Angle’s classification of malocclusion in the 1890s was an important step in the development of orthodontics because it not only subdivided major types of malocclusion but also included the first clear and simple definition of normal occlusion in the natural dentition. Angle’s postulate was that the upper first molars were the key to occlusion and that the upper and lower molars should be related so that the mesiobuccal cusp of the upper molar occludes in the buccal groove of the lower molar. If the teeth were arranged on a smoothly curving line of occlusion (Fig. 1.2) and this molar relationship existed (Fig. 1.3), then normal occlusion would result.3 This statement, which 100 years of experience has proved to be correct except when there are aberrations in the size of teeth, brilliantly simplified normal occlusion.
Angle then described three classes of malocclusion, based on the occlusal relationships of the first molars:
- • Class I: Normal relationship of the molars, but line of occlusion incorrect because of malposed teeth, rotations, or other causes
- • Class II: Lower molar distally positioned relative to upper molar, line of occlusion not specified
- • Class III: Lower molar mesially positioned relative to upper molar, line of occlusion not specified
Note that the Angle classification has four classes: normal occlusion, Class I malocclusion, Class II malocclusion, and Class III malocclusion (see Fig. 1.3). Normal occlusion and Class I malocclusion share the same molar relationship but differ in the arrangement of the teeth relative to the line of occlusion. The line of occlusion may or may not be correct in Class II and Class III malocclusion.
With the establishment of a concept of normal occlusion and a classification scheme that incorporated the line of occlusion, by the early 1900s orthodontics was no longer just the alignment of irregular teeth. Instead, it had evolved into the treatment of malocclusion, defined as any deviation from the ideal occlusal scheme described by Angle. Because precisely defined relationships required a full complement of teeth in both arches, maintaining an intact dentition became an important goal of orthodontic treatment. Angle and his followers strongly opposed extraction for orthodontic purposes. With the emphasis on dental occlusion that followed, however, less attention came to be paid to facial proportions and esthetics. Angle abandoned extraoral force because he decided this was not necessary to achieve proper occlusal relationships. He solved the problem of dental and facial appearance by simply postulating that the best esthetics always were achieved when the patient had ideal occlusion.
As time passed, it became clear that even an excellent occlusion was unsatisfactory if it was achieved at the expense of proper facial proportions. Not only were there esthetic problems, it often proved impossible to maintain an occlusal relationship achieved by prolonged use of heavy elastics to pull the teeth together as Angle and his followers had suggested. Under the leadership of Charles Tweed in the United States and Raymond Begg in Australia (both of whom had studied with Angle), extraction of teeth was reintroduced into orthodontics in the 1940s and 1950s to enhance facial esthetics and achieve better stability of the occlusal relationships.
Cephalometric radiography, which enabled orthodontists to measure the changes in tooth and jaw positions produced by growth and treatment, came into widespread use after World War II. These radiographs made it clear that many Class II and Class III malocclusions resulted from faulty jaw relationships, not just malposed teeth. By use of cephalometrics, it also was possible to see that jaw growth could be altered by orthodontic treatment. In Europe, the method of “functional jaw orthopedics” was developed to enhance growth changes, while in the United States, extraoral force came to be used for this purpose. At present, both functional and extraoral appliances are used internationally to control and modify growth and form. Obtaining correct or at least improved jaw relationships became a goal of treatment by the mid-20th century.
The changes in the goals of orthodontic treatment, which now focus on facial proportions and the impact of the dentition on facial appearance, have been codified in the form of the soft tissue paradigm.4
Modern Treatment Goals: The Soft Tissue Paradigm
A paradigm can be defined as “a set of shared beliefs and assumptions that represent the conceptual foundation of an area of science or clinical practice.” The soft tissue paradigm states that both the goals and limitations of modern orthodontic and orthognathic treatment are determined by the soft tissues of the face, not by the teeth and bones. This reorientation of orthodontics away from the Angle paradigm that dominated the 20th century is most easily understood by comparing treatment goals, diagnostic emphasis, and treatment approach in the two paradigms (Table 1.1). With the soft tissue paradigm, the increased focus on clinical examination rather than examination of dental casts and radiographs leads to a different approach to obtaining important diagnostic information, and that information is used to develop treatment plans that would not have been considered without it.
TMJ, Temporomandibular joint.
More specifically, what difference does the soft tissue paradigm make in planning treatment? There are several major effects:
- 1. The primary goal of treatment becomes soft tissue relationships and adaptations, not Angle’s ideal occlusion. This broader goal is not incompatible with Angle’s ideal occlusion, but it acknowledges that to provide maximum benefit for the patient, ideal occlusion cannot always be the major focus of a treatment plan. Soft tissue relationships, both the proportions of the soft tissue integument of the face and the relationship of the dentition to the lips and face, are the major determinants of facial appearance. Soft tissue adaptations to the position of the teeth (or lack thereof) determine whether the orthodontic result will be stable. Keeping this in mind while planning treatment is critically important.
- 2. The secondary goal of treatment becomes functional occlusion. What does that have to do with soft tissues? Temporomandibular (TM) dysfunction, to the extent that it relates to the dental occlusion, is best thought of as the result of injury to the soft tissues around the temporomandibular joint (TMJ) caused by clenching and grinding the teeth. Given that, an important goal of treatment is to arrange the occlusion to minimize the chance of injury. In this also, Angle’s ideal occlusion is not incompatible with the broader goal, but deviations from the Angle ideal may provide greater benefit for some patients and should be considered when treatment is planned.
- 3. The thought process that goes into “solving the patient’s problems” is reversed. In the past, the clinician’s focus was on dental and skeletal relationships, with the tacit assumption that if these were correct, soft tissue relationships would take care of themselves. With the broader focus on facial and oral soft tissues, the thought process is to establish what these soft tissue relationships should be and then determine how the teeth and jaws would have to be arranged to meet the soft tissue goals. Why is this important in establishing the goals of treatment? It relates very much to why patients and parents seek orthodontic treatment and what they expect to gain from it.
The following sections of this chapter provide some background on the prevalence of malocclusion, what we know about the need for treatment of malocclusion and dentofacial deformity, and how soft tissue considerations, as well as teeth and bone, affect both need and demand for orthodontic treatment. It must be kept in mind that orthodontics is shaped by biological, psychosocial, and cultural determinants. For that reason, when defining the goals of orthodontic treatment, one has to consider not only morphologic and functional factors, but a wide range of psychosocial and bioethical issues as well. All these topics are discussed in much greater detail in the following chapters on diagnosis, treatment planning and treatment.
The Usual Orthodontic Problems: Epidemiology of Malocclusion
Angle’s “normal occlusion” more properly should be considered the ideal. In fact, perfectly interdigitating teeth arranged along a perfectly regular line of occlusion are quite rare. For many years, epidemiologic studies of malocclusion suffered from considerable disagreement among investigators about how much deviation from the ideal should be accepted within the bounds of normal. By the 1970s, a series of studies by public health or university groups in most developed countries provided a reasonably clear worldwide picture of the prevalence of the various types of malocclusion by degree of severity.
In the United States, two large-scale surveys carried out by the U.S. Public Health Service (USPHS) covered children ages 6 to 11 years from 1963 to 1965 and youths ages 12 to 17 years in 1969 and 1970.5,6 As part of a large-scale national survey of health care problems and needs in the United States in 1989 through 1994 (Third National Health and Nutrition Examination Survey [NHANES III]), estimates of malocclusion again were obtained. This study of some 14,000 individuals was statistically designed to provide weighted estimates for approximately 150 million persons in the sampled racial or ethnic and age groups. The data provide reasonably current information for U.S. children and youths and include the first good data set for malocclusion in adults, with separate estimates for the major racial or ethnic groups.7
The characteristics of malocclusion evaluated in NHANES III included the irregularity index, which is a measure of incisor alignment (Fig. 1.4); the prevalence of midline diastema larger than 2 mm (Fig. 1.5); and the prevalence of posterior crossbite (Fig. 1.6). In addition, overjet (Fig. 1.7) and overbite or open bite (Fig. 1.8) were measured. Overjet reflects Angle’s Class II and Class III molar relationships. Because overjet can be evaluated much more precisely than molar relationship in a clinical examination, molar relationship was not evaluated directly.
Data for these characteristics of malocclusion for children (age 8 to 11), youths (age 12 to 17), and adults (age 18 to 50) in the U.S. population, taken from NHANES III, are displayed graphically in Figs. 1.9 to 1.11.
Note in Fig. 1.10 that in the age 8 to 11 group, just over half of U.S. children have well-aligned incisors. The rest have varying degrees of malalignment and crowding. The percentage with excellent alignment decreases in the age 12 to 17 group as the remaining permanent teeth erupt, then remains essentially stable in the upper arch but worsens in the lower arch for adults. Only 34% of adults have well-aligned lower incisors. Nearly 15% of adolescents and adults have severely or extremely irregular incisors, so that major arch expansion or extraction of some teeth would be necessary to align them (see Fig. 1.10).
A midline diastema (see Fig. 1.5) often is present in childhood (26% have >2 mm space). Although this space tends to close, over 6% of youths and adults still have a noticeable diastema that compromises the appearance of the smile. Blacks are more than twice as likely to have a midline diastema as whites or Mexican-Americans (P < .001).
Occlusal relationships must be considered in all three planes of space. Lingual posterior crossbite (i.e., upper teeth lingual to lower teeth; see Fig. 1.6) is the major deviation from the normal transverse dental relationship and reflects deviations from ideal occlusion in the transverse plane of space. According to the NHANES III data,7 it occurs in 9% of the U.S. population, ranging from 7.6% of Mexican-Americans to 9.1% of whites and 9.6% of blacks.
Overjet or reverse overjet indicates anteroposterior deviations in the Class II or Class III direction, respectively, with Class III being much less prevalent (Fig. 1.12). Normal overjet is 2 mm. Overjet of 5 mm or more, suggesting Angle’s Class II malocclusion, occurs in 23% of children, 15% of youths, and 13% of adults. This reflects the greater postnatal growth of the mandible than the maxilla, which is discussed in Chapter 2. Severe Class II problems are less prevalent and severe Class III problems are more prevalent in the Mexican-American than the white or black groups.
Vertical deviations from the ideal overbite of 0 to 2 mm are less frequent in adults than children but occur in half the adult population, with excessive overbite occurring much more frequently than open bite (negative overbite) (Fig. 1.13). There are striking differences between the racial or ethnic groups in vertical dental relationships. Severe deep bite is nearly twice as prevalent in whites as blacks or Mexican-Americans (P < .001), whereas open bite of more than 2 mm is five times more prevalent in blacks than in whites or Mexican-Americans (P < .001). This almost surely reflects the slightly different craniofacial proportions of the black population groups (see Chapter 5 for a more complete discussion). In contrast to the higher prevalence of anteroposterior problems, vertical problems are less prevalent in Mexican-Americans than either blacks or whites.
From the survey data, it is interesting to calculate the percentage of American children and youths who would fall into Angle’s four groups. From this perspective, 30% at most have Angle’s normal occlusion. Class I malocclusion (50% to 55%) is by far the largest single group; there are about half as many Class II malocclusions (approximately 15%) as normal occlusions; and Class III (less than 1%) represents a very small proportion of the total.
Differences in malocclusion characteristics between the United States and other countries would be expected because of differences in racial and ethnic composition. Although the available data are not as extensive as for American populations, it seems clear that Class II problems are most prevalent in whites of northern European descent (for instance, 25% of children in Denmark are reported to have Class II malocclusion), whereas Class III problems are most prevalent in Asian populations (3% to 5% in Japan, nearly 2% in China, with another 2% to 3% pseudo–Class III [i.e., shifting into anterior crossbite because of incisor interferences]). African populations are by no means homogenous, but from the differences found in the United States between blacks and whites, it seems likely that Class III and open bite are more frequent in African than European populations and deep bite less frequent.
Why Is Malocclusion So Prevalent?
Crowded and irregular teeth now occur in a majority of the population; skeletal remains indicate that this was unusual until relatively recently, although not unknown (Fig. 1.14). Because the mandible tends to become separated from the rest of the skull when long-buried skeletal remains are unearthed, it is easier to be sure what has happened to alignment of teeth than to occlusal relationships. The skeletal remains suggest that all members of a group might tend toward a Class III or, less commonly, a Class II jaw relationship. Similar findings are noted in present population groups that have remained largely unaffected by modern development: crowding and malalignment of teeth are uncommon, but the majority of the group may have mild anteroposterior or transverse discrepancies, as in the Class III tendency of South Pacific islanders8 and buccal crossbite (X-occlusion) in aboriginal people of Australia.9
Although 1000 years is a long time relative to a single human life, it is a very short time from an evolutionary perspective. The fossil record documents evolutionary trends over many thousands of years that affect the present dentition, including a decrease in the size of individual teeth, in the number of the teeth, and in the size of the jaws. For example, there has been a steady reduction in the size of both anterior and posterior teeth over at least the last 100,000 years (Fig. 1.15). The number of teeth in the dentition of higher primates has been reduced from the usual mammalian pattern (Fig. 1.16). The third incisor and third premolar have disappeared, as has the fourth molar. At present, the human third molar, second premolar, and second incisor often fail to develop, which indicates that these teeth may be on their way out. Compared with other primates, modern humans have quite underdeveloped jaws.
It is easy to see that the progressive reduction in jaw size, if not well matched to a decrease in tooth size and number, could lead to crowding and malalignment. It is less easy to see why dental crowding should have increased quite recently, but this seems to have paralleled the transition from primitive agricultural to modern urbanized societies. Cardiovascular disease and related health problems appear rapidly when a previously unaffected population group leaves agrarian life for the city and civilization. High blood pressure, heart disease, diabetes, and several other medical problems are so much more prevalent in developed than underdeveloped countries that they have been labeled “diseases of civilization.”
There is some evidence that malocclusion increases within well-defined populations after a transition from rural villages to the city. Corruccini, for instance, reported a higher prevalence of crowding, posterior crossbite, and buccal segment discrepancy in urbanized youths compared with rural Punjabi youths of northern India.10 One can argue that malocclusion is another condition made worse by the changing conditions of modern life, perhaps resulting in part from less use of the masticatory apparatus with softer foods now. Under primitive conditions, of course, excellent function of the jaws and teeth was an important predictor of the ability to survive and reproduce. A capable masticatory apparatus was essential to deal with uncooked or partially cooked meat and plant foods. Watching an Australian aboriginal man using every muscle of his upper body to tear off a piece of kangaroo flesh from the barely cooked animal, for instance, makes one appreciate the decrease in demand on the masticatory apparatus that has accompanied civilization (Fig. 1.17). An interesting proposal by anthropologists is that the introduction of cooking, so that it did not take as much effort and energy to masticate food, was the key to the development of the larger human brain. Without cooked food, it would not have been possible to meet the energy demand of the enlarging brain. With it, excess energy is available for brain development and robust jaws are unnecessary.11