Common orthodontic indices and classification

Angle's classification 1899

Angle's system⁷ is one of the oldest and most well-known indices employed by the majority of clinicians worldwide. This is because it is simple to use, easy to understand and quick to undertake. It was initially devised as a prescription for treatment rather than for diagnostic purposes by clearly conveying the nature of the malpositioned teeth that required correction.

The classification, as we use it today, describes the relationship of the lower first permanent molars relative to the upper first permanent molars in the antero-posterior plane (Table 4 and Figure 1).⁷ Angle had also stressed the importance of looking at the positions of the other teeth and facial lines as part of his planning,⁷ but this is much less commonly used (Table 5).

Critics have argued that the Angle's molar classification system is too simple and potentially inaccurate when describing malocclusions. The classification confines malocclusions to discrete variables, when they are in fact not: leading to variable reliability between different operators, especially in Class II division 2 malocclusions.⁸ Modifications have been suggested to improve reliability by introducing numerical quantification (½ unit, or millimetres) and by even changing the tooth to a premolar instead of the first permanent molar.⁹ These modifications have been proven to be more reliable, but have not been widely accepted.

Angle had thought that the key to occlusion was that the upper permanent first molars (and canines) always erupted in a stable (fixed) position and controlled the position of the other teeth. However, it is now known that this is not the case owing to factors, such as early loss of second deciduous molars, congenitally missing premolars (hypodontia), and other local factors.

British Standards Institute classification of malocclusion (BS4492 1982)

The British Standards Institute's (BSI) classification¹⁰ is based upon Ballard and Wayman's work. They analysed the orthodontic requirements of 310 army apprentices,¹¹ and this has now superseded Angle's classification in the UK for describing a malocclusion. In this, four categories describe the manner in which the lower incisor edges occlude with the upper incisor cingulum plateau, rendering an occlusion Class I, Class II division I, Class II division 2 and Class III (Table 6 and Figure 2).¹⁰

As with Angle's classification, Williams and Stephens¹² found inter-operator disagreement, and therefore unreliability, using the BSI classification. This was particularly notable in Class II incisal relationships. They recommended that the classification be extended to include a Class II intermediate group, where ‘the lower incisor edges lie posterior to the cingulum plateau of the upper central incisors. The upper incisors are upright or slightly retroclined and the overjet lies between 5 and 7 mm’.

Attempts to increase the reproducibility of a diagnostic index have been made, for example, Du et al¹³ and Brin et al¹⁴ found that Katz's classification was the most reproducible above the BSI and Angle's classifications, respectively. In Katz Class I, The most anterior upper premolar fits exactly into the embrasure created by the distal contact of the most anterior lower premolar.⁹ However, since this index still deals with only one aspect of malocclusion, with sole focus on the buccal segment, there is still a degree of unreliability.

Index of Orthodontic Treatment Need (IOTN)¹⁵

The Index of Orthodontic Treatment Need (IOTN)¹⁵ is one of the most widely used indices around the world to rank malocclusions. It has been routinely used in the National Health Service (NHS) since 2006. The index was developed to determine the eligibility for orthodontic treatment and therefore prioritize care and referrals for those with the highest ‘need’. The index is quick and easy to use as a single worst feature is chosen; it can be used chairside or on dental casts; used in inter-disciplinary communication; the index can be modified for use in epidemiological studies^16,17 and quantitative data can be yielded for analysis.

There are two components to the index: the dental health component (DHC) and the aesthetic component (AC). The AC was initially developed by Evans and Shaw in 1987¹⁸ and was based on lay peoples' judgement of 1000 photos of 12 year old occlusions on a scale of 1 (most attractive teeth) to 10 (least attractive teeth). The photo that was the closest match to the patient's occlusion receives that score. A score of 6 or above implies a need for orthodontic treatment.

The DHC is based upon Linder-Aronson's work for the Swedish Public Dental Health Service from 1974.¹⁹ The malocclusion is grouped into five categories where 1 indicates no ‘need’ for orthodontic treatment and 5 indicates a ‘very great’ need for treatment.²⁰ A specific ruler can be used to help assess the DHC score. The categories are further subdivided using letters that describe the feature of the malocclusion that has been scored in a given order of importance: ‘MOCDO’ (missing teeth, overjet, crossbite, displacement of contact points and overbite; Table 7).

Studies have found that both the DHC and AC have high reproducibility, with the former having an inter-clinician kappa score of above 0.80 and the latter of above 0.72.^21,22

Despite the reliability of the IOTN, the index has its own drawbacks and obviates features of malocclusion, such as spacing. The DHC component of the IOTN also reflects some of our previous understanding regarding the health risks of a malocclusion, which are now only weakly correlated at best for most of the features of malocclusion: hypodontia can score a 4 or 5 on the DHC, with no evidence to suggest that this is associated with an oral health problem requiring orthodontics, and similarly little evidence exists connecting crowding to periodontal health issues.²³

The desire for orthodontic treatment is primarily driven by the perception of the patient regarding their own dental or facial aesthetics.^24,25 In the IOTN, the AC allows for this, but only through the frontal view of the occlusion in predominantly Class I/II malocclusions. Thus, it also has limitations through using mostly one incisal class, and not including a full smile or facial views. Additionally, there is ambivalent evidence regarding the psychological impact that the malocclusion and smile of a patient can have on self-esteem. For example, a patient with a moderate treatment need may be more psychologically impacted by their malocclusion than someone with a higher treatment need. This disadvantage has the risk of insensitivity and misjudgement of the needs of the individual patient. These additional nuances can be found in the Dental Aesthetic Index (DAI)²⁶ and are described in Jenny and Cons,²⁷ but this does makes the index more complicated and time consuming.

Some patients also have functional problems owing to their malocclusion, and this is especially true in orthognathic patients. Unfortunately, the IOTN does not completely take into account such issues, and so another index was developed to overcome this, the Index of Orthognathic Functional Treatment Need (IOFTN).²⁸

Questions have arisen over the validity of the IOTN and whether it really does measure the ‘need’ for orthodontic treatment, as ‘need’ depends on many factors that may be subjective and varied between countries owing to the system of remuneration, or social norms.^29,30 The IOTN, perhaps, needs re-evaluation to consider more recent findings related to malocclusion and oral/psychosocial wellbeing and to also take the patient's desire for treatment into account. After all, oral health is defined as being ‘multi-faceted and includes the ability to speak, smile, smell, taste, touch, chew, swallow and convey a range of emotions through facial expressions with confidence and without pain, discomfort and disease of the craniofacial complex’.³

Peer Assessment Rating (PAR)

The Peer Assessment Rating (PAR) was formulated over a series of meetings by 10 orthodontists before its publication.³¹ The PAR index is a quantitative and objective method for measuring malocclusion. In it, a weighted score is assigned to each of the occlusal traits that make up the malocclusion. The weighting of the scores is based on the severity of the trait and the perceived difficulty in correcting it. The sum of the scores then represents the degree to which the malocclusion deviates from the norm. A specific ruler showing the seven occlusal traits that PAR assesses is used, which makes scoring quick and easy to perform.

PAR scores are any easy tool that can be used to measure the efficacy of the clinical treatment. The difference between the post- and pre-treatment scores indicates the improvement (or not) as a result of orthodontic treatment. These PAR scores may be plotted on a graph to show the percentage of patients who are ‘worse or no different,’ ‘improved,’ or ‘greatly improved’ at the end of orthodontic treatment (Figure 3).

Of note, there are slight differences between the PAR developed in the UK and its modified version used in the USA. Hamdan and Rock³³ criticized the index, stating that the PAR weighting appears generic and unrepresentative. The high weighting for overjet, for example, may disproportionately increase the PAR score, while the low weighting for overbite could mean that correcting functional issues, such as a traumatic overbite, is misrepresented in terms of treatment value.³⁴ Much like the IOTN, this index does not consider treatment ‘need’ in its broadest terms: taking the patient's social/cultural background into account as well as the patient's own perceived need for treatment.