The shape of the space, described properly
Two adjacent teeth don’t meet at a single point in the way a diagram suggests. They touch across a small flattened area, and above and below that contact the surfaces curve away from each other, creating a space shaped roughly like two triangles meeting at their tips. Dentists call those spaces embrasures. The gum fills the lower one, rising up between the teeth as a small peak of tissue.
That peak of gum, and the slight valley of tissue running between the cheek side and the tongue side beneath the contact, is where a great deal of gum disease begins. It is protected from the tongue, from the cheek, from chewing and from rinsing, which is exactly the problem.
Why bristles cannot enter
A toothbrush bristle approaches from outside. Where two teeth are in contact, there is no opening from the cheek side to the tongue side that a bristle can pass through, and the curved surfaces immediately beside the contact sit in the bristle’s shadow. No amount of pressure changes this and no design of brush head solves it, because the obstruction is the neighboring tooth.
This is the entire mechanical case for cleaning between teeth, and it holds regardless of what any particular trial found. If a surface is never disturbed, the film on it matures undisturbed, and mature film at a gum margin produces inflammation.
It also explains the pattern of decay that dentists see between teeth. A cavity starting on the surface just below the contact point is invisible from the outside, painless for a long time, and frequently found on an x-ray rather than by looking. The tooth can appear perfectly sound from the front while the damage sits in the shadow.
Gaps differ enormously, which is why one tool does not fit everyone
In a young mouth with healthy gums and tight contacts, the space beneath the contact is almost entirely filled with tissue and the opening is very narrow. Something thin has to slide past the contact and wipe each surface in turn.
In a mouth where gum has receded, or where bone has been lost through periodontal disease, the space opens up and becomes a triangle you can see. Now something thin passes through the middle without touching much, and a small tapered brush that fills the space will clean far more effectively. Sizes vary by site, and getting the size right matters more than most people expect.
Between those extremes sits everybody else, often with different requirements in different parts of the same mouth. Which is why I am wary of blanket instructions. The right approach is decided by looking at your gaps, and that’s a two-minute part of a routine appointment.
The way this goes wrong at home
The commonest error I hear described is a straight downward snap. Floss forced through a tight contact drops sharply once it passes, strikes the gum peak, and over time causes a small cleft in the tissue. The tooth surfaces, meanwhile, barely get wiped.
The motion that works is closer to easing through with a gentle side-to-side pressure, then curving the floss against one tooth surface and moving it up and down before repeating against the neighbor. Each contact has two surfaces facing it, and both need attention. That is a skill, and skills are learned by demonstration, not description.
Forcing a tool that is too large into a tight space is the other frequent problem, particularly with interdental brushes. If it will not pass with light pressure, the answer is a smaller size, not more force.
What to notice, and when to ask
Food packing repeatedly into one particular space is worth mentioning to your dentist. It can happen for several reasons — a lost contact between teeth, a filling that does not restore the shape, drifting after an extraction — and the local irritation it produces is a common source of a sore, tender gum that will not settle.
Bleeding from one specific site every time you clean it, when the rest of the mouth is fine, is also worth reporting rather than tolerating. It may simply be inflammation from an area that has been missed for a while and resolves once it is cleaned properly. It may be something else, and the only way to distinguish them is an examination.
The general point holds throughout. This article can tell you why the space is hard to clean. It can’t tell you which of your own spaces are being missed, and that is precisely the information a check-up produces.
Common questions
Do I need to clean between every tooth?
Wherever two teeth touch, there is a surface a brush cannot reach, so the general answer is yes, including the back of the last molar where there is no neighbor. In practice people skip the back of the mouth most often, which is unfortunate because it is also where problems are least visible.
Should this be done before or after brushing?
Both orders are defended and neither is clearly superior in the evidence. Doing it before brushing means the toothpaste reaches surfaces that have just been cleared, which is a reasonable argument. Doing it at all, once a day, matters far more than the sequence.
My gums are tender between two teeth. What does that mean?
It can mean several things, from a patch that has been missed and is inflamed, to food impacting there, to something requiring treatment. I cannot tell you which from a description, and neither can anyone else without looking. If it has lasted more than a couple of weeks, have it examined.
Dental Surgeon, Dr. Shruti Goel
Dr. Shruti Goel is a dental surgeon in practice since 2006 — around 20 years of clinical work — at Advanced Dental Clinic in Faridabad. She writes here about hygiene and prevention: the part of dentistry that happens at home, between appointments, and that decides how much of the rest is ever needed.