Sâu kẽ răng: vì sao khó thấy, dấu hiệu và cách phòng ngừa

You brush twice a day, your smile looks fine in the mirror, and yet the dentist finds a cavity. Very often the culprit is a cavity between teeth — decay that starts on the side surfaces, right where two teeth touch, a spot that neither your eyes nor your toothbrush can easily reach. Here is how dentists find it and what you can do every day.

What is a cavity between teeth, and why is it hard to see?

A cavity between teeth (an interproximal or proximal cavity) forms on the contact surface between two neighbouring teeth, most often the back teeth. Because the adjacent tooth hides it, the lesion can grow while the chewing surface still looks normal.

The process is the same as any tooth decay: bacteria in plaque turn sugars and starches into acid, and the acid draws minerals out of the enamel. According to the US National Institute of Dental and Craniofacial Research (NIDCR), the first sign may be just a white spot where minerals have been lost, and at that stage decay can still be stopped or reversed. The catch is that a white spot hidden between two teeth is almost impossible to see yourself.

Why do the spaces between teeth decay so easily?

• Toothbrush bristles mainly clean the outer, inner and chewing surfaces; the contact point between teeth is largely missed unless you clean between them separately.
• Food left between teeth ferments and helps decay-causing bacteria thrive, as Vinmec (a Vietnamese hospital source) points out.
• Snacking on sweets and sugary drinks many times a day keeps teeth under repeated acid attack, with no time to regain minerals.
• Saliva supplies the minerals enamel uses to repair itself (NIDCR); people with frequent dry mouth lose part of that protection.

Signs of a cavity between teeth to watch for

The NHS notes that early tooth decay may cause no symptoms at all. As the lesion grows, you may notice:

• Sensitivity in one area when eating sweets or drinking something cold or hot.
• Toothache, especially when chewing.
• A dark spot or small hole appearing at the side of a tooth.

None of these signs is specific: sensitivity can also come from enamel wear or gum recession (see our article on tooth sensitivity when brushing). Avoid self-diagnosis.

How do dentists detect cavities between teeth?

Besides looking and probing, dentists often take bitewing X-rays — intraoral films that capture the upper and lower back teeth of one area at the same time. Cleveland Clinic says bitewings show cavities between teeth and below the gumline.

There is no fixed schedule. The American Dental Association (ADA) stresses that the interval between X-rays depends on each person's oral health, age, disease risk and symptoms. Cleveland Clinic states that adults with healthy teeth and gums typically need new X-rays about every 6–18 months, while children may need them more often.

X-rays have limits too. A Cochrane review by Walsh and colleagues (2021) pooled 77 studies (15,518 tooth sites) on detecting early enamel caries. Imaging had an estimated sensitivity of 0.47 and specificity of 0.88: very early lesions are often missed, but a positive finding is usually right. The certainty of the evidence was rated low. PubMed That is why regular check-ups, compared over time, are worth more than a single X-ray.

Treating a cavity between teeth by stage

Your dentist decides after an exam, but treatment generally follows the extent of the damage:

• Early stage, enamel only demineralised: with no hole yet, the dentist may prescribe fluoride varnish or mouthwash to reverse early decay (NHS). Tam Anh Hospital (a Vietnamese source) also notes that fluoride helps restore enamel and stop decay from progressing.
• A cavity has formed: a filling is usually needed, commonly with composite or glass ionomer (GIC) (Tam Anh).
• Decay reaches the pulp: root canal treatment may be required; a badly damaged tooth may have to be extracted (NHS).

Preventing cavities between teeth: 4 habits to keep

No complicated tools needed — consistency is what counts.

1. Clean between your teeth every day: use floss, interdental brushes or a water flosser at least once a day (NHS, Tam Anh).
2. Brush with fluoride toothpaste: at least twice a day; spit after brushing and do not rinse, so fluoride stays on the teeth (NHS). Read more in our article on fluoride toothpaste.
3. Limit sugary snacks between meals: this is NIDCR's advice. The WHO recommends cutting free sugars (sugars added to food and drinks plus those in honey, syrups and fruit juices) to less than 10% of total energy, ideally below 5% — about 25 g, or 6 teaspoons, a day.
4. Have regular check-ups and X-rays: so lesions between teeth are found while they can still remineralise.

How can a water flosser help between teeth?

A water flosser is one option for interdental cleaning, especially if you find string floss awkward and tend to skip it. Still, it only helps flush plaque and food from between teeth; it does not replace fluoride toothpaste, does not remineralise enamel and cannot detect a cavity the way an X-ray can. The evidence comparing the two is covered in water flosser vs dental floss.

Two Inmax models with suitable specifications:

• M131 water flosser: 9 modes shown on a digital display, 40–130 PSI, 350 ml tank, 2,000 mAh battery. The many levels let you start gently and step up gradually.
• M209 water flosser: 4 modes (Pulse, Soft, Normal, Power); 310 ml tank; 2,000 mAh battery for fewer recharges; IPX7 waterproof.

You can compare specifications or contact Inmax for advice.

Frequently asked questions

Can a cavity between teeth heal on its own?

Once a hole has formed, it will not close by itself. Only very early lesions, where enamel has just lost minerals, can be stopped or reversed with fluoride and good hygiene — and a dentist needs to assess that.

Why do I get cavities between teeth even though I brush well?

Because bristles cannot reach the contact point between two teeth. However thoroughly you brush, you still need a daily interdental cleaning step.

Can a water flosser replace dental floss?

Both are ways to clean between teeth; choose the one you can stick with every day. If a gap has a filling or a cavity being monitored, ask your dentist which tool to use.

This article is for general information only and draws on the linked sources; cavities between teeth must be diagnosed and treated by a dentist, and nothing here replaces your doctor's advice.