Management of Lower Incisor Extraction Cases. Part 1: Case Selection and Planning

Case selection

A lower incisor of poor prognosis

There are many reasons why the prognosis of a lower incisor may be compromised. Although it may result in more challenging treatment, it may be prudent to consider removal of a tooth that has a poor long-term prognosis. In some cases, more than one lower incisor may be severely affected. If two incisors require removal, planning treatment to leave space that is the equivalent of three lower incisors means that, potentially, only one long-term restoration is needed, rather than two.

Reasons for a lower incisor having a poor prognosis, and therefore the extraction of choice, include:

Severe tooth wear;

Dilaceration (Figure 1);

Caries/heavily restored tooth;

Periodontal damage (Figure 2);

Previous trauma (Figure 3);

Gingival recession (Figure 4);

Transposition of a lower incisor with a canine (Figure 5);

Ectopic position of a lower incisor.

Crowding up to 6mm localized to the lower labial segment

This can present early in adolescence either with a severely displaced or excluded incisor, as shown in Figure 6. An incisor may also be associated with localized gingival recession. Extraction of two premolars would require significant distal tooth movement to open sufficient space before being able to align the excluded incisor. Moving teeth a substantial distance increases treatment time and the risk of relapse of the severely displaced tooth.²

Taking the pragmatic approach of extracting a lower incisor provides an alternative treatment plan in cases with moderate lower anterior crowding, especially when a short treatment time is important. It may also be useful in cases with limited objectives, for example for patients with complex medical conditions (Figure 7).

Late lower incisor crowding in adult patients can present de novo, or as a relapse from previous treatment that may have involved premolar extractions. Crowding greater than 3 mm would require substantial proclination or interproximal reduction to create space for alignment. In patients who have had previous premolar extractions, losing a further premolar bilaterally is not a feasible option because it will leave a canine–molar contact, and may result in too much space. By extracting a lower incisor, space is provided closer to the site of crowding and, therefore, aligning the teeth should not take too much time. If the lower incisor crowding is moderate and buccal segments are Class I, then it may be possible to retain this fit. If the case has relapsed from a previous non-extraction treatment plan, then avoiding the loss of lower premolars may also prevent the need to extract in the upper arch.

Which incisor to choose?

The choice of which lower incisor to extract will depend on factors, such as the site of crowding, position, tooth morphology and prognosis. Ideally, it would be better to select a tooth for extraction that is already compromised and of poor prognosis, or is less favourably placed (Figure 8). When all incisors are of equal prognosis, extraction of the tooth nearest to the site of greatest crowding is a sensible option. Uribe et al³ found that loss of a lower lateral incisor has less risk of black triangle formation compared with lower central incisor extraction: 41% of patients exhibited an open gingival embrasure following extraction of a central incisor, compared to 27% in patients who had a lateral incisor extraction.³

Upper canines and a lower incisor

Choosing to extract upper canines, which are generally considered to ‘define the character of a smile’,⁴ is an atypical extraction pattern, but may be a pragmatic choice in severely crowded cases with a 2–4 contact, or when there are ectopic canines that present in a poor position for alignment. Upper first premolars have been shown to aesthetically replace upper permanent canines well, without affecting the attractiveness of a smile.⁵

As upper canines are generally 1–2 mm wider than upper first premolars, extracting them reduces the overall mesio-distal length of the upper labial segment. In a crowded lower labial segment, a single lower incisor extraction will result in a concomitant reduction in the lower labial segment and can improve the fit, as shown in Figure 9.

Class III camouflage

The extraction of a lower incisor for treatment of a Class III malocclusion was popularized by Færøvig and Zachrisson in 1999.⁶ When the Class III malocclusion is mild and there is a reduced overbite, extracting a lower incisor facilitates retroclining the lower labial segment, enabling improvement in the overjet and deepening of the overbite. Optimizing axial inclinations of each lower incisor and use of mesio-distal enamel reduction can also be carried out in maxillary or mandibular arches to idealize the appearance of the final occlusal result and to retain the interproximal gingivae.⁶

This option is more suited to adult patients where there is limited further growth potential and where there is only minimal lower anterior crowding. The available space can then be used to camouflage the Class III incisor relationship rather than for resolution of crowding (Figure 10).

Surgical treatment

During pre-surgical decompensation for Class III patients, normally the lower incisors are proclined and the upper incisors are retroclined to idealize tooth inclinations and reverse overjet, to allow an appropriate surgical movement, and therefore facial change. This reduces the radial width of the upper front teeth and increases that of the lower labial segment; arch co-ordination therefore proves more challenging, particularly in cases where the upper lateral incisors are smaller than average.

When there is moderate lower arch crowding, fully decompensating the lower incisors can create a substantial reverse overjet, which requires significant surgical movements to the maxilla and/or the mandible. In some cases, a larger surgical movement may compromise surgical stability and produce unfavourable facial changes. Loss of a lower incisor can address both these issues (Figure 11), particularly when there is a thin biotype where decompensating the lower labial segment fully could risk pushing the incisors out of the bony envelope.

In some Class II cases where extractions have taken place, crowding and proclination may remain. For full correction, lower arch pre-surgical decompensation requires incisor uprighting. Extracting a lower incisor can resolve the localized crowding without further proclination, as well as providing space to upright and maximize the overjet required for mandibular advancement and ideal facial change.

Hypodontia: missing lower central incisors

A lower central incisor is the fourth most commonly missing tooth.⁷ When this tooth is developmentally absent, it causes similar problems to extracting a lower incisor. If both lower central incisors are missing, it may pose a restorative problem because two adjacent sites are difficult to restore with a bridge, and often, bone and soft tissue morphology is poor for implant placement. A decision can be made to approximate the two incisors and leave the residual spaces in the lateral incisor position because the bounded sites are more easily restored. Alternatively, reducing the space to a single lower incisor space and treating to three incisors makes the space more readily restorable (Figure 12), but the effect of retraction of the lower labial segment needs to be taken into consideration.

Aligner treatment

The rise in popularity of clear aligners, alongside improvements in the outcomes for leading systems, has led to an increase in patient demand for this aesthetic treatment option. Many of these systems were initially used for non-extraction treatment using interproximal enamel reduction where small amounts of space were required.⁸ In cases with more than 3 mm of crowding, aligning teeth on a non-extraction basis even with interproximal reduction, has a potential risk of both instability and damage to the periodontal attachment. Gingival dehiscence can occur if teeth are pushed outside the normal bony and soft tissue envelopes. With localized lower incisor crowding of 3–6 mm, aligners can be considered a potential satisfactory alternative to fixed appliances, with extractions or interproximal reduction.

With many of the systems available, simulated treatment planning can be visualized with a digital set-up that provides information about the potential detrimental effects on the radial fit of the teeth. Additionally, it allows patients to see how their teeth might look with three incisors (Figure 13). This improves the informed consent process when a lower incisor is selected as the extraction of choice.

One of the potential issues with aligner-based treatments in extraction cases is that they provide adequate crown control, but frequently less root control, which can lead to tipping of teeth. If adjacent incisor teeth are tipped substantially into a lower incisor space, this can magnify the potential for an unsightly black triangle at the gingival margin. Using appropriate long vertical attachments to provide root uprighting during space closure can minimize, but often not completely remedy this. This is more likely to occur in patients with long clinical crowns, reduced periodontal support and triangular-shaped teeth.

Diagnostic set-up

There are a number of potential pitfalls for extracting a lower incisor that are noted in part two of this series. As such, it is very helpful for a clinician to visualize the three-dimensional effect of extracting a lower incisor on the occlusion and aesthetics. This also allows the clinician to plan appropriate methods for improving the radial fit and aesthetics. From a patient's perspective, being able to visualize how the centreline will look with three incisors and what the final fit of the teeth could look like, is important for informed consent, particularly if it results in a compromised end result.

Plaster models can be duplicated and teeth repositioned into a diagnostic ‘Kesling’ set-up (Figure 14). This process can be time consuming and requires laboratory input. With many practices now routinely using digital scanners for image capture, rather than impressions, most systems have software that allow for removal and alignment of teeth, as well as colour rendering, making the potential plan more realistic for the patient to view. Kockich and Shapiro⁹ highlighted the importance of this by using diagnostic wax-ups and a Bolton analysis to preview potential changes, but to also decide on how much interdental stripping was required.

When using aligner-based treatments, most systems have integral software allowing the clinician to prescribe an extraction and view potential finishing positions (Figure 15). Patients should be advised, however, that this is only a guide to the final position, and certain important aesthetic considerations, such as black triangles, are unlikely to be accurately visualized in the finished digital view.

Summary

Case selection, choice of lower incisor, space requirements and the optimum final radial fit all have an influence on considering whether a lower incisor should be extracted. In part two of this series, the problems caused when managing orthodontic cases with three lower incisors along with some practical solutions are discussed.