Orthodontic Conundrums Part 3: Indications for the loss of a lower incisor

Mild to moderate Class III malocclusions

The most frequently reported indication is a mild to moderate Class III malocclusion, where the incisors are edge-to-edge, or in anterior crossbite with minimal or reduced overbite.^3,9 The decrease in mandibular intermolar, intercanine and overall arch width following extraction of a lower incisor is beneficial, particularly where the overbite is reduced.^9–11 In these cases the lower incisors should ideally be relatively parallel sided as wide or open interproximal gingival embrasure spaces can result in unaesthetic ‘black triangles’ following space closure.^12,13 Patients must be warned and consented for these potential consequences prior to treatment commencing. In a Class III malocclusion with severe lower incisor crowding, the loss of a single lower incisor may not provide sufficient space for full correction of the malocclusion as most of the space created would be consumed with alignment of the teeth before correction of the incisal relationship can be achieved.^5,9

If a tooth size discrepancy is suspected it may be confirmed with a Bolton's analysis (the percentage mesiodistal width of the lower teeth relative to the upper). In ‘normal’ instances the sum of the widths of the lower anterior teeth divided by the sum width of the upper anterior teeth is 0.772 or 77.2%.¹⁴ An extraction that aids in the correction of a Bolton discrepancy has the potential to greatly improve aesthetics.

In Case 1, a lower incisor extraction was used to relieve moderate lower anterior crowding, and create a positive overjet and overbite in a patient with a Class III incisor relationship. This patient was planned to finish to a Class III canine relationship on the left side while maintaining coincident centrelines and Class I molars (Figure 1a, b).

Compromised lower incisor teeth

Ectopic, periodontally compromised, heavily restored, traumatized, non-vital or malformed lower incisors may prove good candidates for extraction when orthodontic treatment is planned.^{5,10,11,15,16}

The first article in this series considered dental transpositions. If a lower incisor is transposed to a position too aberrant to make orthodontic correction feasible, extraction of this tooth may be sensible. This may avoid the lengthy treatment required to align multiple, severely displaced teeth, reduce the risk of gingival recession and decrease the risk of significant post-treatment relapse.¹¹Figure 2 shows a case of pseudo-transposition, where the position and compromised periodontal support of the lower right central incisor made it a strong candidate for extraction.

Gingival recession may jeopardize the long-term prognosis of a lower incisor and provide an indication for its extraction. The adult patient in Case 3 presented complaining of thermal sensitivity and was found to have gingival recession affecting the lower right central incisor (Figure 3a–c). Loss of this tooth improved the patient's symptoms and dental aesthetics and left her with an acceptable overjet. In patients with Class I buccal segments and no tooth size discrepancy it is prudent to anticipate a residual increased overjet and overbite, as well as non-coincident centrelines, when this extraction pattern is chosen. Again, patients must be informed of these consequences prior to extraction and the commencement of treatment.

In Case 4 (Figure 4a–c), a 13-year-old patient presented with a Class III malocclusion, with severe crowding and an absent lower left second premolar. The initial plan involved the loss of the upper first premolars and the lower left second deciduous molar, but after initial alignment there was evidence of recession around the lower right lateral incisor, and so this tooth was extracted.

Finally, hard tissue damage to a lower incisor from trauma can be another indication for elective removal. In Case 5 (Figure 5a–d) a traumatized lower left central incisor of poor prognosis was extracted as part of the orthodontic treatment plan. One must be aware that extraction of a lower incisor may constrict the archform and can cause a transverse discrepancy. In this case the transverse discrepancy was corrected using lower archwire expansion and unilateral posterior cross elastics (Figure 5e).

Relative lower tooth size excess

Relative lower tooth size excess in a Class III malocclusion has already been presented as a possible indication for loss of a lower incisor (Figure 1a, b). Extraction of this tooth may also be a valid approach where an anterior tooth size discrepancy, confirmed using a Bolton analysis, exists in a patient with a Class I malocclusion, and there is a desire to maintain the antero-posterior buccal segment relationship.

In Case 6 an adult patient presented with a Class I malocclusion, missing upper lateral incisors and a crossbite associated with the lower left canine, first premolar, and first molar (Figure 6a). The missing upper lateral incisors gave rise to an obvious Bolton discrepancy, but with reasonable buccal interdigitation there was no indication for loss of premolar units.

This case was planned with the aid of a Kesling set-up. Kesling described a process where teeth are sectioned from the base of a study model, and then repositioned onto the bases in wax.¹⁷ This allows visual assessment of both the teeth in their new positions and the distribution of space.¹⁸ A Kesling set-up not only aids in treatment planning, but provides a useful tool to help demonstrate the intended treatment aims to the patient, thereby making the consent process more robust.^19,20 Kesling set-ups are more useful still, in those cases that may be treated through lower incisor extraction, or the less destructive approach of interproximal stripping. Here they facilitate visual assessment of the suitability of the two available approaches.

To fabricate the Kesling set-up in this case, the lower left lateral incisor, and adjacent teeth, were removed from a plaster cast of the lower arch. The teeth were then replaced aligned in wax, with the exception of the lower left lateral incisor, which was being considered for extraction. Using a similar approach, the upper left canine was removed and replaced in an improved position. The intended outcome of treatment could then be assessed by the orthodontist and also presented to the patient (Figure 6b, c).

Extraction of the lower left lateral incisor and good wear of intermaxillary elastics allowed for efficient alignment of the teeth and correction of the transverse discrepancy (Figures 6d, e). The choice of extraction pattern in this case, with its fanned lower incisors, was also sparing on the high anchorage demands in correcting the alignment and crossbites.

Mandibular tooth size–arch length discrepancy

In Class I patients, the presence of localized lower labial segment crowding and insufficient space available for all four lower incisors, may be an indication for a single lower incisor extraction. Reports in the literature suggest this may be appropriate when the mandibular tooth size/arch length discrepancy exceeds 5 mm, ie the approximate width of a lower incisor.^3,5,11 The alternative of producing excessive proclination with fixed appliances may have a detrimental effect on the periodontal support for the lower incisors, as they may be moved outside the alveolar bone. Furthermore, in these situations, extraction of a lower incisor may be preferable to the loss of posterior teeth, which may provide excess space and negatively impact the posterior occlusion. A similar indication may present when localized lower labial segment relapse occurs following completion of an earlier course of orthodontic treatment with ineffective retention (Figure 8).

In these scenarios, extraction of a lower incisor does, however, have the potential to create an anterior Bolton discrepancy and interproximal stripping of the maxillary anterior teeth may help to restore normal proportions as necessary.^10,21

Case 7 is an adult patient who presented with a Class I malocclusion and problems including the lower left canine in crossbite, lower incisor crowding, and recession associated with the lower left central incisor (Figure 7a). The loss of this compromised tooth and treatment with fixed appliances gave a favourable outcome without disruption to the posterior occlusion (Figure 7b).

The patient in Case 8 had a history of orthodontic treatment as a child that involved extraction of the upper first premolars (Figure 8a, b). Significant post-treatment relapse, localized to the lower labial segment, was evident. Alignment of these teeth on a non-extraction basis would most likely have worsened the periodontal recession, so extraction of the lower right lateral incisor was planned. A bonded retainer was fitted to reduce future relapse and space re-opening following treatment.

Disadvantages and contraindications for lower incisor extraction

Some authors have considered an increased overjet to be a contraindication for extraction of a lower incisor.^11,16 However, in growing patients, mechanics to correct the Class II relationship could be used alongside extraction of a lower incisor as required. In adult patients with a Class II division 1 malocclusion, crowded or proclined lower incisors may be managed through extraction of a lower incisor and upper premolars.¹⁵ Alternatively, where surgical advancement of the mandible is required, extraction of a lower incisor alone may provide the space required for alignment prior to surgery.²²

Lower incisor extraction has the potential to increase the overbite, therefore it may be contraindicated where there is an existing increased overbite.^15,16 It may also be contraindicated in cases with an anterior tooth size discrepancy, where the upper incisors are broad and the lower incisors are narrow. Here, loss of a lower incisor would further increase the discrepancy and overall case complexity.

Loss of a single lower incisor may result in midline discordance. Some patients may find this unaesthetic and so the consent process should include discussion of this outcome.

Where the lower incisors are triangular in form, and there is minimal crowding, extraction risks creating open gingival embrasure spaces or ‘black triangles’. In such instances, interproximal stripping may be a more appropriate means of creating space. Even in cases where lower incisor extraction is undertaken, interproximal stripping may help to move the contact point closer to the crest of the alveolar bone making complete infill of the interdental space with papilla more likely.

Tarnow et al. found a distance greater than 5 mm, between the contact point and crest of alveolar bone, to be associated with loss of the interdental papillae and consequently development of ‘black triangles’.¹² Research suggests orthodontic treatment may cause a loss of alveolar crestal bone height,¹³ and in older patients, where some alveolar bone may already have been lost to chronic periodontal disease, one must approach these extractions with caution. However, age itself does not appear to predict the likelihood of ‘black triangles’ developing.²³ Other possible contraindications include, poor buccal segment relationship, mesially angulated canines, mild (less than 4 mm) or severe (8 mm or over) localized lower labial segment crowding, or a high lower labial frenum in the region of the planned extraction site.

Retention protocols

Some reports in the literature suggest cases involving the extraction of a lower incisor may be more stable than those that used a premolar extraction pattern.^7,11 Conversely, other researchers have highlighted the possibility of space re-opening in the aesthetic zone and advocate long-term fixed retention.⁹ In light of the limited evidence supporting any single retention protocol,²⁴ and the risk of aesthetic compromise posed by relapse in the anterior segment, the authors advocate provision of a lower bonded retainer in cases involving the loss of a lower incisor.

Conclusion

Lower incisor extraction can be an effective approach in carefully selected cases, despite loss of this tooth making achievement of an ‘ideal’ occlusion somewhat more difficult.

This extraction pattern presents a pragmatic and rational approach, especially where a single lower incisor is compromised and of poor prognosis, and is an effective tool in the orthodontist's armamentarium.