Clear Aligners for Severe Crowding: What Is Realistic
For adults considering aligners for badly crowded teeth: what studies measured for IPR, arch widening, moving back teeth, extraction cases and treatment time. The core point: archived UK guidance names severe crowding among cases where aligners may call for extra techniques, fixed braces among them, for an ideal result, and a 2022 retrospective study of 40 adults found widening and IPR fell short of plan after the first aligner set.
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Medical disclaimer: This page is for general information only. It does not replace an examination or treatment by a dentist.
Can clear aligners correct severe crowding?
Archived UK guidance gives severe crowding as an example where aligner treatment may call for extra techniques, such as fixed braces, for an ideal result. That 2014 British Orthodontic Society (BOS) page lists among routine aligner treatments: mild spacing, mild to moderate irregularity, and crowding that is correctable by mild to moderate arch widening or by narrowing the teeth.
Its other examples are extraction treatment, complex tooth movements, and problems that stem from an underlying difference in jaw size or position. See crowded teeth for crowding itself, and the options in this section for other suitability factors.
How do aligners create space for crowded teeth?
A 2022 study measured three ways aligners gained space without extractions: arch widening, front-tooth position and IPR (slimming the sides of teeth). Its 40 adults in Italy, selected for good wear and treated by 10 experienced clinicians with one aligner system, averaged irregularity of 7.3 mm (upper) and 8.1 mm (lower), which the authors called severe.
By the end of the first aligner set, crowding reduction reached 87% of plan (upper) and 81% (lower), widening 59% to 83% (upper) and 49% to 67% (lower) depending on the tooth, and front-tooth position 68% and 69%. The study was retrospective, with self-reported wear.
How much space can IPR make, and what are its limits?
BOS's statement on IPR puts the enamel removed at up to 0.5 mm, taken from the sides of teeth. It cautions that taking off excess enamel can damage the teeth and, given IPR's risks, that patients should understand it fully before agreeing.
In a 2022 retrospective Italian study of 40 adults treated without extractions with one aligner system, 1.09 mm (upper) and 1.43 mm (lower) of IPR was planned per arch, up to 0.5 mm per tooth; by the end of the first aligner set, 0.55 and 0.82 mm were achieved, 49% and 42%. The authors called IPR the least predictable method, perhaps as it depends on the operator. Our page on interproximal reduction during aligner treatment explains the procedure.
How far can aligners widen the dental arch?
Aligner arch widening often fell short of plan in a 2025 retrospective study of 98 adults. They were treated without extractions by one clinician in Barcelona, with one aligner system, and measured at the first refinement.
Planned widening averaged 2.46 mm; results differed from plan by 0.92 mm on average: 1.24 mm (upper, 3.00 mm planned) and 0.61 mm (lower, 1.92 mm planned). Some under-expansion showed in 72.2% of measurements, and large planned expansions or an initial back-tooth crossbite seemed less predictable. Widening was more reliable in the lower jaw and at the canines; in a 2022 Italian study, it was less reliable at both.
To ask about orthodontic treatment, book online or call 0114 288 2121.
Can aligners move upper back teeth backwards?
Aligners can move upper back teeth backwards, a 2026 systematic review concluded, but that effectiveness falls as more movement is prescribed. In 16 non-randomised studies of 461 patients needing it (mean ages 21.6 to 36), the share of planned movement achieved ranged from 31.1% to 88.4%, about 2 mm on average. The studies were not pooled; certainty was very low.
What happens in extraction cases treated with aligners?
Extraction cases can drift from the plan: in a 2022 study, upper first molars moved forward 2.2 mm more than designed. Its 31 patients, aged 14 to 44, were treated with one aligner system at a Chinese hospital after two upper premolars were removed; their canines moved back 5.2 mm against 6.5 mm planned. It was retrospective and small.
When are fixed braces more predictable?
BOS warns that in more severe cases, especially those needing extractions, aligners do not have the control needed to give consistently good outcomes. They often lead to disappointment there, it says, adding that they can be very effective when all that is needed is aligning mildly irregular teeth, are not well suited to correcting prominent upper front teeth, and in selected cases are very effective but limited in scope.
A 2019 review of 8 studies (2 randomised trials, 706 people, mean ages 15.5 to 35.2, one aligner system) found aligners might be less effective than braces at making the teeth meet well, controlling torque (root inclination), widening the arch and retention, the last from one study. Two studies measured widening: one favoured braces at the upper canines and premolars, with molars similar, the other a trend for aligners to increase the width between the lower canines.
On its overall score, pooled across two studies with very different results, the difference was not significant, and the authors advised caution; in both studies fewer aligner cases reached a passing score, significantly in one.
In its single extraction trial, of Class I adults, overall finishing scores were similar, but aligners scored consistently worse for tooth inclination and for how the teeth met. Our pages on choosing between fixed braces and aligners and fixed braces go further.
How long does aligner treatment take with and without extractions?
Aligner treatment averaged 43 months with extractions and 32 without in a 2025 Canadian retrospective study of 46 patients with moderate to severe crowding. Mean ages were 20.0 and 17.3, respectively. A 2019 review's single extraction trial, of Class I adults, took 31.5 months with aligners, against 22 with braces. Of its seven studies without extractions, four reported aligner treatment significantly shorter than braces, while three reported no significant difference, one of them the only randomised trial in that group.
Who is orthodontic treatment at Deepcar Dental Care for?
Orthodontic treatment at Deepcar Dental Care is for adults only.
The practice's aligner treatment is described on our clear aligner therapy page.
Frequently asked questions
How do studies decide whether crowding is severe?
Studies define severity differently. A 2025 Canadian study called a 4 to 7 mm arch length shortfall moderate to severe, a 2022 Italian study called average irregularity of 7.3 and 8.1 mm severe, and a 2022 US study called 2 to 6 mm mild to moderate.
Will crowded teeth line up fully with the first set of aligners?
Not always, in a 2022 Italian study of 40 adults treated without extractions. After the first aligner set, irregularity averaged 2.8 mm (upper) and 2.7 mm (lower), against 1.8 and 1.4 mm in the digital plan, and achieved arch shapes did not match planned ones.
Does arch widening with aligners tip the back teeth outwards?
It can. In a 2025 retrospective Canadian study of 46 patients with moderate to severe crowding (mean ages 17.3 without extractions, 20.0 with), upper back teeth tipped outwards 5.56 degrees on average without extractions, against 0.43 with; lower back teeth about 5.6 degrees in both groups. The authors linked tipping without extractions to arch widening.
Can crowding come back after aligner treatment?
It can. An archived BOS page says every aligner treatment must be followed by retainers. In a 2019 review, the retention evidence was a single study of 22 people: alignment relapsed more after aligners than after braces, but the overall score showed no significant difference.
Which way of making space was most linked to less crowding?
Arch widening, in a 2022 Italian study of 40 adults treated without extractions. Of the three methods measured, only widening was significantly linked with crowding improvement, across the canines in both arches; IPR and front-tooth position were not. This shows association, not cause.
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