A back tooth has five surfaces, and a toothbrush cleans only three of them well. The two it misses are the sides where neighbouring teeth touch, and that is where interproximal decay, a cavity between teeth, begins. Because the spot is hidden and enamel cannot feel, these cavities often grow for a long time without any warning.
The contact point is a sheltered pocket for plaque
Just below where two teeth meet, there is a small area protected from the tongue, cheeks and brush bristles. Plaque settles there undisturbed. Each time you have something sugary or starchy, the bacteria in that plaque make acid, and the acid draws minerals out of the enamel on one or both of the touching surfaces.
Several things make this more likely: crowded or overlapping teeth, very tight contacts, food that packs between teeth, gaps under the edges of old fillings or crowns, and simply not cleaning between the teeth. If plaque stays long enough it can also harden into tartar, which is covered in plaque versus tartar.
Finding a cavity you cannot see or feel
Looking into the mouth, a dentist sees the outer and inner faces of each tooth, but the contact area is blocked by the neighbouring tooth. Early decay there is rarely visible until it is large enough to show as a grey shadow through the enamel, or to break through as a hole.
Bitewing X-rays are the usual way around this. They show the back teeth from the side, and decay appears as a dark area spreading in from the contact surface. Comparing bitewings taken over time also shows whether a small lesion is growing or holding steady. How often you need them depends on your decay risk.
Symptoms, if they come at all, tend to arrive late: floss that shreds in one spot, food catching where it never used to, sensitivity to sweet or cold, or an ache once the decay nears the nerve.
Shallow, deeper and deep lesions are managed differently
A 2020 systematic review in Clinical Oral Investigations (Splieth and colleagues) looked at how to manage decay between teeth in adolescents and adults and concluded that the right approach depends on how deep the lesion is and whether it is active.
Decay confined to the enamel, with the surface intact, can often be held back without drilling. Better cleaning between the teeth, fluoride and fewer sugar exposures are the main tools, followed by X-ray checks. The same review found moderate evidence that micro-invasive treatments, such as sealing the contact surface or infiltrating it with a thin resin, helped slow these early lesions compared with non-invasive care or a placebo. Our article on reversing early decay explains when this approach fits.
Decay that has reached the dentine or broken the surface usually needs a filling. Because the cavity sits between teeth, the dentist reaches it from the biting surface and rebuilds the side wall so it contacts the neighbouring tooth properly. Getting a filling walks through the appointment.
Deep decay close to the nerve may need a larger restoration, and if the nerve is inflamed or infected, root canal treatment.
The touching tooth is checked as well, since the same plaque sits against both surfaces.
Cleaning between the teeth
Floss and interdental brushes are the tools that reach the contact area. A 2019 Cochrane review (Worthington and colleagues) found low to very low certainty evidence that adding floss or interdental brushes to toothbrushing may reduce gum inflammation or plaque, with interdental brushes possibly more effective than floss. None of the trials it found ran long enough to measure decay between teeth, so the case for flossing against cavities rests on how decay forms rather than on direct trial evidence. Removing plaque from where cavities start remains sensible advice.
In practice:
- Clean between your teeth once a day, using floss where the gaps are tight and interdental brushes where they fit.
- Brush twice a day with fluoride toothpaste and spit rather than rinse.
- Keep sweet food and drinks to mealtimes.
- Have check-ups, with bitewing X-rays when your dentist advises them.
Technique matters, because floss needs to curve around each tooth and slide just under the gum. How to floss properly shows the method, and a dentist or OHT can show you which interdental brush sizes suit your gaps.
Common questions
Will a cavity between my teeth go away?
Very early decay in the enamel can stop progressing and may partly remineralise. Once the surface has broken down into a cavity, it will not heal and needs repair.
Can I tell by looking in the mirror?
Usually not. By the time decay between teeth is visible, it is often well into the dentine. X-rays are the reliable way to find it early.
Does a filling between teeth take longer?
It can, because the dentist has to reach the side of the tooth and rebuild a tight contact with the neighbour. Small lesions caught early generally mean a smaller filling.
Check what is happening between your teeth
A check-up with bitewing X-rays can show decay between the teeth while it is still shallow and easier to manage. Our dentists and OHTs can also help you find a cleaning routine that works for your gaps. See current offers to book.
This article is general information and is not a substitute for personal dental advice.
