From Volume 15, Issue 1, January 2022 | Pages 7-10
In part one of this series we described situations where extracting a lower incisor could provide a useful alternative to premolar extractions, in addition to solutions for treatment planning certain types of cases. In part two, we look at some of the problems caused by opting to extract a lower incisor and how they may be overcome.
Extraction of a lower incisor results in loss of 5–7 mm of tooth tissue from the lower arch. This reduces the intercanine width and, therefore, affects the radial fit of the upper teeth around the remaining lower teeth. With normally proportioned upper anterior teeth, this will lead to an increased overbite and overjet,1 unless steps are taken to reduce the upper labial segment width, or increase that of the lower incisors. Alternatively, a compromise in vertical and horizontal overlap can be accepted by the clinician and patient.
In Class II cases, particularly where there is already a deep bite, any residual space after relieving crowding may result in the lower incisors retroclining further, with a concomitant increase in both overbite and overjet. The commonly used MBT prescription has a -6° torque in the lower incisors, which will result in lingual crown torque of the lower incisors and can compound a Class II incisor relationship.
Conversely, in a Class III incisor relationship or reduced overbite, this tendency to increase the overbite and overjet can be used to aid camouflage of mild Class III cases with mild crowding. However, if there is greater than 2–3 mm of crowding, extraction of the single incisor will most probably only relieve space, and may not provide sufficient space for correcting the reverse overjet.
Essentially, when extracting a lower incisor, improving arch co-ordination can be achieved by either reducing the width of the upper labial segment teeth, or increasing the amount of space that the residual three lower incisors occupy.
In cases with narrow upper incisors, this feature alone may improve the occlusal fit sufficiently without the need for any further arch length reduction (Figure 1).
Alternatively, tooth morphology may lend itself to interproximal reduction. Up to 50% of enamel reduction is the maximum amount of proximal reduction that can be carried out without risk of damage to the periodontium and teeth.2 Upper incisors that are triangular in shape or those that have pre-existing interproximal black triangles are ideal for enamel reduction to improve their shape. Precautions must be taken, however, as variations in enamel thickness are common when considering difference in tooth morphology and ethnic groups.2 While it may be possible to safely remove up to 0.25 mm per proximal surface in the lower labial segment,3 the aesthetics of the crown morphology must also be kept in mind. Interdental stripping is more feasible in adults where greater secondary dentine has been laid down and is less likely to cause sensitivity.4 If there are restorations in the upper labial segment, planning for adjustments to existing restorations, such as placement of new crowns (Figure 2) or composite alterations, can also help to achieve a reduction of the upper arch length.
When using a pre-adjusted edgewise prescription, working up to a full-size rectangular steel wire will express torque to the upper labial segment and result in the upper incisors occupying more space and, therefore, increasing the overjet further, due to only having three incisors in the lower arch. This effect can be minimized by reducing the palatal root torque on the upper incisors. Round wires could be used in the upper arch or the corners of the rectangular archwire could be smoothed in the upper labial segment to reduce the effective torque.
Ensuring the three remaining lower incisor teeth occupy additional space is a more challenging procedure. It is unlikely that this will be necessary in a Class III case, where achieving lingual movement of the lower incisors is usually beneficial. In Class I cases, however, and even more so in Class II cases, maintaining a more anterior position of the lower labial segment will help to maintain the antero-posterior relationship with the upper incisors. While it is possible to broaden the remaining incisors using composite build-ups, this raises the question ‘Why remove an incisor in the first place?’ If the plan is to resolve the anterior crowding, but prevent them tipping lingually, this may be achieved by inverting the three remaining lower incisor brackets (Figure 3). Effectively, by placing an inverted MBT lower incisor bracket, the torque value will be changed from -6° to +6° buccal crown torque. Although there is 10° of slop in an 0.022’ x 0.028’ bracket slot, this is a 12° difference that will tend to keep the lower incisor crowns forward, but only when using a full size 0.019’ x 0.025’ archwire. This extra proclination should allow the three remaining incisors to occupy more space than if they were retroclined. The overbite is also a consideration though, as a lack of chamfer of the gingival bracket wings could cause an anterior interference if the bite is deep.
As the lower incisor space is closed, the canines will tend to be in a more mesial position, which leads to a slight Class III discrepancy in the buccal segments. The relative size of the upper teeth and inclination of the labial segments will determine the final occlusal fit of the teeth. A plaster or digital set up can provide valuable information about the final fit of the teeth, and this can aid a useful discussion with the patient to ensure fully informed consent.
In protrusive excursions, the lower canine cusp tips could cause an interference with the upper lateral incisors and lead to detrimental forces or wear of the upper incisor edges, as shown in Figure 4. This interference can be prevented by placing the canine brackets slightly more incisal to intrude them to the same height as the lower incisal edges. Alternatively, progressively grinding the lower canine tip can remove the interferences, or a combination of the two techniques can be employed. This will ensure that in protrusive excursion, the upper lateral incisors are protected from heavy contacts.5
The upper dental centreline is an important aesthetic component of the smile. Significant asymmetry and deviations beyond 3 mm have a detrimental effect on the perception of smile aesthetics.6 Patients will often ask ‘won't it look funny with just three lower front teeth?’ but the effect of changes to the lower centreline with three lower incisors, appears to have little impact on smile aesthetics. The lower teeth are smaller and more uniform in size and more likely to be partially hidden by the lower lip, resulting in the lower centreline seldomly being a cause for concern. However, this has not been fully investigated within the literature. A suitable approach to inform a patient about their final occlusal result may be to show them previously treated cases, or to use a diagnostic set up to demonstrate the intended treatment outcome.
To maintain symmetry of the centrelines, it is common to align the midpoint of the middle of the three lower incisors to the upper midline, as shown in Figure 5. Occasionally, when the lower dental centreline is already displaced by a full unit to one side owing to long-standing crowding, it is possible to finish the case with the centrelines appearing coincident, as in Figure 6.
Black triangles are the dark spaces at the gingival embrasure as the teeth taper towards the root, these can often be unsightly. They may be more common in adult patients where there is loss of the interdental papilla and gingival attachment.7 When teeth are crowded and overlap, black triangles are often hidden; aligning the teeth re-establishes the contact point, closer to the incisal edge, which can make the black triangle much more noticeable. When assessing the aesthetic effect of black triangles with mandibular extractions as judged by laypersons, dental professionals, and dental students, Pithon et al found poorer aesthetic scores with larger black triangles, ranging from 0.5 mm to 2.5 mm.8
Uribe et al investigated the presence of black triangles in a cohort of patients without periodontal disease. They reported black triangles in 68% of the 51 mandibular extraction cases they reviewed, with just over half of them being obvious in photographs of the smile.9 Black triangles were more obvious with lower central incisor extraction versus lower lateral incisors, but no clear predictors for this finding were identified. It is therefore important to assess the smile line and visibility of the lower anterior teeth prior to treatment. All patients should be warned about the potential for black triangles to appear at the end of treatment, prior to undertaking this extraction option. In general, the older the patient, the greater likelihood that this area will be visible on smiling, and increased chances that there will have already been some loss of periodontal support that would compound this effect.
Aesthetically, the largest space that patients are most often concerned about is the initial extraction space of the incisor. If the patient is particularly concerned about this large, but temporary initial gap, then options include:
Black triangles are magnified when there is either triangular crown morphology, reduced periodontal support or tipped teeth with divergent roots. Towards the end of treatment, spending time in full-size rectangular archwires will allow better root uprighting and parallelism. Choosing a bracket prescription, such as MBT, where there is no tip in the anterior segment, will help facilitate root uprighting.
With triangular crown morphology, the teeth can be reshaped using interproximal reduction. When reshaping in this way, the radial width will be reduced and cause the lower incisors to retrocline further, thereby causing further increase in the overbite and overjet, which may be counterproductive depending on whether the case is Class II or Class III.1 Alternatively, composite bonding can be used to fill out the space, as long as patients are fully informed that they are committing to long-term restorative maintenance (Figure 8).
There is limited robust evidence to demonstrate the stability of lower incisor extraction therapy within the literature. When extracting a lower incisor there is always concern that the intercanine width can continue to reduce further after treatment, which has been shown in both incisor and premolar extractions cases.10, 11 Interestingly, this research also found that a greater proportion of the lower incisor extraction cases demonstrated acceptable alignment at 10 years post treatment compared to four premolar extractions cases. Currently, there is not enough evidence to universally agree on the best retention regimen, regardless of the type of orthodontic case treated, and, therefore, as in all cases, indefinite retention is advised.12,13
In the presence of excellent oral health, placement of a bonded retainer from canine to canine is a reasonable recommendation, particularly when there have been corrections of rotations to the incisors. Where oral hygiene is less than ideal, having a bonded retainer just across the extraction site and remaining lower incisors is advisable (Figure 9).14
When there are large black triangles and small contact points, the risk of contacts slipping will be greater; however, black triangles can also facilitate interdental cleaning beneath bonded retainers and, if they are not readily visible, this may be a good reason to leave these areas accessible for cleaning.
In either case, a dual retention regimen with a bonded retainer and part-time wear of a vacuum-formed retainer at night is advisable to maintain the occlusal result and changes in the arch form.
Extracting a lower incisor may not be a first-line choice extraction pattern, but it can be a pragmatic option in appropriately selected cases. Inevitably, extracting a single incisor tooth and treating to three lower incisors is likely to cause a compromise with the fit or treatment mechanics; however, knowledge of these potential problems can help a clinician both educate and consent patients to achieve a successful outcome.
