The British Standards definition of Class III incisor relationship includes those malocclusions where the lower incisor edge occludes anterior to the cingulum plateau of the upper incisors. Class III malocclusions affect around 3 per cent of Caucasians.
11.1.1. Skeletal pattern
The skeletal relationship is the most important factor in the aetiology of most Class III malocclusions, and the majority of Class III incisor relationships are associated with an underlying Class III skeletal relationship. Cephalometric studies have shown that, compared with Class I occlusions, Class III malocclusions exhibit the following:
increased mandibular length;
a more anteriorly placed glenoid fossa so that the condylar head is positioned more anteriorly leading to mandibular prognathism;
reduced maxillary length;
a more retruded position of the maxilla leading to maxillary retrusion.
The first two of these factors are the most influential. Figure 11.1 shows a patient with a Class III malocclusion with mandibular prognathism and Fig. 11.2 illustrates maxillary retrognathia (maxillary retrusion).
Class III malocclusions occur in association with a range of vertical skeletal proportions, ranging from increased to reduced. A backward opening rotation pattern of facial growth will tend to result in a reduction of overbite; however, a forward rotating pattern of facial growth will lead to an increase in the prominence of the chin.
11.1.2. Soft tissues
In the majority of Class III malocclusions the soft tissues do not play a major aetiological role. In fact the reverse is often the case, with the soft tissues tending to tilt the upper and lower incisors towards each other so that the incisor relationship is often less severe than the underlying skeletal pattern. This dento-alveolar compensation occurs in Class III malocclusions because an anterior oral seal can frequently be achieved by upper to lower lip contact. This has the effect of moulding the upper and lower labial segments towards each other. The main exception occurs in patients with increased vertical skeletal proportions where the lips are more likely to be incompetent and an anterior oral seal is often accomplished by tongue to lower lip contact.
11.1.3. Dental factors
Class III malocclusions are often associated with a narrow upper arch and a broad lower arch, with the result that crowding is seen more commonly, and to a greater degree, in the upper arch than in the lower. Frequently, the lower arch is well aligned or evenly spaced.
11.2. OCCLUSAL FEATURES
By definition Class III malocclusions occur when the lower incisors are positioned more labially relative to the upper incisors. Therefore an anterior crossbite of one or more of the incisors is a common feature of Class III malocclusions. As with any crossbite, it is essential to check for a displacement of the mandible on closure from a premature contact into maximal interdigitation. In Class III malocclusions this can be ascertained by asking the patient to try to achieve an edge-to-edge incisor position. If such a displacement is present, the prognosis for correction of the incisor relationship is more favourable. In the past it was thought that such a displacement led to overclosure and greater prominence of the mandible, with the condylar head displaced forward. In fact cephalometric studies suggest that in most cases, although there is a forward displacement of the mandible to disengage the premature contact of the incisors as closure into occlusion occurs, the mandible moves backwards until the condyles regain their normal position within the glenoid fossa (Fig. 11.3).
Fig. 11.3. Diagram illustrating the path of closure in a Class III malocclusion from an edge-to-edge incisor relationship into maximal occlusion. Although the mandible is displaced forwards from the initial contact of the incisors to achieve maximal interdigitation, the condylar head is not displaced out of the glenoid fossa.
Another common feature of Class III malocclusions is buccal crossbite, which is usually due to a discrepancy in the relative width of the arches. This occurs because the lower arch is positioned relatively more anteriorly in Class III malocclusions and is often well developed, while the upper arch is narrow. This is also reflected in the relative crowding within the arches, with the upper arch commonly more crowded (Fig. 11.4).
As mentioned above, Class III malocclusions often exhibit dento-alveolar compensation with the upper incisors proclined and the lower incisors retroclined, which reduces the severity of the incisor relationship (Fig. 11.5).
11.3. TREATMENT PLANNING IN CLASS III MALOCCLUSIONS
A number of factors should be considered before planning treatment.
The patient’s opinion regarding their occlusion and facial appearance must be taken into account. This subject needs to be approached with some tact.
The severity of the skeletal pattern both anteroposteriorly and vertically should be assessed. This is the major determinant of the difficulty and prognosis of orthodontic treatment.
The expected pattern of future growth both anteroposteriorly and vertically should be considered. It is important to remember that average growth will tend to result in a worsening of the relationship between the arches, and a significant deterioration can be anticipated if growth is unfavourable. When evaluating the likely direction and extent of facial growth, the patient’s age, sex, and facial pattern should be taken into consideration (see Chapter 4). Children with increased vertical skeletal proportions often continue to exhibit a vertical pattern of growth, which will have the effect of reducing incisor overbite. Obviously for patients on the borderline between different management regimes it is wise to err on the side of pessimism (as growth will often prove this to be correct).
In Class III malocclusions a normal or increased overbite is an advantage, as a vertical overlap of the upper incisors with the lower incisors post-treatment is vital for stability.
If the patient can achieve an edge-to-edge incisor position, this increases the prognosis for correction of the incisor relationship.
In general, orthodontic management of Class III malocclusion will aim to increase dento-alveolar compensation. Therefore, if considerable dentoalveolar compensation is already present, trying to increase it further may not be an aesthetic or stable treatment option.
The degree of crowding in each arch should be considered. In Class III malocclusions crowding occurs more frequently, and to a greater degree, in the upper arch than in the lower. Extractions in the upper arch only should be resisted as this will often lead to a worsening of the incisor relationship. Where upper arch extractions are necessary, it is advisable to extract at least as far forwards in the lower arch.
Orthodontic correction of a Class III incisor relationship can be achieved by either proclination of the upper incisors alone or retroclination of the lower incisors with or without proclination of the upper incisors. The approach applicable to a particular malocclusion is largely determined by the skeletal pattern and the amount of overbite present before treatment, as proclination of the upper incisors reduces the overbite (Fig. 11.6) whereas retroclination of the lower incisors helps to increase overbite (Fig. 11.7). A prognosis tracing (see Chapter 6, Section 6.8) may be helpful in deciding between the two approaches (Fig. 11.8).
If the lower arch is moderately crowded, consideration should be given to extracting the lower first premolars to allow the lower labial segment to drop lingually, thereby aiding dento-alveolar compensation. This can result in residual space in the lower arch if fixed appliances are not used.
Additional space for relief of crowding in the upper arch can often be gained by expansion of the arch anteriorly to correct the incisor relationship and/or buccolingually to correct buccal segment crossbites. Therefore, where possible, it may be prudent to delay permanent extractions until after the crossbite is corrected and the degree of crowding is reassessed. Expansion of the upper arch to correct a crossbite will have the effect of reducing overbite, which is a disadvantage in Class III cases. This reduction in overbite occurs because expansion of the upper arch is achieved primarily by tilting the upper premolars and molars buccally, which results in the palatal cusps of these teeth swinging down and ‘propping open’ the occlusion. Therefore, if upper arch expansion is indicated and the over-bite is reduced, fixed appliances should be used to try and limit tilting of the upper molars buccally during the expansion.
Distal movement of the upper buccal segments with headgear to gain space for alignment is inadvisable as this will have the effect of restraining growth of the maxilla. However, in Class III cases with mild to moderate mid-arch crowding, space can be made by a combination of forward movement of the incisors as well as some distal movement of the remaining buccal segment teeth. This can be accomplished by using a removable appliance with a screw positioned at the site of crowding or with fixed appliances.
Another approach is to use a functional appliance, but it is difficult for patients to posture posteriorly to achieve an active working bite. Therefore functional appliances are less widely used in Class III malocclusions, although they can be useful in mild cases in the mixed dentition where a combination of proclination of the upper incisors together with retroclination of the lower incisors is required.
11.4. TREATMENT OPTIONS
11.4.1. Accepting the incisor relationship
In mild Class III malocclusions, particularly those cases where the overbite is minimal, it may be preferable to accept the incisor relationship and direct treatment towards achieving arch alignment (Fig. 11.9).
Occasionally patients with more severe Class III incisor relationships are unconcerned about their malocclusion, particularly if the remainder of the family have a similar facial appearance. In this situation, and also where a patient is unwilling to undergo the fixed appliance treatment necessary to correct the incisor relationship, treatment can be limited to achieving alignment only.
Sometimes upper arch crowding results in the lateral incisors erupting palatally and the canines buccally. If the upper lateral incisors are markedly displaced then their extraction may make treatment more straightforward (Fig. 11.10). Some patients are happy to accept a smile with the canines adjacent to the central incisors. However, veneers can be used to make the canines resemble lateral incisors more closely.
11.4.2. Proclination of the upper labial segment
Correction of the incisor relationship by proclination of the upper incisors only can be considered in cases with the following features:
a Class I or mild Class III skeletal pattern
the upper incisors are not already significantly proclined
an adequate overbite will be present at the end of treatment to retain the corrected position of the upper incisors, given that a reduction of overbite will occur as the incisors are tipped labially (see Section 11.3 and Fig. 11.6).
If indicated, this approach is often best carried out in the mixed dentition when the unerupted permanent canines are high above the roots of the upper lateral incisors (Fig. 11.11). Extraction of the lower deciduous canines at the same time may allow the lower labial segment to move lingually slightly, thereby aiding correction of the incisor relationship. Early correction of a Class III incisor relationship has the additional advantage that further forward mandibular growth may be counterbalanced by dento-alveolar compensation (Fig. 11.12).
Later in the mixed dentition, when the developing permanent canines drop down into a buccal position relative to the lateral incisor root, there may be a risk of resorption if the incisors are moved labially. In this situation correction is best deferred until the permanent canines have erupted.
Where the upper labial segment is mildly crowded, permanent extractions should be delayed until after the incisor relationship is corrected as proclination of the upper incisors will provide additional space. If the lower arch is at all crowded, consideration should be given to relieving the crowding by extractions as this will allow some lingual movement of the lower labial segment teeth.
Proclination of the upper labial segment can often be accomplished successfully with a removable appliance, particularly as buccal capping can be incorporated to free the occlusion with the lower arch. A screw type design is particularly useful in the mixed dentition as then the upper incisors can be utilized for retention of the appliance (see Chapter 16). Fixed appliances can also be
used to advance the upper labial segment and are useful when other features of the malocclusion dictate their use.
11.4.3. Retroclination of the lower labial segment with or without proclination of the upper labial segment
In those cases with a mild to moderate Class III skeletal pattern, or where there is a reduced overbite, a combination of retroclination of the lower incisors and proclination of the upper incisors will achieve correction of the incisor relationship (see Fig. 11.8). Although the pitfalls of significant movement of the lower labial segment have been emphasized in earlier chapters, in the correction of Class III malocclusions the positions of the upper and lower incisors are changed around within the zone of soft tissue balance and, provided that there is an adequate overbite and further growth is not unfavourable, the corrected incisor relationship has a good chance of stability. Although removable and functional appliances can be used to advance the upper incisors and retrocline the lower incisors, in practice these tooth movements are accomplished more efficiently with fixed appliances.
Space is required in the lower arch for retroclination of the lower labial segment, and extractions are required unless the arch is spaced naturally. Use of a round archwire in the lower arch and a rectangular arch in the upper arch along with judicious space closure can be used to help correct the incisor relationship (Fig. 11.13).
Intermaxillary Class III elastic traction (see Chapter 15, Section 15.6.1) from the lower labial segment to the upper molars (Fig. 11.14) can also be used to help move the upper arch forwards and the lower arch backwards, but care is required to avoid extrusion of the molars which will reduce overbite.
Reverse-pull headgear, also known as a face-mask (Fig. 11.15), is used to apply an anteriorly directed force, via elastics, on the maxillary teeth and maxilla. Although some have claimed that this appliance can change the position of the maxilla, a very cooperative patient is necessary in view of the prolonged daily wear required, often over several years. Nevertheless, this technique is occasionally useful in the management of Class III malocclusions, particularly those associated with a cleft lip and palate anomaly, and also in cases of hypodontia where forward movement of the buccal segment teeth to close space is desirable.
Correction of a Class III malocclusion by retroclination of the lower incisors and proclination of the upper incisors using fixed appliances with relief of crowding by the extraction of all four first premolars: (a) pretreatment; (b) fixed appliances in situ. (note the use of rectangular archwire in the upper arch and a round wire in the lower arch during space closure to help achieve the desired movements); (c) post-treatment result.
In some cases the severity of the skeletal pattern and/or the presence of a reduced overbite or an anterior open bite precludes orthodontics alone, and surgery is necessary to correct the underlying skeletal discrepancy. It is impossible to produce hard and fast guidelines as to when to choose surgery rather than orthodontics, but it has been suggested that surgery is almost always required if the value for the ANB angle is below – 4° and the inclination of the lower incisors to the mandibular plane is less than 83°. However, the cepahalometric findings should be considered in conjunction with other features of the malocclusion and the patient’s facial appearance.
For those patients where orthodontic treatment will be challenging owing to the severity of the skeletal pattern and/or a lack of overbite, a surgical approach should be explored before any permanent extractions are carried out, and preferably before any appliance treatment. The reason for this is that management of Class III malocclusions by orthodontics alone involves dento-alveolar compensation for the underlying skeletal pattern. However, in order to achieve a satisfactory occlusal and facial result with a surgical approach, any dento-alveolar compensation must first be removed or reduced (Fig. 11.16). For example, if
lower premolars are extracted in an attempt to retract the lower labial segment but this fails and a surgical approach is subsequently necessary, the presurgical orthodontic phase will probably involve proclination of the incisors to a more average inclination with reopening of the extraction spaces. This is a frustrating experience for both patient and operator.
Severe Class III malocclusion with dento-alveolar compensation. (b) Without reduction of the dento-alveolar compensation, surgery to produce a Class I incisor relationship will only achieve a limited correction of the underlying skeletal pattern, thus constraining the overall aesthetic result. (c) Decompensation of the incisors to bring them nearer to their correct axial inclination allows a complete correction of the underlying skeletal pattern.
Some patients with marked skeletal III malocclusions are unwilling to wear appliances. Management by surgery alone is unsatisfactory as the resulting occlusion is poor, and in addition a full correction of the underlying skeletal problem is not possible without dento-alveolar decompensation. Therefore patients should be encouraged to undergo the appliance therapy necessary for the best result.
An example of a patient treated by a combination of orthodontics and surgery is shown in Fig. 11.17. Surgical approaches to the correction of Class III malocclusions are considered in Chapter 20.
PRINCIPAL SOURCES AND FURTHER READING
Battagel, J. M. (1993). Discriminant analysis: a model for the prediction of relapse in Class III children treated orthodontically by a non-extraction technique. European Journal of Orthodontics, 15, 199–209.
Battagel, J. M. (1993). The aetiological factors in Class III malocclusion. European Journal of Orthodontics, 15, 347–70.
Battagel, J. M. and Orton, H. S. (1993). Class III malocclusion: the post-retention findings following a non-extraction treatment approach. European Journal of Orthodontics, 15, 45–55.
Bryant, P. M. F. (1981). Mandibular rotation and Class III malocclusion. British Journal of Orthodontics, 8, 61–75.
This paper is worth reading for the introduction alone, which contains a very good discussion of growth rotations. The study itself looks at the effect of growth rotations and treatment upon Class III malocclusions.
Dibbets, J. M. (1996). Morphological differences between the Angle classes. European Journal of Orthodontics, 18, 111–18.
Gravely, J. F. (1984). A study of the mandibular closure path in Angle Class III relationship. British Journal of Orthodontics, 11, 85–91.
A very readable and clever article which examines the displacement element of Class III malocclusions.
Kerr, W. J. S. and Tenhave, T. R. (1988) A comparison of three appliance systems in the treatment of Class III malocclusion. European Journal of Orthodontics, 10, 203–14.
Kerr, W. J. S., Miller, S., and Dawber, J. E. (1992). Class III malocclusion: surgery or orthodontics? British Journal of Orthodontics, 19, 21–4.
An interesting study which compares the pretreatment lateral cephalometric radiographs of two groups of Class III cases treated by either surgery or orthodontics alone. The authors report the thresholds for three cephalometric values which would indicate when surgery is required.
Kim, J. H. et al. (1999). The effectiveness of protraction face mask therapy: a metaanalysis. American Journal of Orthodontics and Dentofacial Orthopedics, 115, 675–85.