the occlusion. Some of the ideas have been developed principally in relation to orthodontics or complete dentures, and others for full mouth rehabilitation. None are completely applicable to the natural dentition; some provide for specific occlusal contact relations and joint positions, and few concepts of occlusion consider in principle, or in practical ways, related muscle and oromotor functions. The idea of a functional rather than simply a static relationship of occlusal surfaces has become increasingly important because of the recognition that functional disturbances of the masticatory system can be related to malocclusion, occlusal dysfunction, and disturbances of oral motor behavior, including bruxism. Thus, for example, the occlusal contact relationships shown in Figure 16-1 reflect oromotor behavior consistent with bruxism (grinding of the teeth) that requires preventive occlusal therapy to control its adverse effects (e.g., an occlusal bite plane stabilization-type splint) (Figure 16-2). The restoration of lost tooth structure due to aggressive bruxism, as for other losses from other causes, requires knowledge of dental morphology, physiology, restorative materials, occlusion, esthetics, and personal habits (e.g., pipe smoking).
FIGURE 16-1 Effects of bruxism in a young patient.
Figure 16-2 Stabilization occlusal bite plane splint to prevent the effects of bruxism.
Development of the Dentitions
Functional disturbances of the masticatory system may have their beginning during the development of occlusion, a time when the substrate for tongue and swallowing habits, chewing patterns, teeth clenching, and bruxism may be established. It is also the time when occlusal discontinuity, occlusal interferences to function, and occlusal instability often develop. The development of malocclusion is a reflection of disturbances in the normal processes of occlusal development. Therefore some knowledge of the process of occlusal development is necessary for the practice of dentistry.
Primary Dentition
Any consideration of the development of the occlusions should begin with the primary dentition. It is during this period in the development of the oral-facial complex that oral motor behavior reflects learning related to the advent of the teeth. Human oral functions that are acquired or modified during the natural progression from birth through infancy to adulthood are in part related to the development of occlusion, both of the deciduous and permanent teeth (i.e., occlusion defined in its broadest sense).
Perhaps many of the reflex mechanisms of the oral-facial area and sensory and higher-center influences are important for the acquisition of masticatory skills, just one of the many motor behaviors that come under the phrase oral motor function. Thus of particular importance is the novel sensory apparatus of the teeth that makes their appearance with the primary/deciduous teeth at an important time in the maturation of the nervous system and its interface with the environment.
The development of the muscle matrix and the active growth of the facial skeleton occur at a very strategic time for the maturation of the nervous system and the development of oral motor functions involving the teeth and chewing. It is also at this time that jaw positions and posturing of the mandible in relationship to the teeth takes place.
OVERVIEW OF THE PRIMARY OCCLUSION
The primary teeth are arranged in the jaws in the form of two arches: a maxillary and a mandibular. An outline following the labial and buccal surfaces of the maxillary teeth describes the segment of an ellipse and is larger than the segment following the same surfaces on the mandibular teeth (see Figure 1-2, A).
The relation between the maxillary and mandibular primary teeth when in occlusion is such that each tooth, with the exception of the mandibular central incisor and the maxillary second molar, occludes with two teeth of the opposing jaw. The primary teeth should be in normal alignment and occlusion shortly after the age of 2, with all the roots fully formed by the time the child is 3 years old. A year or so after the teeth have fully erupted and have assumed their respective positions in the arches, the rapid development of the jaws is sufficient to create an interdental space, or diastema, between some of them.
The anterior teeth separate and usually show greater separation as time goes on by a process that is caused by the growth of the jaws and the approach of the permanent teeth from the lingual side. This separation usually begins between the ages of 4 and 5 years. The canines and molars are supposed to keep their positive contact relation during all the jaw growth. However, some shifting and separation are seen quite often. Because the teeth do not hold their relative positions for long, they are worn off rapidly on incisal ridges and occlusal surfaces. As an example, when a primary canine is lost 8 years or more after its eruption, its long, sharp cusp has in most instances been worn down. If the primary teeth are in good alignment, the occlusion is most efficient during the time that these teeth are in their original positions. This situation exists for only a relatively short time. After normal jaw growth has resulted in considerable separation, the occlusion is supported and made more efficient by the eruption and occlusion of the first permanent molars immediately distal to the primary second molars. The child is now approximately 6 years of age and will use some of the primary teeth for 6 more years.
ERUPTION CHRONOLOGY OF THE PRIMARY TEETH
The timing of the eruption or emergence of the teeth is due in large part to heredity and only somewhat to environmental factors. According to Falkner12 the development and eruption of the primary dentition are quite independent of the development and maturation of the child as a whole. The significance of local environmental factors for the development of occlusion considered in its broadest sense is relatively unknown. Thus learning of mastication may be highly dependent on the stage of development of the occlusion (type and number of teeth present), maturation of the neuromuscular system, and factors such as diet.
Table 16-1 Eruption of Primary Teeth*
 |
Tooth |
Emergence of Teeth (Months) |
|
|
il |
E, F |
10 (8-12) |
|
|
i 2 |
D, G |
11 (9-13) |
|
|
c |
C, H |
19 (16-22) |
|
|
ml |
B, I |
16 (13-19)$ (14-18)2 |
|
|
m2 |
A, T |
29 (25-33) |
|
 |
|||
 |
il |
P, O |
8 (6-10) |
|
i2 |
Q, N |
13 (10-16) |
|
|
c |
R, M |
20 (17-23) |
|
|
ml |
S, L |
16 (14-18) |
|
|
m2 |
T, K |
27 (23-31)$ (24-30)2 |
|
il, Central incisor; i2, lateral incisor; C, canine; ml, first molar; m2, second molar. ^Universal numbering system for primary/deciduous dentition.
MEAN AGE OF ERUPTION
The mean age of eruption of the primary teeth as shown in Table 16-1 is demonstrated schematically in Figure 16-3, although one should keep in mind the range of values seen in Table 16-1 and the idea that a 6-month acceleration or delay is considered normal. Even so, on average the sequence generally follows that shown in Figure 16-3.
CONTACT RELATIONS
The contact relations of the teeth tend to vary with the degree of bruxism present in the child. A number of factors appear to be related to the development of contact relations at the time of eruption of the teeth, including the position of the tooth germ, presence of permanent teeth, development of the condyles, cuspal inclines, and neuromuscular influences. Generally, little attention has been paid to the cusp-fossa and temporomandibular joint (TMJ) relationships, and usually only tooth-to-tooth observations have been made, namely, the mesiolingual cusp of the maxillary molar occludes in the central fossa of the deciduous mandibular molar. Jaw-to-jaw relations with regard to the position of the condyle have received little attention. Perhaps discrepancies between condylar position and tooth position have little meaning in a relatively plastic, rapidly growing and changing system, including the nervous system.
PRIMARY ARCH FORM
The arch form and width for both the primary and permanent dentitions has been largely established by the age of 9 months.13 This observation may seem untrue in view of the apparent difference in facial appearance between the child of that age and a young adult with a permanent dentition. What does change substantially, of course, is the anteropos-terior dimension of the jaws, an increase in which is necessary for the incorporation of the permanent molars into the occlusion. It must be kept in mind that the alveolar bone and basal bone determine the shape of the dental arches. Arch form will be considered in more detail in the section on the permanent dentition.
Figure 16-3 Mean age (in months) of emergence of the primary/ deciduous dentition.
INTERDENTAL SPACING
The position of the deciduous teeth in the arches generally shows some degree of interdental spacing, which tends to decrease slightly with age. The size of the primary teeth and the spacing between them has a relationship to the position of the permanent teeth and the size of the dental arches (e.g., sufficient interdental space is needed for the permanent teeth to erupt into an uncrowded position). One of the indicators of future sufficiency or insufficiency of space in the dental arches for the permanent teeth is the presence or absence of spacing between the teeth of the primary denti-tion14,15 (i.e., spacing between the primary teeth [Figure 164] is necessary for the proper alignment of the permanent dentition).
The probability of crowding of the permanent dentition based on the amount of interdental spacing of the primary teeth is given in Table 16-2.
PRIMARY MOLAR RELATIONSHIPS
Primary molar relationships have been described by Moyers as the flush terminal plane, mesial step, and distal step (Figure 16-5, A, B, and C).14
Figure 16-4 Primary dentition in a child 5 years of age with adequate (greater than 6 mm) interdental spacing in both arches.
Table 16-2 Probability of Crowding of Permanent Teeth Based on Available Spaces Between Primary Teeth
|
Primary Teeth |
Chances of Crowding |
|
>6 mm |
None |
|
3-5 mm |
1 in 5 |
|
>3 mm |
1 in 2 |
|
No spacing |
2 in 3 |
|
Crowded |
1 in 1 |
Data from Leighton BC: The early signs of malocclusion, Trans Eur Orthod Soc 45:353, 1969; and Leighton BC: Early recognition of normal occlusion. In McNamara JA Jr, editor: Craniofacial growth seríes: the biology of occlusal development, Monograph 7, Ann Arbor, 1977, Center for Human Growth and Development, University of Michigan.
Of particular interest to the orthodontist is the fact that the mandibular second primary molar has a greater mesiodistal diameter than the maxillary second molar. As a result of this difference in dimensions of the two teeth, the distal surfaces of these two molars are in the same plane; a flush terminal plane is located at the end of the deciduous dentition. It has been reported that if a "step" (deviation of the flush terminal plane) occurs because of carious lesions or other disturbances, a tendency to interfere with the development of normal occlusal relations of the permanent first molars is evident. Also, it has been shown that the natural wearing away of the cusps in the deciduous dentition allows the mandible to assume a more forward position during a period when the mandible is growing more rapidly than the maxilla. In the absence of cuspal interferences, there is some evidence that the permanent incisors erupt with less vertical overlap and the permanent molars erupt into a more favorable occlusion. Several orthodontic techniques have been directed toward the functional protraction of the mandible during growth in patients with anteroposterior jaw discrepancies.
EFFECTS OF TERMINAL PLANE RELATIONSHIPS
The effect of these deciduous molar relationships (see Figure 16-5) on the development of relationships of the permanent molars (Figure 16-6) is influenced to some degree by the presence or absence of several factors: differential growth of the jaws, forward growth of the mandible, and sufficient leeway space to accommodate a mesial shift of the permanent molars.16
Figure 16-5 Terminal plane relationship in the primary dentition.
Figure 16-6 A, Angle Class I molar relationship. B, Class II molar relationship. C, Class III molar relationship.
Morphological growth studies on the growth process of a primary dentition having a distal step have been reported by Inuzuka17 and Bishara et al.18 In the latter study it was found that more than 60% of the subjects in the study developed a Class I molar relationship (see Figure 16-6, A; see also Figure 16-31), almost 35% developed a Class II relationship (see Figure 16-6, B), and slightly more than 4% developed a Class III molar relationship (see Figure 16-6, C). The sides of the arch that started with a distal step in the deciduous dentition proceeded to develop into a Class II molar relationship in the permanent dentition. Of the sides with a flush terminal plane, 56% progressed into a Class I molar relationship and 44% into a Class II molar relationship. The mesial step in the deciduous dentition indicates a greater probability for a Class I molar relationship and a lesser probability for a Class III molar relationship. The Class I molar relationship is considered to be a "normal" relationship (i.e., the mesial buccal cusp of the maxillary molar occludes in the mesiobuccal developmental groove of the first permanent molar) (see Figure 16-6, A). The completion of the eruption of the primary teeth into occlusion results in the contact relations shown in the section on the permanent dentition.
DETAILS OF THE PRIMARY OCCLUSION
The occlusion of the primary teeth in a 3-year-old child is described. After separation has begun, the migration of the teeth changes the occlusion. Nevertheless, if development is normal, the spacing of the teeth is rather uniform (Figure 16-7). This biological change opens up contacts in the arch between teeth and increases occlusal wear. These changes anticipate the child’s needs, however, because if normal healthy reactions are in effect, the child seldom suffers from mechanical irritations during this severe adjustment period.
The normal occlusion of the primary teeth at the age of 3 years is as follows.
1. Mesial surfaces of maxillary and mandibular central incisors are in line with each other at the median line.
2. The maxillary central incisor occludes with the mandibular central incisor and the mesial third of the mandibular lateral incisor. The mandibular anterior teeth strike the maxillary anterior teeth lingually above the level of the incisal ridges.
Figure 16-7 Primary/deciduous dentition.
3. The maxillary lateral incisor occludes with the distal two thirds of the mandibular lateral incisor and that portion of the mandibular canine which is mesial to the point of its cusp.
4. The maxillary canine occludes with that portion of the mandibular canine distal to its cusp tip and the mesial third of the mandibular first molar (that portion mesial to the tip of the mesiobuccal cusp).
5. The maxillary first molar occludes with the distal two thirds of the mandibular first molar and the mesial portion of the mandibular second molar, which portion is represented by the mesial marginal ridge and the mesial triangular fossa.
6. The maxillary second molar occludes with the remainder of the mandibular second molar, with the distal surface of the maxillary molar projecting slightly over the distal portion of the mandibular second molar.
The interrelation of cusps and incisal ridges of the opposing arches of primary teeth may be studied in Figure 16-8. The size relation of deciduous and permanent arches is illustrated in Figure 16-9 (see also Figure 1-2).
