The short answer

You finish straightening your teeth, look closely in the mirror, and notice tiny dark triangles between your front teeth that you never saw before. It is one of the most common "nobody warned me" moments in adult orthodontics, and it can be unsettling. The good news is that black triangles are well understood, they are rarely a sign that something has gone wrong, and there are several ways to make them smaller or close them completely.

This guide explains what black triangles are, why they appear, how clear aligner treatment fits in, and what a dentist can do about them. It is written for patients, so we have kept the jargon to a minimum and linked to the research where we quote it.

What are black triangles?

Between each pair of teeth there is a small triangle of gum called the interdental papilla. In a healthy young mouth it rises up to the point where the two teeth touch and fills the space completely. When the papilla is shorter than the space it needs to fill, a dark gap is left behind. Dentists call this an open gingival embrasure; most people simply call it a black triangle.

They are most noticeable between the upper and lower front teeth because that is where light and shadow are most obvious when you smile. They can also collect food and plaque, which is why some people notice them first when flossing or using interdental brushes.

Why black triangles appear

There is rarely a single cause. Most black triangles come from a combination of the factors below.

The most important factor is how far the point where two teeth touch sits above the bone that supports the gum. In a classic study published in the Journal of Periodontology in 1992, researchers measured 288 sites in 30 patients. When that distance was 5 mm or less, the papilla filled the space almost 100% of the time. At 6 mm it was present 56% of the time, and at 7 mm or more only 27% of the time or less. In simple terms: every extra millimetre makes a black triangle more likely.

If the bone between the teeth shrinks, the gum follows it down. Gum disease (periodontitis) is a common reason for this, which is why the NHS advice on gum disease stresses regular brushing, cleaning between teeth and routine dental check-ups. Brushing too hard, smoking and simply getting older can all contribute to recession as well. You can read more in our guide to the stages of gum disease.

Some teeth are wide at the biting edge and narrow at the neck, a bit like an upside-down triangle. When two of these teeth sit side by side they touch only near the tip, leaving a larger space near the gum for the papilla to fill. Square or rectangular teeth have longer contact areas and are less likely to show black triangles.

When teeth overlap, the gum between them is often squashed or poorly formed, and the overlap hides it. Once the teeth are lined up, the space that was always there becomes visible for the first time. This is why many people notice black triangles after straightening, even though the treatment did not "create" the gum loss.

If the roots of neighbouring teeth tilt away from each other, the space between them near the gum widens. Dentists can often see this on an X-ray and may plan tooth movements to bring the roots more parallel.

Gum tissue tends to thin and recede slowly with age, so adults are more likely to see black triangles than teenagers. That does not mean adults should avoid straightening; it just means a gum check is especially worthwhile before you start.

Do clear aligners cause black triangles?

Black triangles can appear after any type of orthodontic treatment, including fixed braces and clear aligners. The aligners themselves do not remove gum. What usually happens is that straightening reveals a space that was already there, or moves the contact point between two teeth to a new position further from the bone.

A 2025 study in BMC Oral Health looked at 82 adults who had finished clear aligner treatment. At least one open gingival embrasure was found in the front teeth of 73% of patients, most often between the lower central incisors. The factors linked with a higher risk were older age, the amount of crowding before treatment, the gap between the teeth at the gum line, and, most strongly, the distance from the contact point to the bone. The same paper notes that earlier research on fixed braces reported black triangles in roughly 38% to 43.7% of treated patients, so this is not a problem unique to one type of appliance.

Many of these triangles are small. The practical message is that your chance of seeing one depends mainly on your gums, bone levels and tooth shape, and that a good treatment plan should look at these before the first aligner is made. At Smileie, every plan starts with a free e-consultation, followed by an in-person dental check and 3D scan so that a GDC-registered dentist can assess your gums and tooth shape as well as your alignment.

How black triangles can be fixed

The right option depends on why the triangle is there and how big it is. A dentist will usually look at your gum health first, then at tooth shape and position. Here are the main approaches.

If gum disease is present, it needs to be treated and stable before any cosmetic work. Sometimes inflamed, swollen gums hide a problem that becomes visible once they settle, and sometimes healthier gums slightly improve the look. Either way, healthy gums are the foundation for every other option.

Orthodontic movement can bring roots closer together or move the contact point towards the gum, which gives the papilla less space to fill. This is often planned into treatment from the start when a dentist can see a triangle is likely.

Very small amounts of enamel can be polished from the sides of triangular teeth. This turns a pin-point contact into a longer, flatter contact, and the teeth are then moved together, which pulls the contact point down towards the gum. It is a common, well-established technique when done within safe limits by a dentist.

Tooth-coloured resin can be added to the sides of the teeth near the gum to fill the triangle. It does not need drilling in most cases, can be matched to your shade and can be polished or repaired later. It is a popular option once teeth are in their final position and your retainer is fitted.

For larger triangles, or where teeth also need a change in shape or colour, porcelain or composite veneers can reshape the teeth more fully. These involve more commitment, as some enamel may be removed. See our comparison of clear aligners and veneers for how the two approaches differ.

Specialist periodontal surgery and gum injections are sometimes used to rebuild the papilla. Results are less predictable than tooth-based options, so they are usually considered case by case by a specialist.

Reducing your risk before and during treatment

You cannot change your tooth shape or your age, but you can make black triangles less likely or less noticeable.

• Get your gums checked first. Any gum disease should be treated and stable before straightening starts. Tell the dental team if your gums bleed when you brush.
• Ask about your risk at the planning stage. If you have triangular teeth, crowded lower front teeth or some recession, ask whether the plan includes steps such as enamel reshaping or root positioning to reduce the chance of triangles.
• Clean between your teeth every day. Use floss or interdental brushes of the right size. They keep the papilla healthy and remove plaque that aligners can trap against the teeth.
• Brush gently. Use a soft brush and small circular movements. Hard scrubbing at the gum line can speed up recession.
• Keep your aligners and mouth clean. Brush before putting aligners back in, and clean the trays daily. Our guide on how to clean clear aligners covers the basics.
• Avoid smoking and vaping. Both are linked with gum problems, and stopping helps gum health whatever your treatment plan.
• Wear your retainer. Teeth that drift apart after treatment can make triangles look bigger. Our guide to teeth shifting after braces explains why retention matters.

When to speak to a dentist

A small black triangle on its own is usually a cosmetic issue. It is worth booking a check with your dentist sooner rather than later if you notice any of the following:

• Bleeding, swollen or tender gums, especially when brushing or flossing.
• Gums that are pulling back or teeth that look longer than they used to.
• Teeth that feel loose after treatment has finished, or a bite that feels different.
• Persistent bad breath or a bad taste, which can be a sign of gum disease.
• Triangles that are getting bigger over months rather than staying the same.

If you are currently in aligner treatment and notice a new triangle, contact your treating dental team. They can check whether it is expected at your current stage, whether later aligners will close it, or whether a small change to the plan would help. You can always check that a dentist is registered on the General Dental Council register.

FAQs

Disclaimer: This article is for informational purposes only and does not constitute medical or dental advice. Black triangles have several possible causes, and the right treatment depends on your gum health, bone levels and tooth shape. Clear aligners are not suitable for every patient and cannot correct skeletal jaw problems on their own. Suitability and likely outcomes must be confirmed by a qualified dental professional following an appropriate clinical assessment. Treatment details, package inclusions, prices, finance and promotions may change; check the relevant Smileie UK pages for current information.