An Introduction to Orthodontics, 2nd Edition

8. Class I


A Class I incisor relationship is defined by the British Standards incisor classification as follows: ‘the lower incisor edges occlude with or lie immediately below the cingulum plateau of the upper central incisors’. Therefore Class I malocclusions include those where the anteroposterior occlusal relationship is normal and there is a discrepancy either within the arches and/or in the transverse or vertical relationship between the arches.


8.1.1. Skeletal

In Class I malocclusions the skeletal pattern is usually Class I, but it can also be Class II or Class III with the inclination of the incisors compensating for the underlying skeletal discrepancy (Fig. 8.1), i.e. dento-alveolar compensation. Marked transverse skeletal discrepancies between the arches are more commonly associated with Class II or Class III occlusions, but milder transverse discrepancies are often seen in Class I cases. Increased vertical skeletal proportions and anterior open bite can also occur where the anteroposterior incisor relationship is Class I.

8.1.2. Soft tissues

In most Class I cases the soft tissue environment is favourable (for example resulting in dento-alveolar compensation) and is not an aetiological factor. The major exception to this is bimaxillary proclination, where the upper and lower incisors are proclined. This may be racial in origin and can also occur because lack of lip tonicity results in the incisors being moulded forwards under tongue pressure.

Fig. 8.1. (a) Class I incisor relationship on Class I skeletal pattern; (b) Class I incisor relationship on a Class II skeletal pattern; (c) Class I incisor relationship on a Class III skeletal pattern.




8.1.3. Dental factors

Dental factors are the main aetiological agent in Class I malocclusions. The most common are tooth/arch size discrepancies, leading to crowding or, less frequently, spacing.

The size of the teeth is genetically determined and so, to a great extent, is the size of the jaws. Environmental factors can also contribute to crowding or spacing. For example, premature loss of a deciduous tooth can lead to a localization of any pre-existing crowding.

Local factors also include displaced or impacted teeth, and anomalies in the size, number, and form of the teeth, all of which can lead to a localized malocclusion. However, it is important to remember that these factors can also be found in association with Class II or Class III malocclusions.


Crowding occurs where there is a discrepancy between the size of the teeth and the size of the arches. Approximately 60 per cent of Caucasian children exhibit crowding to some degree. In a crowded arch loss of a permanent or deciduous tooth will result in the remaining teeth tilting or drifting into the space created. This tendency is greatest when the adjacent teeth are erupting.

Crowding can either be accepted or relieved. Before deciding between these alternatives the following should be considered:

·     the position, presence, and prognosis of remaining permanent teeth;

·     the degree of crowding which is usually calculated in millimetres per arch or quadrant;

·     the patient's malocclusion and any orthodontic treatment planned, including anchorage requirements;

·     the patient's age and the likelihood of the crowding increasing or reducing with growth;

·     the patient's profile.

These aspects of treatment planning are considered in more detail in Chapter 7, Sections 7.6 and 7.7.

In a Class I case with mild crowding (1–2 mm per quadrant) acceptance, or perhaps extraction of second molars, should be considered unless a significant increase in crowding is anticipated. In cases with moderate crowding (3–5 mm per quadrant) extraction of premolars is usually indicated. Where the crowding is severe (more than 5 mm per quadrant) space maintenance is definitely indicated prior to the extraction of, probably, the first premolars. Occasionally the extraction of two teeth per quadrant is indicated, but this severity of crowding is the province of the specialist.

After relief of crowding a degree of natural spontaneous movement will take place. In general, this is greater under the following conditions:

·     in a growing child;

·     if the extractions are carried out just prior to eruption of the adjacent teeth;

·     where the adjacent teeth are favourably positioned to upright if space is made available (for example considerable improvement will often occur in a crowded lower labial segment provided that the mandibular canines are mesially inclined);

·     there are no occlusal interferences with the anticipated tooth movement.

Fig. 8.2. Class I malocclusion treated by extraction of all four first premolars and no appliances: (a)–(c) prior to extractions; (d)–(f) 3 years after extractions.







Fig. 8.3. Class I malocclusion with upper arch crowding, treated by extraction of upper first premolars and use of fixed appliances: (a) pretreatment; (b) during treatment; (c) 1 year after the end of retention.




Most spontaneous improvement occurs in the first 6 months after the extractions. If alignment is not complete after 1 year, then further improvement will require active tooth movement with appliances. Figure 8.2 shows a case which was treated by extraction of all four first premolars without appliances, and Fig. 8.3 shows a patient whose management required the extraction of upper first premolars and the use of fixed appliances.

8.2.1. Late lower incisor crowding

In most individuals intercanine width increases up to around 12 to 13 years of age, and this is followed by a very gradual diminution throughout adult life. The rate of decrease is most noticeable during the mid to late teens. This reduction in intercanine width results in an increase of any pre-existing lower labial crowding, or the emergence of crowding in arches which were well aligned or even spaced in the early teens. Therefore, to some extent, lower incisor crowding can be considered as an age change. Certainly, patients who have undergone orthodontic treatment (including extractions) are not immune from lower labial segment crowding unless steps are taken to retain alignment subsequently; for example with a bonded lingual retainer.

The aetiology of late lower incisor crowding is not fully understood, and considerable controversy still exists as to the role of the third permanent molar. Most authors acknowledge that the aetiology is multifactorial. Nevertheless the following have all been proposed as major influences in the development of this phenomenon:

·     Forward growth of the mandible (either horizontally or manifesting as a growth rotation) when maxillary growth has ceased, together with soft tissue pressures, which result in a reduction in lower arch perimeter and labial segment crowding.

·     Mesial migration of the posterior teeth owing to forces from the interseptal fibres and/or from the anterior component of the forces of occlusion.

·     The presence of an erupting third molar pushes the dentition anteriorly, i.e. the third molar plays an active role.

·     The presence of a third molar prevents pressure developed anteriorly (due to either mandibular growth or soft tissue pressures) from being dissipated distally around the arch, i.e. the third molar plays a passive role.

Reviews of the many studies that have been carried out indicate that the third permanent molar has a statistically weak association with late lower incisor crowding.

Removal of symtomless lower third molars has been advocated in the past in order to prevent lower labial segment crowding. A recent prospective study found that there was a (non-significant) reduction in the presence of crowding in patients who had had the lower wisdom teeth extracted, but concluded that removing the lower third molar to reduce the degree of lower labial segment crowding could not be justified. Management of lower labial segment crowding should be considered together with other aspects of the malocclusion (see Chapter 7), bearing in mind the propensity of this problem to worsen with age. However, lower labial segment crowding is occasionally seen in arches, which are otherwise well aligned with a good Class I buccal segment interdigitation and a slightly increased overbite (Fig. 8.4). These cases are best kept under observation until the late teens when the fate of the third permanent molars, if present, has been determined. At that stage mild lower labial segment crowding can be accepted. If the crowding is more marked and upper extractions are contraindicated, it may be better to consider extraction of the most displaced lower incisor and use of a sectional fixed appliance to align and upright the remaining lower labial segment teeth (Fig. 8.5). However, patients should be warned that this may result in the labial segments dropping lingually, to the detriment of alignment in the upper arch.

Fig. 8.4. Class I occlusion with acceptable mild lower labial segment crowding.

Fig. 8.5. Adult with severe lower labial segment crowding despite the previous loss of a lower incisor. Management involved the extraction of the most displaced incisor and a lower sectional fixed appliance: (a), (b) pretreatment; (c), (d) post-treatment.











Generalized spacing is rare and is due to either hypodontia or small teeth in well-developed arches. Interestingly, an association between small teeth and hypodontia has been demonstrated. Orthodontic management of generalized spacing is frequently difficult as there is usually a tendency for the spaces to reopen unless permanently retained. In milder cases it may be wiser to encourage the patient to accept the spacing, or if the teeth are narrower than average, acid-etch composite additions or porcelain veneers can be used to widen them and thus improve aesthetics. In severe cases of hypodontia a combined orthodontic–restorative approach to localize space for the provision of prostheses, or perhaps implants, may be required (Fig. 8.6).

Localized spacing may be due to hypodontia or loss of a tooth as a result of trauma, or because extraction was indicated because of displacement, morphology, or pathology. This problem is most noticeable if an upper incisor is missing as the symmetry of the smile is affected, a feature which is usually noticed more by the lay public than other aspects of a malocclusion.

Fig. 8.6. Patient with hypodontia (the upper right second premolar and all four lateral incisors were absent) treated with fixed appliances to localize space for prosthetic replacement of the lateral incisors: (a) pretreatment; (b) fixed appliance; (c) post-treatment, before replacement of the lateral incisors by dentures cum retainers.




8.3.1. Median diastema

A median diastema is a space between the central incisors, which is more common in the upper arch (Fig. 8.7). A diastema is a normal physiological stage in the early mixed dentition when the fraenal attachment passes between the upper central incisors to attach to the incisive papilla. In normal development, as the lateral incisors and canines erupt this gap closes and the fraenal attachment migrates labially to the labial attached mucosa. If the upper arch is spaced or the lateral incisors are dimunitive or absent, there is less pressure forcing the upper central incisors together and the diastema will tend to persist. Rarely, the fraenal attachment appears to prevent the central incisors from moving together. In these cases, blanching of the incisive papilla can be observed if tension is applied to the fraenum, and on radiographic examination a V-shaped notch of the interdental bone can be seen between the incisors indicating the attachment of the fraenum (see Chapter 3, Fig. 3.26).

Fig. 8.7. Upper midline diastema.

Management (see also Chapter 3, Section 3.3.9)

It is important to take a periapical radiograph to exclude the presence of a super-numerary tooth which, if present, should be removed before closure of the diastema is undertaken. As median diastemas tend to reduce or close with the eruption of the canines, management can be subdivided as follows.

·     Before eruption of the permanent canines intervention is only necessary if the diastema is greater than 3 mm and there is a lack of space for the lateral incisors to erupt. Care is required not to cause resorption of the incisor roots against the unerupted canines.

·     After eruption of the permanent canines space closure is usually straightforward. Either fixed or removable appliances are used as indicated by the angulation of the incisors. Prolonged retention is usually necessary as diastemas exhibit a great tendency to reopen, particularly if there is a familial tendency; the upper arch is spaced or the initial diastema was greater than 2 mm. In view of this it may be better to accept a minimal diastema, particularly if no other orthodontic treatment is required. Alternatively, if the central incisors are narrow a restorative solution, for example veneers, can be considered (Fig. 8.8).

If it is thought that the fraenum is an contributory factor, then fraenectomy is best carried out during space closure as scar tissue contraction will aid space closure.

Fig. 8.8. Adult with narrow proclined upper central incisors with a midline diastema. An upper removable appliance was used to reduce the overbite, and then to retract and move 1/1 a little closer together: (a) pretreatment; (b) at the completion of active appliance therapy, following veneering of 21/1.



8.3.2. Management of missing upper incisors

Upper central incisors are rarely congenitally absent. They can be lost as a result of trauma, or occasionally their extraction may be indicated because of dilaceration. Upper lateral incisors are congenitally absent in approximately 2 per cent of a Caucasian population, but can also be lost following trauma. Both can occur unilaterally, bilaterally, or together. Whatever the reason for their absence, there are two treatment options:

·     closure of the space

·     opening of the space and placement of a denture or a bridge.

The choice for a particular patient will depend upon a number of factors, which are listed below. However, this is a difficult area of treatment planning and specialist advice should be sought.

·     Skeletal relationship: if the skeletal pattern is Class III, space closure in the upper labial segment may compromise the incisor relationship; conversely, for a Class II division 1 pattern space closure may be preferable as it will aid overjet reduction.

·     Presence of crowding or spacing.

·     Colour and form of adjacent teeth: if the permanent canines are much darker than the incisors and/or particularly caniniform in shape, modification to make them resemble lateral incisors will be difficult; also, if a lateral incisor is to be brought forward to replace a missing single upper central incisor, an aesthetically pleasing result will only be possible if the lateral is fairly large and has a good gingival circumference.

·     The inclination of adjacent teeth, as this will influence whether it is easier to open or close the space.

·     The desired buccal segment occlusion at the end of treatment; for example if the lower arch is well aligned and the buccal segment relationship is Class I, space opening is preferable.

·     The patient's wishes and ability to cooperate with complex treatment: some patients have definite ideas about whether they are willing to proceed with appliance treatment, and whether they wish to have the space closed or opened for a prosthetic replacement.

Fig. 8.9. Trial (Kesling's) set-up.

Trial (Kesling's) set-up

To investigate the feasibility of different options a trial set-up can be carried out using duplicate models. The teeth to be moved are cut off the model and repositioned in the desired place using wax (Fig. 8.9). This allows any number of options to be tested and also gives an opportunity to evaluate in more detail the amount and nature of any orthodontic and restorative treatment required by a particular option. This exercise is often helpful in describing the outcome of different options to the patient.

After assessment of the above factors a provisional plan can be discussed with the patient. It is often possible to draw up more than one plan and these should all be thoroughly discussed, including the advantages and disadvantages, and the long-term maintenance of any prosthetic replacements.

Space closure

This can be facilitated by early extraction of any deciduous teeth to allow forward movement of the first permanent molars in that quadrant(s). In crowded mouths, if this step is carried out early it may be possible to achieve a satisfactory result without appliances, but usually fixed appliances are necessary to correct the axial inclinations. If any masking procedures (for example contouring a canine incisally, palatally, and interproximally to resemble a lateral incisor) or acid-etch composite additions are required, these should be carried out prior to the placement of appliances to facilitate final tooth alignment. Placement of a bonded retainer post-treatment is advisable in the majority of cases (Fig. 8.10).

Fig. 8.10. (a) Patient with missing lateral incisors treated by space closure and modification of the upper canines. (b) Occlusal view of same patient to show bonded retainer.



Space maintenance or opening

If an incisor is extracted electively or a patient seen soon after loss has occurred, ideally a space maintenance should be fitted forthwith. In cases where space closure has occurred as a result either of early tooth loss or congenital absence, appliances will be required to open the space. The angulation of the adjacent teeth will determine whether fixed or removable appliances are required (Fig. 8.11). Whenever space is opened prior to bridgework, it is important to retain with a partial denture for at least 3 to 6 months (Fig. 8.12), particularly if an adhesive acid-etch retained bridge is to be used. Research has shown that acid-etch bridges placed immediately after the completion of tooth movement have a greater incidence of failure than those placed following a period of retention with a removable retainer.

Fig. 8.11. Class I incisor relationship on a Class III skeletal pattern with congenital absence of all four lateral incisors. Since space closure would run the risk of retracting the upper incisors into a Class III relationship, it was decided to open space for prosthetic replacement of 2/2 and to accept the spacing in the lower arch. Because of the axial inclination of the central incisors and canines, an upper fixed appliance was used in conjunction with a clip-over bite plane (using plint clips engaging the bands on 6/6). (a) Pretreatment lateral view showing mild Class III skeletal pattern; (b)–(d) pretreatment intra-oral photographs; (e) post-treatment intra-oral photographs (a partial denture cum retainer was fitted replacing 2/2).






Fig. 8.12. (a) Patient with early traumatic loss of 1/ and partial space closure. Space for prosthetic replacement of 1/ was gained using a fixed appliance. (b) Result on completion of active treatment. (c) Partial denture cum retainer (NB: Stops were placed mesial to both 2/ and /1 to help prevent relapse).




Implant technology is improving rapidly and it is hoped that it will become cheaper in the future, allowing this option to be more readily available.


Teeth can be displaced for a variety of reasons including the following:

·     Abnormal position of the tooth germ: canines (Chapter 14) and second premolars are the most commonly affected teeth. Management depends upon the degree of displacement. If this is mild, extraction of the associated primary tooth plus space maintenance, if indicated, may result in an improvement in position in some cases. Alternatively, exposure and the application of orthodontic traction may be used to bring the mildly displaced tooth into the arch. If the displacement is severe, extraction is usually necessary.

·     Crowding: lack of space for a permanent tooth to erupt within the arch can lead to or contribute to displacement. Those teeth that erupt last in a segment, for example upper lateral incisors, upper canines (Fig. 8.13), second premolars, and third molars, are most commonly affected. Management involves relief of crowding, followed by active tooth movement where necessary. However, if the displacement is severe it may be prudent to extract the displaced tooth (Fig. 8.14).

·     Retention of a deciduous predecessor: extraction of the retained primary tooth should be carried out as soon as possible provided that the permanent successor is not displaced.

·     Secondary to the presence of a supernumerary tooth or teeth (see Chapter 3): management involves extraction of the supernumerary followed by tooth alignment, usually with fixed appliances. Displacements due to supernumeraries have a tendency to relapse and prolonged retention is required.

·     Caused by a habit (see Chapter 9).

·     Secondary to pathology, for example a dentigerous cyst. This is the rarest cause.

Fig. 8.13. Class I malocclusion with mild lower and marked upper arch crowding. In crowded arches the last teeth in a segment to erupt, in this case the upper canines, are the most likely to be short of space. The maxillary second premolars are also crowded, probably owing to early loss of the upper second deciduous molars.

Fig. 8.14. Occasionally it may be prudent to extract the most displaced tooth. In this case all four canines were extracted: (a) prior to extractions; (b) after extractions. (NB: Patient is posturing forwards to show lower arch alignment.)




Variations in the vertical dimension can occur in association with any anteroposterior skeletal relationship. Increased vertical skeletal proportions are discussed in Chapter 9 in relation to Class II division 1; in Chapter 11 in relation to Class III, and in Chapter 12 on anterior open bite.


A transverse discrepancy between the arches results in a crossbite and can occur in association with Class I, Class II, and Class III malocclusions. Classification and management of crossbite is discussed in Chapter 13.


As the name suggests, bimaxillary proclination is the term used to describe occlusions where both the upper and lower incisors are proclined. Bimaxillary proclination is seen more commonly in some racial groups (for example Afro-Caribbean)), and so when an assessment is carried out the patient should be assessed bearing in mind what is normal for their ethnic background. This is particularly pertinent in cephalometric analysis.

When bimaxillary proclination occurs in a Class I malocclusion the overjet is increased because of the angulation of the incisors (Fig. 8.15). Management is difficult because both upper and lower incisors need to be retroclined to reduce the overjet. Retroclination of the lower labial segment will encroach on tongue space and therefore has a high likelihood of relapse following removal of appliances. For these reasons, treatment of bimaxillary proclination should be approached with caution and consideration should be given to accepting the incisor relationship. If the lips are incompetent, but have a good muscle tone and are likely to achieve a lip-to-lip seal if the incisors are retracted, the chances of a stable result are increased. However, the patient should still be warned that the prognosis for stability is guarded. Where bimaxillary proclination is associated with competent lips, or with grossly incompetent lips which are unlikely to retain the corrected incisor position, it may be wiser not to proceed. However, if, treatment is decided upon, permanent retention is advisable.

Fig. 8.15. (a) Class I incisor relationship with normal axial inclination (inter-incisal angle is 137°); (b) Class I incisor relationship with bimaxillary inclination showing increased overjet (inter-incisal angle is 107°).

Bimaxillary proclination can also occur in association with Class II division 1 and Class III malocclusions.


Bishara, S. E. (1999). Third molars: a dilemma: Or is it? American Journal of Orthodontics and Dentofacial Orthopedics115, 628–33.

Harradine, N. W. T., Pearson, M. H., and Toth, B. (1998). The effect of extraction of third molars on late lower incisor crowding: A randomised controlled trial. British Journal of Orthodontics25, 117–22.

This excellent study is essential reading.

Little, R. M., Reidel, R. A., and Artun, J. (1981). An evaluation of changes in mandibular anterior alignment from 10–20 years postretention. American Journal of Orthodontics and Dentofacial Orthopedics93, 423–8.

Classic paper. The authors found that lower labial segment crowding tends to increase even following extractions and appliance therapy.

Richardson, M. E. (1989). The role of the third molar in the cause of late lower arch crowding: a review. American Journal of Orthodontics and Dentofacial Orthopedics95, 79–83.

The evidence in support of the theory that the presence of a third molar is one of the aetiological factors in late lower incisor crowding is reviewed in this paper.

Shashua, D. and Artun, J. (1999). Relapse after orthodontic correction of maxillary median diastema: a follow-up evaluation of consecutive cases. The Angle Orthodontist69, 257–63.

Stephens, C. D. (1989). The use of natural spontaneous tooth movement in the treatment of malocclusion. Dental Update16, 337–42.

An interesting paper in which the role of interceptive extractions in Class I malocclusions is discussed.

Vasir, N. S., and Robinson, R. J. (1991). The mandibular third molar and late crowding of the mandibular incisors — a review. British Journal of Orthodontics18, 59–66.

An unbiased review of the literature regarding the role of third molars in late lower incisor crowding. The authors conclude that the wisdom tooth has a small, but variable, effect.

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