Nangia A, Darendeliler MA. Finishing occlusion in Class II or Class III molar relation: therapeutic Class II and III. Aust Orthod J. 2001; 17:89-94
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Andrews LF. The six keys to normal occlusion. Am J Orthod Dentofac Orthoped. 1972; 62:296-309
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This is the second article in a series on conundrums in orthodontics. A Class I molar relationship is a frequent aim of orthodontic treatment. There are, however, examples where intentionally finishing to a Class III molar relationship is the more pragmatic and preferable option. Pursuing this approach in the appropriate circumstances may, for example, prevent deleterious consequences to the facial profile or avoid the need for further dental extractions. This article explores the indications for finishing cases to a Class III molar relationship with illustrated clinical cases.
CPD/Clinical Relevance: Dogmatic attempts to deliver Class I molar relationships, in all cases, without consideration of the consequences, have the potential to cause the patient harm. The orthodontist must then consider all possible options available, including those resulting in a Class III molar relationship.
Article
In this second paper on conundrums in orthodontics, we explore the uncommon approach of finishing treatment to a Class III molar relationship. A Class I molar relationship is often a key aim of treatment, but there are instances where dogmatic attempts to deliver this may beunwise because they have the potential to cause harm. Orthodontic treatment may intentionally culminate ina molar relationship that is either a full unit Class II or Class III.1,2 There are severalreasons why this may be the preferred option, and frequently, it is theresult of pragmatic treatment planning. While finishing to a Class II molar relationship is relatively common, this is not true for Class III molarrelationships. The latter is the focus of this article and here we will consider the indications for finishing cases to Class III molar relationship and illustrate these with clinical cases.
Over a century ago Edward Angle published his classification of malocclusion.3 In this seminal work, he described a Class I relationship as occuring when the mesiobuccal cusp of the maxillary first molar occludes with the buccal groove of the mandibular first molar. This molar relationship was to be considered ‘normal’ and therefore ideal. In contrast, a Class III molar relation occurs where the mesiobuccal cusp of the maxillary first molar lies one unit posteriorly to the Class I position.
Decades later, Andrews4 described the six keys of an ‘ideal’ static occlusion based on observations in 120 non-orthodontic cases with ‘normal’ occlusion. These were: correct molar relation; correct crown angulation; correct crown inclination; tight interproximal contacts; absence of rotations; and a flat occlusal plane. Most orthodontic treatment aims to provide patients with an occlusion that conforms to these six keys while also providing optimal dental and facial aesthetics; however, the literature suggests this is rarely the orthodontic treatment aim of choice.5
Problems with a Class III molar relationship
There is good reason why the Class I molar relationship has long been considered as the ideal. This arrangement, when combined with a Class I canine and incisor relationship, has the greatest potential to deliver a functional and harmonious static and dynamic occlusion.6,7 In a Class I relationship, the mandibular teeth are a half cusp width in front of the maxillary teeth in the intercuspal position (centric occlusion). This results in all teeth, except for the mandibular central incisors and maxillary third molars, occluding with two antagonists.2,8
Over the long term, the majority of orthodontic cases will develop some form of relapse.9 In a Class III molar relationship, there is a decrease in the number of contact points between the upper and lower arches and this may compromise occlusal stability.10 Furthermore, where a Class III molar relationship has been an intentional aim of treatment, and is accompanied by a Class I canine relationship, there is a tendency towards a cusp to cusp relationship between the maxillary second premolar and the mandibular first molar. Consequently, there is less intercuspation of teeth in static occlusion and a propensity for interferences in dynamic occlusion.2
Occlusal interferences, and their impact on temporomandibular dysfunction (TMD) have been explored in the literature, with research finding occlusal interferences to play, at most, only a minor role in the aetiology of TMD.7,11 However, finishing cases to a Class III molar relationship is still considered ’less than ideal’ by some, and is only attempted after careful evaluation of the malocclusion and consideration of all the available treatment alternatives.
Indications: congenitally absent lower second premolars
Apart from the third molars, mandibular second premolars are the most commonly congenitally absent teeth in the dentition.12 Where they are absent, and the maxillary teeth are all present and relatively well-aligned, a treatment plan that avoids maxillary extractions and closes space in the mandible through mesial movement of the buccal segments, may be appropriate. This is all the more pragmatic when the patient has a facial profile with limited upper lip support.13,14,15 Such a situation is demonstrated in Figures 1 and 2, where the retained primary molars were extracted, the resultant space was used to correct the lower labial segment crowding, and the case finished to Class III molar relation bilaterally.
Figure 1. (a–e) Case 1: a patient with congenitally absent lower second premolars, and a relatively well-aligned maxillary arch.Figure 2. (a–e) Case 1 on completion of treatment showing Class III molar relation.
This approach should be undertaken with caution because the planned mesial movement of the lower first molars is, of course, accompanied by reciprocal retroclination of the lower labial segment, which, in itself, can detrimentally impact on the facial profile.2
Indications: skeletal Class III with proclined lower incisors
Where patients have a mild Class III malocclusion, with proclination of the lower labial segment in the presence of an uncrowded or mildly crowded maxillary arch, there may be an opportunity to camouflage the case with lower arch extractions only, finishing to Class III molars. Certainly in the presence of maxillary retrognathia, extraction of upper premolars, followed by attempts to mesialize the upper first molars into a Class I position can be detrimental to an already compromised facial profile.14
Figure 3 shows a Class III skeletal and incisor relationship, proclined lower incisors and mild crowding in the upper arch. A Class I molar relationship was present prior to the commencement of treatment; however, space was required to create a positive overjet and it was decided to obtain this through the extraction of the lower second premolars. The upper arch was treated on a non-extraction basis. The outcome, shown in Figure 4, also illustrates the benefits of this treatment in improving the lower lip prominence.
Figure 3. (a–e) Case 2: a Class III incisor relationship on a Class III skeletal base with moderate upper arch crowding. Note the Class I molar relation on presentation.Figure 4. (a–e) Case 2 after treatment, with Class I incisors and canines, and a class III molar relationship.
Indications: skeletal class II with no indication for upper extractions
Orthodontic treatment must consider the patient as a whole and bear in mind the negative effects on the facial profile that can result from extractions. Figures 5 and 6 show a patient presenting with a Class II division 1 incisor relationship with moderate crowding in the lower arch. The narrow upper arch had no crowding, adequate upper lip support, and the upper incisors were not significantly proclined. These salient features meant there was the need for extraction of lower premolars to provide space for relief of crowding and correction of a centreline shift, but no such indication for upper arch extractions. Furthermore, when one considers the facial profile in this case (Figure 8), extraction of upper teeth and loss of support to the upper lip would have been detrimental given the increased naso-labial angle at the outset.15,16
Figure 5. (a–e) Case 3: A Class II division 1 incisor relationship, narrow upper archform, and lower labial segment crowding.Figure 6. (a–e) Case 3 midway through treatment. Note the space created through extractions in the lower arch, with no compensating extractions in the upper arch.Figure 7. (a–e) Case 3 at the end of treatment. The overjet and crowding have both been corrected and the buccal segments occlude in Class III molar relation.Figure 8. (a, b) The pre- and post-treatment profile views of case 3 showing profile improvement.
Figure 6 shows the patient mid-treatment, following A-P correction and anchorage management with a Twin Block functional appliance. At this stage the narrow upper arch has been expanded and there has been an asymmetric lower arch extraction to allow for centreline correction.
On completion of orthodontic treatment, the patient has a well-aligned Class I occlusion with a bilateral Class III relationship (Figure 7) and, as well as correcting the salient features of the presenting malocclusion, this pragmatic approach to treatment planning has avoided negative effects on the facial profile and an increase of the naso-labial angle (Figure 8).
Indications: lower incisor decompensation before orthognathic surgery in a Class II skeletal relationship
The pre-surgical orthodontic treatment for a patient undergoing orthognathic surgery typically involves dental decompensation. The extent of decompensation directly determines the extent of surgical correction that is possible as the occlusion is used as the guide for surgical movement.17Figure 9 shows a case with Class II skeletal discrepancy and proclined and crowded lower incisors that was planned for a mandibular osteotomy.
(a–e) Case 4: a Class II skeletal relationship before decompensation in anticipation of orthognathic surgery.
The space to decompensate the lower incisors was created through extraction of the lower second premolars. The mild crowding in the upper arch was used to procline these teeth and decompensate on a non-extraction basis. The loss of two compensatory premolars in the upper arch, with the principle aim to deliver a Class I molar relationship, would have involved bimaxillary surgery rather than a single jaw osteotomy. The dental outcome following completion of both orthodontic and surgical treatment phases is shown in Figure 10.
Figure 10. (a–e) Case 4 after completion of orthodontic and surgical treatment. A Class III molar relationship has been established.
Conclusion
Orthodontic treatment must always be undertaken in the patient's best interests. It is our role as orthodontists to ensure that treatment aims involve correcting the malocclusion without negative consequences to the facial profile. There are long-established ideals for orthodontic treatment outcomes, including finishing to a Class I molar relationship; however, these ideals can often result in less-than-ideal patient outcomes in terms of facial aesthetics. In selected cases, such as the ones illustrated here, finishing to a Class III molar relationship is certainly the treatment of choice.
