An Introduction to Orthodontics, 2nd Edition

2. The aetiology and classification 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.


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).


2.3.1. 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.

Fig. 2.1. Angle's classification.

Fig. 2.2. Incisor classification Class I.

Fig. 2.3. Incisor classification Class II division 1.

Fig. 2.4. Incisor classification Class II division 2.

Fig. 2.5. Incisor classification Class III.

Fig. 2.6. IOTN ruler.

2.3.3. Summers occlusal index

This index was developed by Summers, in the USA, during the 1960s. It is popular in the USA, particularly for research purposes. Good reproducibility has been reported and it has also been employed to determine the success of treatment with acceptable results. The index scores nine defined parameters including molar relationship, overbite, overjet, posterior crossbite, posterior open bite, tooth displacement, midline relation, maxillary median diastema, and absent upper incisors. Allowance is made for different stages of development by varying the weighting applied to certain parameters in the deciduous, mixed, and permanent dentition.

2.3.4. Index of Orthodontic Treatment Need (IOTN)

The Index of Orthodontic Treatment Need was developed as a result of a government initiative. The purpose of the index was to help determine the likely impact of a malocclusion on an individual's dental health and psychosocial well-being. It comprises two elements.

Dental health component

This was developed from an index used by the Dental Board in Sweden designed to reflect those occlusal traits, which could affect the function and longevity of the dentition. The single worst feature of a malocclusion is noted (the index is not cumulative) and categorized into one of five grades reflecting need for treatment (Table 2.1):

·     Grade 1 — no need

·     Grade 2 — little need

·     Grade 3 — moderate need

·     Grade 4 — great need

·     Grade 5 — very great need.

A ruler has been developed to help with assessment of the dental health component (Fig. 2.6), and these are available commercially.

Aesthetic component

This aspect of the index was developed in an attempt to assess the aesthetic handicap posed by a malocclusion and thus the likely psychosocial impact upon the patient a task fraught with potential pitfalls (see Chapter 1). The aesthetic component comprises a set of ten standard photographs (Fig. 2.7), which are also graded from score 1, the most aesthetically pleasing, to score 10, the least aesthetically pleasing. Colour photographs are available for use with a patient in the clinical situation and black-and-white photographs for scoring from study 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

Grade 1 (None)


Extremely minor malocclusions including displacements less than 1 mm

Grade 2 (Little)


Increased overjet 3.66 mm with competent lips


Reverse overjet 0.1–1 mm


Anterior or posterior crossbite with up to 1 mm discrepancy between retruded contact position and intercuspal position


Displacement of teeth 1.1–2 mm


Anterior or posterior open bite 1.12 mm


Increased overbite 3.5 mm or more, without gingival contact


Prenormal or postnormal occlusions with no other anomalies; includes up to half a unit discrepancy.

Grade 3 (Moderate)


Increased overjet 3.66 mm with incompetent lips


Reverse overjet 1.1–3.5 mm


Anterior or posterior crossbites with 1.12 mm discrepancy


Displacement of teeth 2.1–4 mm


Lateral or anterior open bite 2.14 mm


Increased and complete overbite without gingival trauma

Grade 4 (Great)


Increased overjet 6.1–9 mm


Reversed overjet greater than 3.5 mm with no masticatory or speech difficulties


Anterior or posterior crossbites with greater than 2 mm discrepancy between retruded contact position and intercuspal position.


Severe displacement of teeth, greater than 4 mm.


Extreme lateral or anterior open bites, greater than 4 mm.


Increased and complete overbite with gingival or palatal trauma.


Less extensive hypodontia requiring pre-restorative orthodontic space closure to obviate the need for a prosthesis


Posterior lingual crossbite with no functional occlusal contact in one or both buccal segments


Reverse overjet 1.13.5 mm with recorded masticatory and speech difficulties


Partially erupted teeth, tipped and impacted against adjacent teeth


Supplemental teeth

Grade 5 (Very Great)


Increased overjet greater than 9 mm


Extensive hypodontia with restorative implications (more than one tooth missing in any quadrant) requiring pre-restorative orthodontics


Impeded eruption of teeth (with the exception of third molars) due to crowding, displacement, the presence of supernumerary teeth, retained deciduous teeth, and any pathological cause


Reverse overjet greater than 3.5 mm with reported masticatory and speech difficulties


Defects of cleft lip and palate


Submerged deciduous teeth

Reproduced with the kind permission of Vuman Ltd.

Fig. 2.7. Aesthetic component of IOTN.

An average score can be taken from the two components, but the dental health component alone is more widely used. The aesthetic component has been criticized for being subjective, and particular difficulty is experienced in accurately assessing Class III malocclusions or anterior open bites, as the photographs are composed of Class I and Class II cases.

2.3.5. Peer Assessment Rating (PAR)

The PAR index was developed primarily to measure the success (or otherwise) of treatment. Scores are recorded for a number of parameters (listed below), before and at the end of treatment, using study models. Unlike IOTN, the scores are cumulative; however, a weighting is accorded to each component to reflect 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)

·     overjet (×6)

·     overbite (×2)

·     centrelines (×4).

The difference between the PAR scores at the start and on completion of treatment can be calculated, and from this the percentage change in PAR score, which is a reflection of the success of treatment, is derived. A high standard of treatment is indicated by a mean percentage reduction of greater than 70 per cent. A change of 30 per cent or less indicates that no appreciable improvement has been achieved. The size of the PAR score at the beginning of treatment gives an indication of the severity of a malocclusion. Obviously it is difficult to achieve an significant reduction in PAR in cases with a low pretreatment score.

2.3.6 Index of Complexity Outcome and Need (ICON)

This new index incorporates features of both the Index of Orthodontic Need (IOTN) and the Peer Assessment Rating (PAR). The aesthetic component of IOTN is included along with scores for upper arch crowding/spacing; presence of cross-bite; overbite/open bite, and buccal segment relationship. As in the PAR, weightings are added to reflect current orthodontic opinion. The sum of the scores and their weightings gives a pretreatment score, which is said to reflect the need for, and likely complexity of, the treatment required. Following treatment the index is scored again to give an improvement grade (pretreatment score minus 4 × post-treatment score) and thus the outcome of treatment. This ambitious index is currently undergoing evaluation.


Angle, E. H. (1899). Classification of malocclusion. Dental Cosmos41, 24864.

Markovic, M. (1992). At the crossroads of oral facial genetics. European Journal of Orthodontics14, 46981.

A fascinating study of twins and triplets with Class II/2 malocclusions.

Mossey, P. A. (1999). The heritability of malocclusion. British Journal of Orthodontics26, 10313, 195203.

Richmond, S., Shaw, W. C., OBrien, K. D., Buchanan, I. B., Jones, R., Stephens, C. D., et al. (1992). The development of the PAR index (Peer Assessment Rating): reliability and validity. European Journal of Orthodontics14, 12539.

The PAR index, part 1.

Richmond, S., Shaw, W. C., Roberts, C. T., and Andrews, M. (1992). The PAR index (Peer Assessment Rating): methods to determine the outcome of orthodontic treatment in terms of improvements and standards. European Journal of Orthodontics14, 1807.

The PAR index, part 2.

Summers, C. J. (1971). A system for identifying and scoring occlusal disorders. American Journal of Orthodontics59, 55267.

For readers requiring further information on Summers occlusal index.

Shaw, W. C., OBrien, K. D., and Richmond, S. (1991). Quality control in orthodontics: indices of treatment need and treatment standards. British Dental Journal170, 10712.

An interesting paper on the role of indices, with good explanations of the IOTN and the PAR index.

Tang, E. L. K. and Wei, S. H. Y. (1993). Recording and measuring malocclusion: a review of the literature. American Journal of Orthodontics and Dentofacial Orthopedics103, 34451.

Useful for those researching the subject.

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