Those dark spaces at the corners of a smile have a name, they are normal, and the research on whether anyone actually notices them is genuinely split. They are called buccal corridors, and they turn up constantly in cosmetic dentistry marketing as a problem to be solved. The evidence is a lot less certain than the marketing. Some studies find that larger buccal corridors reduce how attractive a smile is rated. Other studies find no meaningful effect at all. A patient reading about them deserves to be told that, rather than sold a fix for something that may not be bothering anyone but the person selling it.
Key takeaways
- Buccal corridors are the dark negative spaces between the outer surfaces of the upper back teeth and the corners of the mouth during a full smile. Everyone has some.
- A 2022 systematic review on orthodontic treatment, midline position, buccal corridor and smile arc states plainly that the influence of the buccal corridor on facial aesthetics has not been conclusively clarified, because the data still gives partly divergent results.
- Hulsey concluded buccal corridors do not meaningfully influence smile aesthetics. Moore and colleagues found the opposite, that buccal corridor size does affect perceived attractiveness and that laypeople can perceive it. Both findings are in the literature.
- Where a difference is found, orthodontists tend to detect and penalise larger buccal corridors more readily than laypeople do, so professionals and the public are not using the same yardstick.
- Pronounced corridors usually come from a narrow upper arch, a retruded or underdeveloped upper jaw, or back teeth that are small relative to the arch. Orthodontic expansion changes the arch itself.
- Veneers or crowns change only the appearance of the corridor, typically by a modest amount per tooth. They cannot correct a skeletal transverse deficiency or a crossbite.
What a buccal corridor actually is
A buccal corridor is the dark space you can see between the outside surfaces of your upper back teeth and the inner corners of your mouth when you smile fully. Buccal simply means towards the cheek. When someone smiles broadly, the lips pull back further than the dental arch curves outward, and the gap between the two shows as shadow.
Everybody has buccal corridors. They are a normal consequence of the fact that a dental arch is a curve and a mouth is wider than that curve at its corners. What varies is how large they are, and that varies with arch width, jaw position, tooth size, lip mobility and how broadly a person smiles on the day. The same person photographed mid-laugh and photographed in a polite posed smile can show noticeably different corridors.
Where this article fits
Three articles on this site cover neighbouring ground, and the distinction between them matters. The article on why a smile looks off centre is about the centre line, that is, whether the join between the two front teeth lines up with the middle of the face. The article on tooth width to length ratio is about the proportions of one individual tooth and why teeth can look short or square.
This article is about neither. Its scope is the width of the whole smile and the space beside the teeth. If your concern is the centre line, read the midline article. If your concern is the shape of a single tooth, read the proportion article. If your concern is that your smile looks narrow or that you see dark triangles at the corners, you are in the right place. Related reading on the mechanics of moving teeth is in the articles on how crooked teeth are straightened and braces compared with aligners for adults.
How buccal corridors are measured
Buccal corridors are measured as a ratio rather than as a raw width, because faces differ in size. The standard calculation is the inner commissure width, meaning the distance between the inner corners of the mouth, minus the visible upper dental arch width, divided by the inner commissure width, expressed as a percentage. In plain terms, it is the proportion of the visible smile width taken up by shadow rather than by teeth.
Some studies then band smiles using that percentage: narrow at around 28 per cent, medium narrow at around 22, medium at around 15, medium broad at around 10 and broad at around 2. It is worth being clear about what that list is. It is one classification drawn from the literature, used to group smiles for research purposes. It is not a target, not a diagnosis and not a standard that anyone is failing to meet. Treating those numbers as goals is a misreading of what they were built for.
The evidence genuinely conflicts, and that is the point
The honest answer to whether buccal corridors matter is that the research does not agree, and pretending otherwise would be misleading. A 2022 systematic review examining the influence of orthodontic treatment, midline position, buccal corridor and smile arc on smile attractiveness reached that conclusion directly: the influence of the buccal corridor on facial aesthetics has not been conclusively clarified, because the available data still produces partly divergent results. That is the position of a review looking across the whole body of work, not the opinion of one study.
The conflict is easy to trace. Hulsey's study concluded that buccal corridors do not meaningfully influence smile aesthetics. Moore and colleagues found the opposite, reporting that buccal corridor size does affect perceived attractiveness and that laypeople are able to perceive the difference. Further studies across different populations have found no significant difference in attractiveness scores related to buccal corridor space at all. These are not fringe papers being contradicted by a consensus. They are the field.
What this means for a patient is straightforward. If a treatment plan is presented to you with buccal corridor reduction as a headline benefit, it is reasonable to ask what the expected change actually is and whether anyone other than a clinician would notice it. That is not a hostile question. It is the question the evidence invites.
Clinicians and the public do not use the same yardstick
Where studies do find a difference, they often find that orthodontists detect and penalise larger buccal corridors more readily than laypeople do. One study placed the layperson threshold for maximum attractiveness at around a 10 per cent buccal width reduction relative to iris pupillary distance, compared with around 20 per cent for orthodontists. In other words, the trained eye reacted to a change that the untrained eye tolerated.
This is one of the more useful findings for a patient to hear, because it reframes the whole question. A clinician looking at your smile is working from a professional frame of reference built by years of comparing smiles. Your friends, colleagues and family are not. Something that reads as an obvious finding on a clinical assessment may be invisible in ordinary life. It also cuts the other way: if something genuinely bothers you when you look at your own photographs, that is a legitimate reason to ask about it, regardless of what a rating study says about average perception.
Why the studies disagree
The divergence is at least partly methodological rather than mysterious. Studies differ in whether they photographed a posed smile or captured a spontaneous one, and those produce different lip positions and therefore different corridors on the same person. They differ in the populations doing the rating, and aesthetic preference is not uniform across cultural groups. They differ in the rating scales used, in whether images were altered digitally or drawn from real patients, and in whether raters saw the whole face or a cropped smile.
Change any of those variables and you can plausibly change the result. That does not make the research worthless. It means the honest summary is that buccal corridor size probably has some effect on perception under some conditions, that the effect is not large or consistent enough to have been pinned down, and that no one should be told a specific percentage is the number to aim for.
What makes buccal corridors more pronounced
Larger buccal corridors usually come down to arch width, jaw position or tooth size. A narrow or constricted upper arch, described clinically as transverse maxillary deficiency, is the most common structural cause. A retruded or underdeveloped upper jaw sets the whole arch further back, which also widens the shadow at the corners. Back teeth that are small relative to the width of the arch can contribute as well.
Transverse maxillary deficiency is not only an aesthetic finding. It typically presents with a narrow palate, a posterior crossbite where the upper back teeth bite inside the lower ones, and crowding at the front. That combination is why an assessment looks at the bite and the palate rather than only at the smile photograph. If a crossbite is present, the conversation stops being cosmetic and becomes functional.
What can and cannot change your buccal corridors
Some approaches change the arch itself and some only change how it looks, and the difference is important when you are weighing up options.
| Approach | What it actually changes | Typical candidate | Limitations |
|---|---|---|---|
| Orthodontic arch expansion | The real width of the upper arch, through slow or rapid maxillary expansion and torque expression using fixed appliances | Someone with a genuinely narrow arch, often with a posterior crossbite or front crowding | Takes time, needs retention, and skeletal change is easier while growth is available. In adults, non surgical dental expansion has been reported to achieve roughly 7mm to 11mm of arch widening with reported stability |
| Clear aligners with expansion movements | Tooth position and some dental arch width, mostly by tipping teeth outward rather than moving bone | Milder narrowness where the underlying skeleton is close to normal | More limited than fixed appliances for true expansion, and not a substitute for skeletal correction |
| Surgically assisted rapid palatal expansion | The skeletal width of the upper jaw, by releasing the bony resistance first | Adults with a more severe skeletal transverse deficiency | A surgical procedure with a recovery period, planned with a specialist team. Not something Lumi Dental provides in house |
| Veneers or crowns | Only the apparent width of individual teeth, commonly cited as roughly 1mm to 2mm per tooth, which narrows the visible shadow | Someone whose arch width is acceptable but who wants the corridor to look less obvious | An optical effect only. It does not widen the arch, correct a crossbite or change the skeleton, and there is a limit before a tooth looks unnatural or needs more preparation |
| Changing the photograph angle or smile pose | Nothing at all about your teeth, only what the camera records | Anyone who mainly dislikes how corridors look in photographs | Free and reversible, but it does not address a functional finding such as a crossbite |
| Doing nothing | Nothing, which is a legitimate outcome given how divided the evidence is | Anyone without a functional problem who is not personally troubled by the appearance | Not appropriate if a crossbite, breathing concern or crowding needs attention for other reasons |
The limits of widening teeth rather than the arch
Restorative widening reduces the appearance of a buccal corridor without changing arch width at all, and it is important not to blur that line. Adding a small amount of width to several upper teeth with veneers or crowns brings the visible tooth surface closer to the corner of the mouth, so less shadow shows. Commonly cited figures sit at roughly 1mm to 2mm per tooth, which across several teeth can be visible.
What it cannot do is correct a skeletal transverse deficiency, resolve a crossbite or make a genuinely narrow arch wider. The bone is where it was. There is also a practical ceiling: a tooth can only be widened so far before the proportions look wrong, before the contact points sit awkwardly, or before more tooth structure has to be removed to accommodate the shape. If the underlying issue is arch width, a restorative approach is a cosmetic workaround rather than a correction, and it should be described that way when the options are laid out. Planning tools such as digital smile design can help show what a change of that size would actually look like before anything is committed to.
Common questions
What are buccal corridors and why does my smile look narrow?
Buccal corridors are the dark spaces between the outer surfaces of your upper back teeth and the corners of your mouth when you smile fully. A smile can look narrow because the upper arch is genuinely narrow, because the upper jaw sits further back, because the back teeth are small relative to the arch, or simply because of how broadly you are smiling in that particular photograph.
Can braces or aligners fix the dark spaces at the corners of my smile?
Orthodontic arch expansion can reduce them by widening the arch itself, using slow or rapid maxillary expansion and torque expression through fixed appliances. Clear aligners can achieve some dental expansion, mostly by tipping teeth outward, which is more limited. Whether either is appropriate depends on whether the narrowness is dental or skeletal, which needs an assessment rather than a photograph.
Do dentists and normal people notice buccal corridors the same way?
Often not. Where studies find a difference, orthodontists tend to detect and penalise larger buccal corridors more readily than laypeople. One study put the layperson threshold for maximum attractiveness at around a 10 per cent buccal width reduction relative to iris pupillary distance, against around 20 per cent for orthodontists. A finding that stands out clinically may not register in ordinary conversation.
Can veneers make a narrow smile look wider?
They can make it look wider, which is not the same as making it wider. Veneers or crowns can add a modest amount of apparent width per tooth, commonly cited as roughly 1mm to 2mm, which reduces how much shadow shows. The arch itself is unchanged, so this does not address a crossbite, a narrow palate or a skeletal deficiency.
What causes a narrow upper jaw in adults?
Transverse maxillary deficiency is the usual description, and it reflects how the upper jaw developed rather than something that happened suddenly. It commonly presents with a narrow palate, a posterior crossbite and crowding of the front teeth alongside the narrow looking smile. An assessment of the bite and palate is needed to tell a skeletal narrowness from a purely dental one.
Is arch expansion possible for adults or only children?
It is possible in adults, with limits. Non surgical dental expansion in adults has been reported to achieve roughly 7mm to 11mm of arch widening with reported stability. For more severe skeletal cases, surgically assisted rapid palatal expansion is used to release the bony resistance first. Growing patients generally have more skeletal change available to them, which is why timing is part of the discussion.
Should I treat buccal corridors at all?
That depends on whether there is a functional finding and on whether it genuinely bothers you. Given that the evidence on whether buccal corridors affect perceived attractiveness is divided, no one should be told that reducing them is a universal goal. If there is a crossbite, crowding or a bite concern, those are reasons to act on their own merits.
Talking it through
If your smile looks narrower than you would like, or someone has raised buccal corridors with you and you want a second opinion on whether it is worth treating, the team at Lumi Dental in Melrose Park offers a complimentary cosmetic consultation. That includes an honest assessment of whether the finding is dental, skeletal or simply a feature of how you smile, and what each option can and cannot achieve. Other current offers are listed on the current deals page, and a written quote can be prepared before anything is decided.
This article is general information only and is not personal dental or orthodontic advice. Arch width, bite and facial proportions differ from person to person, and treatment suitability and outcomes vary. Please speak with a dentist or orthodontist about your own situation before making a decision.
