16 May

  • Crossbite: a discrepancy in the buccolingual relationship of the upper and lower teeth.
By convention the transverse relationship of the arches is described in terms of the position of the lower teeth relative to the upper teeth.
  • Buccal crossbite: the buccal cusps of the lower teeth occlude buccal to the buccal cusps of the upper teeth (Fig. 13.1).
  • Lingual crossbite: the buccal cusps of the lower teeth occlude lingual to the lingual cusps of the upper teeth. This is also known as a scissors bite (Fig. 13.2).
  • Displacement: on closing from the rest position the mandible encounters a deflecting contact(s) and is displaced to the left or the right, and/or anteriorly, into maximum interdigitation (Fig. 13.3).

A variety of factors acting either singly or in combination can lead to the development of a crossbite.
13.2.1. Local causes
The most common local cause is crowding where one or two teeth are displaced from the arch. For example, a crossbite of an upper lateral incisor often arises owing to lack of space between the upper central incisor and the deciduous canine, which forces the lateral incisor to erupt palatally and in linguo-occlusion with the opposing teeth. Posteriorly, early loss of a second deciduous molar in a crowded mouth may result in forward movement of the first permanent molar, forcing the second premolar to erupt palatally. Also, retention of a primary tooth can deflect the eruption of the permanent successor leading to a crossbite.
13.2.2. Skeletal
Generally, the greater the number of teeth in crossbite, the greater is the skeletal component of the aetiology. A crossbite of the buccal segments may be due purely to a mismatch in the relative width of the arches, or to an anteroposterior discrepancy, which results in a wider part of one arch occluding with a narrower part of the opposing jaw. For this reason buccal crossbites of an entire buccal segment are most commonly associated with Class III malocclusions (Fig. 13.4), and lingual crossbites are associated with Class II malocclusions. Anterior crossbites are associated with Class III skeletal patterns.
13.2.3. Soft tissues
A posterior crossbite is often associated with a digit-sucking habit, as the position of the tongue is lowered and a negative pressure is generated intra-orally.
13.2.4. Rarer causes
These include cleft lip and palate, where growth in the width of the upper arch is restrained by the scar tissue of the cleft repair. Trauma to, or pathology of, the temporomandibular joints can lead to restriction of growth of the mandible on one side, leading to asymmetry.
13.3.1. Anterior crossbite
An anterior crossbite is present when one or more of the upper incisors is in linguo-occlusion (i.e. in reverse overjet) relative to the lower arch (Fig. 13.5). Anterior crossbites involving only one or two incisors are considered in this chapter, whereas management of more than two incisors in crossbite is considered in Chapter 11 on Class III malocclusions. Anterior crossbites are frequently associated with displacement on closure (see Fig. 13.3).
13.3.2. Posterior crossbites
Crossbites of the premolar and molar region involving one or two teeth or an entire buccal segment can be subdivided as follows.
Unilateral buccal crossbite with displacement
This type of crossbite can affect only one or two teeth per quadrant, or the whole of the buccal segment. When a single tooth is affected, the problem usually arises because of the displacement of one or both teeth from the arch, leading to a deflecting contact on closure into the crossbite.
When the whole of the buccal segment is involved, the underlying aetiology is usually that the maxillary arch is of a similar width to the mandibular arch (i.e. it is too narrow) with the result that on closure from the rest position the buccal segment teeth meet cusp to cusp. In order to achieve a more comfortable and efficient intercuspation, the patient displaces their mandible to the left or right (see Chapter 5, Fig. 5.12). It is often difficult to detect this displacement on

closure as the patient soon learns to close straight into the position of maximal interdigitation. This type of crossbite may be associated with a centreline shift in the lower arch in the direction of the mandibular displacement (Fig. 13.6).
Unilateral buccal crossbite with no displacement
This category of crossbite is less common. It can arise as a result of deflection of two (or more) opposing teeth during eruption, but the greater the number of teeth in a segment that are involved, the greater is the likelihood that there is an underlying skeletal asymmetry.
Bilateral buccal crossbite
Bilateral crossbites (Fig. 13.7) are more likely to be associated with a skeletal discrepancy, either in the anteroposterior or transverse dimension, or in both.
Unilateral lingual crossbite
This type of crossbite is most commonly due to displacement of an individual tooth as a result of crowding or retention of the deciduous predecessor.
Bilateral lingual crossbite (scissors bite)
Again, this crossbite is typically associated with an underlying skeletal discrepancy, often a Class II malocclusion with the upper arch further forward relative to the lower so that the lower buccal teeth occlude with a wider segment of the upper arch.
13.4.1. Rationale for treatment
Research has shown that displacing contacts may predispose towards temporomandibular joint dysfunction syndrome in a susceptible individual (see Chapter 1, Section 1.7).


Therefore a crossbite associated with a displacement is a functional indication for orthodontic treatment. Similarly, treatment for a bilateral crossbite without displacement should be approached with caution, as partial relapse may result in a unilateral crossbite with displacement. In addition, a bilateral crossbite is probably as efficient for chewing as the normal buccolingual relationship of the teeth. However, the same cannot be said of a lingual crossbite where the cusps of affected teeth do not meet together at all.

Anterior crossbites, as well as being frequently associated with displacement, can lead to movement of a lower incisor labially through the labial supporting tissues, resulting in gingival recession. In this case early treatment is advisable (see Fig. 13.5).
13.4.2. Treatment of anterior crossbite
The following factors should be considered:
  • What type of movement is required? If tipping movements will suffice, a removable appliance can be considered, however, if bodily or apical movement is required then fixed appliances are indicated.
  • How much overbite is expected at the end of treatment? For treatment to be successful there must be some overbite present to retain the corrected incisor position. However, when planning treatment it should be remembered that proclination of an upper incisor will result in a reduction of overbite compared with the pretreatment position.
  • Is there space available within the arch to accommodate the tooth/teeth to be moved? If not, are extractions required and if so which teeth?
  • Is movement of the opposing tooth/teeth required? If reciprocal movement is required, a fixed appliance is indicated.
Provided that there is sufficient overbite and tilting movements will suffice, treatment can often be accomplished with a removable appliance. The appliance should incorporate the following features:
  • good anterior retention to counteract the displacing effect of the active element (where two or more teeth are to be proclined, a screw appliance may circumvent this problem);
  • buccal capping just thick enough to free the occlusion with the opposing arch (if the overbite is significantly increased a flat anterior bite-plane may be utilized instead);
  • an active element, for example a Z-spring (see Chapter 16).
    Fixed appliances are indicated in the following cases:
  • The apex of the incisor in crossbite is palatally positioned.
  • If there will be insufficient overbite to retain the corrected incisor(s), consideration should be given to using fixed appliances to move the lower incisor(s) lingually at the same time as the upper incisor(s) is moved labially in order to try and increase overbite.
  • Other features of a malocclusion necessitate the use of fixed appliances (Fig. 13.8).
If the upper arch is crowded, the upper lateral incisor often erupts in a palatal position relative to the arch. If the lateral incisor is markedly bodily displaced,


relief of crowding by extraction of the displaced tooth itself may sometimes be an option, but it is wise to seek a specialist opinion before taking this step.

13.4.3. Treatment of posterior crossbite
It is important to consider the aetiology of this feature before embarking on treatment. For example, is the crossbite due to displacement of one tooth from the arch, in which case correction will involve aligning this tooth, or is reciprocal movement of two or more opposing teeth required? Also, if there is a skeletal component, will it be possible to compensate for this by tooth movement? The inclination of the affected teeth should also be evaluated. Upper arch expansion is more likely to be stable if the teeth to be moved were tilted palatally initially. As expansion will create additional space, it may be advisable to defer a decision regarding extractions until after the expansion phase has been completed.
Even when fixed appliances are used, expansion of the upper buccal segment teeth will result in some tipping down of the palatal cusps (Fig. 13.9). This has the effect of hinging the mandible downwards leading to an increase in lower face height, which may be undesirable in patients who already have an increased lower facial height and/or reduced overbite. If expansion is indicated in these patients, fixed appliances are required to apply buccal root torque to the buccal segment teeth in order to try and resist this tendency, perhaps with high-pull headgear as well.
Unilateral buccal crossbite
Where this problem has arisen owing to the displacement of one tooth from the arch, for example an upper premolar tooth which has been crowded palatally, treatment will involve movement of the displaced tooth into the line of the arch, relieving crowding where and if necessary. If the displacement is marked, consideration can be given to extracting the displaced tooth itself or using fixed appliances to try and achieve bodily movement. Mild displacement of an upper premolar palatally can often be corrected using a T-spring on a removable appliance, but a screw type of appliance is preferable if buccal movement of a molar is required.
If correction of a crossbite requires movement of the opposing teeth in opposite directions, this can be achieved by the use of cross elastics (Fig. 13.10) attached to bands or bonded brackets on the teeth involved. If this is the only feature of a malocclusion requiring treatment, it is wise to leave the attachments in situ following correction, stopping the elastics for a month to review whether the corrected position is stable. If the crossbite relapses, the cross elastics can be re-instituted and an alternative means of retention considered.
A unilateral crossbite involving all the teeth in the buccal segment is usually associated with a displacement, and treatment is directed towards expanding the upper arch so that it fits around the lower arch at the end of treatment. If the

upper buccal teeth are not already tilted buccally, this can be accomplished with an upper removable appliance incorporating a midline screw and buccal capping. Alternatively, a quadhelix appliance can be used (see below). As a degree of relapse can be anticipated, some overexpansion of the upper arch is advisable, but not to the degree where a lingual crossbite or fenestration of the buccal periodontal support results. Retention should be continued for approximately 3 months full time followed by 3 months nights only with the removable appliance, and the quadhelix should be made passive and recemented as a retainer for 3 to 6 months.
Bilateral buccal crossbite
Unless the upper buccal segment teeth are tilted palatally to a significant degree, bilateral buccal crossbites are usually accepted. Rapid maxillary expansion can be used to try and expand the maxillary basal bone, but even with this technique a degree of relapse in the buccopalatal tooth position occurs following treatment, with the risk of development of a unilateral crossbite with displacement.
Bilateral buccal crossbites are common in patients with a repaired cleft of the palate. Expansion of the upper arch by stretching of the scar tissue is often indicated in these cases (see Chapter 21) and is readily achieved using a quadhelix appliance (Fig. 13.11).
Lingual crossbite
If a single tooth is affected, this is often the result of displacement due to crowding. If extraction of the displaced tooth itself is not indicated to relieve crowding, then provided that space can be made available it is often possible to use an upper removable appliance to free the occlusion with the lower arch and move the affected upper tooth palatally with a buccal spring. More severe cases with a greater skeletal element usually need a combination of buccal movement of the affected lower teeth and palatal movement of the upper teeth with fixed appliances. Treatment is not straightforward and should only be tackled by the experienced orthodontist, particularly as a scissors bite will often dislodge fixed attachments on the buccal aspect of the lower teeth until the crossbite is eliminated.
13.4.4. The quadhelix appliance
The quadhelix is a very efficient fixed slow expansion appliance (Fig. 13.12). The quadhelix appliance can also be adjusted to give more expansion anteriorly or posteriorly as required, and when active treatment is complete it can be made passive and recemented to act as a retainer.
A quadhelix is fabricated in 1 mm stainless steel wire and attached to the teeth by bands cemented to a molar tooth on each side. Preformed types are available which slot into palatal attachments welded onto bands on the molars and can be readily removed by the operator for adjustment. However, the appliance can also be custom-made in a laboratory. The usual activation is about half a tooth width each side. Overexpansion can occur readily if the appliance is overactivated, and therefore its use should be limited to those who are experienced with fixed appliances.
13.4.5. Rapid maxillary expansion
This upper appliance incorporates a screw similar to the type used for expansion in removable appliances except that it is soldered to bands, usually to both a pre-molar and molar tooth on both sides. The screw is turned twice daily, usually


over an active treatment period of 2 weeks (Fig. 13.13). The large force generated is designed to open the midline suture and expand the upper arch by skeletal expansion rather than by movement of the teeth. For this reason its use is really limited to patients in their early teens before the suture fuses, or cleft palate patients where it can be utilized to expand the cleft segments by stretching the scar tissue.

Once expansion is complete the appliance is left in situ as a retainer, usually for several months. Bony infill of the expanded suture has been demonstrated but on removing the appliance approximately 50 per cent relapse due to soft tissue pressures can be anticipated, and for this reason some overexpansion is indicated.
This appliance should only be used by the experienced.
The management of posterior crossbites is one of the few areas in orthodontics, which has been the subject of a systematic review. This process involves studying all the available literature on a subject and selecting only those randomized, controlled clinical trails, which have been carried out to the highest scientific standards (with no bias, adequate sample size, etc.). Disappointingly, only a few studies were suitable for inclusion. The authors concluded that removal of premature contacts of the deciduous teeth is effective in preventing a posterior cross-bite being perpetuated into the mixed dentition. In those cases where it is not effective, an upper removable appliance can be used to expand the upper arch to reduce the risk of the crossbite continuing into the permanent dentition. The paucity of good quality research in this area meant that clear recommendations could not be made regarding treatment in the late mixed and permanent dentition. This does not mean that the management approaches discussed above are wrong. In fact they reflect currently accepted good practice, but further studies with appropriate sample sizes and methodology are required.
Birnie, D. J. and McNamara, T. G. (1980). The quadhelix appliance. British Journal of Orthodontics, 7, 115–20.
The fabrication, management, and modifications of the quadhelix appliance are described in this paper.
Harrison, J. E. and Ashby, D. (1998). Orthodontic treatment for posterior crossbites (Cocrane Review), The Cochrane Library, Issue 4. Update Software, Oxford.
This is a systematic review of the effectiveness of different treatment modalities used in the correction of a posterior crossbite. Well worth the trouble taken to find it (try the Cochrane Collaboration on the Internet
Hermanson, H., Kurol, J., and Ronnerman, A. (1985). Treatment of unilateral posterior crossbites with quadhelix and removable plates. A retrospective study. European Journal of Orthodontics, 7, 97–102.
In this study it was found that the clinical results achieved were similar with the two types of appliance. However, the number of visits and chairside time were greater for the removable appliance. The authors calculated that the mean cost of treatment was 40 per cent greater for the removable appliance compared with the quadhelix.
Linder-Aronson, S. and Lindgren, J. (1979). The skeletal and dental effects of rapid maxillary expansion. British Journal of Orthodontics, 6, 25–9.
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