Introduction

Class II subdivision malocclusion is a common orthodontic presentation characterized by the presence of asymmetric molar relationships. The etiology of Class II subdivision is varied involving asymmetric dental positions, a skeletal asymmetry, or a combination of these (Cassidy et al. 2014). The majority of subdivisions, however, appear to have posterior positioning of the mandibular molar being contributory often reflecting an underlying skeletal issue (Alavi et al. 1988; Rose et al. 1994; Janson et al. 2001; Azevedo et al. 2006). Moreover, aberrant positioning of the glenoid fossa allied to the presence of a functional displacement may also be contributory (Li et al. 2015).

Dental Arch Form

Dental arch form asymmetry may involve the presence of asymmetric crowding or spacing due to underlying dento‐alveolar disproportion. This may also be aggravated by the presence of hypodontia, or the premature loss of primary teeth. Asymmetric crowding in the maxillary arch may, for example, introduce associated space loss with mesial movement of the contralateral maxillary molar and deviation of the maxillary midline to the same side. Correction may therefore necessitate space creation on the contralateral (Class II) side in order to address the maxillary midline shift.

Up to 60% of subdivisions involve mandibular midline shift, in isolation, with 20% having upper midline deviation alone and the remaining 20% mixed characteristics (Janson et al. 2007). Mandibular midline shift may present due to uneven mandibular space conditions with the midline deviating to the crowded quadrant. As such, space creation in the side away from which the midline has shifted (Class I) may be considered. Bilateral removal of maxillary bicuspids may be considered in order to preserve the pre‐existing molar relationships (Janson et al. 2017).

Mandibular asymmetry is a common finding in cases with asymmetric molar relationships and predisposes to the deviation of the lower dental midline to the side to which the chin‐point is deviated. While more severe skeletal asymmetry may be associated with vertical elements and may necessitate a combined orthodontic‐surgical approach, the occlusion can be corrected by tooth movement in the presence of milder skeletal asymmetry. Extractions may be required in both arches in order to provide sufficient space for lower midline correction while also permitting molar correction and the achievement of Class I incisor relationship. Non‐extraction approaches may also be used particularly in the presence of limited lower arch crowding (Akın et al. 2019).

It is important to appreciate that Class II subdivision may present a complex array of features not limited to midline shift and/or molar asymmetry. In particular, vertical issues and asymmetric positioning of the canines are to be expected. Moreover, arch form changes including transverse mismatch are common findings. During orthodontic treatment in symmetric situations, increases in arch dimensions may be planned to alleviate crowding, to address transverse discrepancies, and to a lesser extent alter smile esthetics (Fleming et al. 2008). In the presence of subdivisions, the transverse dimension may require careful management in order to maintain optimal arch coordination, particularly in the canine region.

In this chapter, case reports of two patients with Class II subdivision are presented and compared. Neither of the cases presented with a facial asymmetry that might indicate a combined orthodontic‐surgical approach. The malocclusions were both Class II subdivision and allied goals of maintaining a Class II molar relationship on the subdivision side, neutral canine relationship, and coincident dental midlines. Both cases were treated with one premolar extraction on the Class II side. The influence of the arch form and the canine position on the appropriate planning of patients exhibiting Class II subdivision will be emphasized.

Case 1

An adult male presented with a Class II subdivision on a mild skeletal Class II pattern with mandibular retrognathia. There was no obvious mandibular asymmetry. Lips were competent with normal soft tissue protrusion. Both upper and lower arches were crowded with palatal displacement of the maxillary left lateral incisor. The maxillary canines were both buccally positioned. The upper midline was displaced to the Class I (neutral) side and the lower midline coincided with the facial midline (Figures 12.1.1–12.1.3).

One upper premolar (#14) was extracted on the Class II side in order to provide space to address the maxillary midline deviation while correcting the Class II canine relationship on the right side. The Class II molar relationship on the right side was to be preserved (Figure 12.1.4). Following canine retraction, the maxillary midline was corrected and space generated for the correction of the palatally‐displaced lateral incisor (Figure 12.1.5). Alignment of the upper left lateral incisor was accomplished with an overlay arch (0.016‐inch NiTi) (Figure 12.1.6