The aetiology and classification of malocclusion
2.1. THE AETIOLOGY OF MALOCCLUSION
The aetiology of malocclusion is a fascinating subject about which there is still much to elucidate and understand. At a basic level, malocclusion can occur as a result of genetically determined factors, which are inherited, or environmental factors, or more commonly a combination of both inherited and environmental factors acting together. For example, failure of eruption of an upper central incisor may arise as a result of dilaceration following an episode of trauma during the deciduous dentition which led to intrusion of the primary predecessor — an example of environmental aetiology. Failure of eruption of an upper central incisor can also occur as a result of the presence of a super-numerary tooth — a scenario which questioning may reveal also affected the patient’s parent, suggesting an inherited problem. However, if in the latter example caries (an environmental factor) has led to early loss of many of the deciduous teeth, then forward drift of the first permanent molar teeth may also lead to superimposition of the additional problem of crowding.
While it is relatively straightforward to trace the inheritance of syndromes such as cleft lip and palate (see Chapter 21), it is more difficult to determine the aetiology of features which are in essence part of normal variation, and the picture is further complicated by the compensatory mechanisms that exist. Evidence for the role of inherited factors in the aetiology of malocclusion has come from studies of families and twins. The facial similarity of members of a family, for example the prognathic mandible of the Hapsburg royal family, is easily appreciated. However, more direct testimony is provided in studies of twins and triplets, which indicate that skeletal pattern and tooth size and number are largely genetically determined.
Examples of environmental influences include digit-sucking habits and premature loss of teeth as a result of either caries or trauma. Soft tissue pressures acting upon the teeth for more than 6 hours per day can also influence tooth position. However, because the soft tissues including the lips are by necessity attached to the underlying skeletal framework, their effect is also mediated by the skeletal pattern.
Crowding is extremely common in Caucasians, affecting approximately two-thirds of the population. As was mentioned above, the size of the jaws and teeth are mainly genetically determined; however, environmental factors, for example premature deciduous tooth loss, can precipitate or exacerbate crowding. In evolutionary terms both jaw size and tooth size appear to be reducing. However, crowding is much more prevalent in modern populations than it was in prehistoric times. It has been postulated that this is due to the introduction of a less abrasive diet, so that less interproximal tooth wear occurs during the lifetime of an individual. However, this is not the whole story, as a change from a rural to
an urban life-style can also apparently lead to an increase in crowding after about two generations.
Although this discussion may at first seem rather theoretical, the aetiology of malocclusion is a vigorously debated subject. This is because if one believes that the basis of malocclusion is genetically determined, then it follows that orthodontics is limited in what it can achieve. However, the opposite viewpoint is that every individual has the potential for ideal occlusion and that orthodontic intervention is required to eliminate those environmental factors that have led to a particular malocclusion. Research suggests that for the majority of malocclusions the aetiology is multifactorial, and orthodontic treatment can effect only limited skeletal change. Therefore, as a patient’s skeletal and growth pattern is largely genetically determined, if orthodontic treatment is to be successful clinicians must recognize and work within those parameters.
Of necessity, the above is a brief summary, but it can be appreciated that the aetiology of malocclusion is a complex subject, much of which is still not fully understood. The reader seeking more information is advised to consult the publications listed in the section on further reading.
2.2. CLASSIFYING MALOCCLUSION
The categorization of a malocclusion by its salient features is helpful for describing and documenting a patient’s occlusion. In addition, classifications and indices allow the prevalence of a malocclusion within a population to be recorded, and also aid in the assessment of need, difficulty, and success of orthodontic treatment.
Malocclusion can be recorded qualitatively and quantitatively. However, the large number of classifications and indices which have been devised, are testimony to the problems inherent in both these approaches. All have their limitations, and these should be borne in mind when they are applied.
Two terms are often mentioned in relation to indices:
Validity — Can the index measure what it was designed to measure?
Reproducibility — Does the index give the same result when recorded on two different occasions, and by different examiners?
2.2.1. Qualitative assessment of malocclusion
Essentially, a qualitative assessment is descriptive and therefore this category includes the diagnostic classifications of maloccusion. The main drawback to a qualitative approach is that malocclusion is a continuous variable so that clear cut-off points between different categories do not always exist. This can lead to problems when classifying borderline malocclusions. In addition, although a qualitative classification is a helpful shorthand method of describing the salient features of a malocclusion, it does not provide any indication of the difficulty of treatment.
Qualitative evaluation of malocclusion was attempted historically before quantative analysis. One of the better known classifications was devised by Angle in 1899, but other classifications are now more widely used, for example the British Standards Institute (1983) classification of incisor relationship.
2.2.2. Quantitative assessment of malocclusion
In quantitative indices two differing approaches can be used:
Each feature of a malocclusion is given a score and the summed total is then recorded (e.g. the PAR Index).
The worst feature of a malocclusion is recorded (e.g. the Index of Orthodontic Treatment Need).
2.3. COMMONLY USED CLASSIFICATIONS AND INDICES
2.3.1. Angle’s classification
Angle’s classification was based upon the premise that the first permanent molars erupted into a constant position within the facial skeleton, which could be used to assess the anteroposterior relationship of the arches. In addition to the fact that Angle’s classification was based upon an incorrect assumption, the problems experienced in categorizing cases with forward drift or loss of the first permanent molars have resulted in this particular approach being superseded by other classifications. However, Angle’s classification is still used to describe molar relationship, and the terms used to describe incisor relationship have been adapted into incisor classification.
Angle described three groups (Fig. 2.1):
Class I or neutrocclusion — the mesiobuccal cusp of the upper first molar occludes with the mesiobuccal groove of the lower first molar. In practice discrepancies of up to half a cusp width either way were also included in this category.
Class II or distocclusion — the mesiobuccal cusp of the lower first molar occludes distal to the Class I position. This is also known as a postnormal relationship.
Class III or mesiocclusion — the mesiobuccal cusp of the lower first molar occludes mesial to the Class I position. This is also known as a prenormal relationship.
2.3.2. British Standards Institute classification
This is based upon incisor relationship and is the most widely used descriptive classification. The terms used are similar to those of Angle’s classification, which can be a little confusing as no regard is taken of molar relationship. The categories defined by British Standard 4492 are as follows:
Class I — the lower incisor edges occlude with or lie immediately below the cingulum plateau of the upper central incisors (Fig. 2.2).
Class II — the lower incisor edges lie posterior to the cingulum plateau of the upper incisors. There are two subdivisions of this category:
Division 1 — the upper central incisors are proclined or of average inclination and there is an increase in overjet (Fig. 2.3).
Division 2 — The upper central incisors are retroclined. The overjet is usually minimal or may be increased (Fig. 2.4).
Class III — The lower incisor edges lie anterior to the cingulum plateau of the upper incisors. The overjet is reduced or reversed (Fig. 2.5).
As with any descriptive analysis it is difficult to classify borderline cases. Some workers have suggested introducing a Class II intermediate category for those cases where the upper incisors are upright and the overjet increased between 4 and 6 mm. However, this suggestion has not gained widespread acceptance.
Grade 1 — no need
Grade 2 — little need
Grade 3 — moderate need
Grade 4 — great need
Grade 5 — very great need.
models alone. The patient’s teeth (or study models), in occlusion, are viewed from the anterior aspect and the appropriate score determined by choosing the photograph that is thought to pose an equivalent aesthetic handicap. The scores are categorized acccording to need for treatment as follows:
score 1 or 2 — none
score 3 or 4 — slight
score 5, 6, or 7 — moderate/borderline
score 8, 9, or 10 — definite.
Table 2.1 The Index of Orthodontic Treatment Need
current opinion in the UK as to their relative importance. The features recorded are listed below, with the current weightings in parenthesis:
crowding — by contact point displacement (×1)
buccal segment relationship — in the anteroposterior, vertical, and transverse planes (×1)