Interdental brushes: size is chosen on the narrowest space, not the most comfortable

The interdental brush has better evidence than floss where space allows, and the reason many patients derive no benefit is almost always the wrong size.

An undersized brush enters easily and touches nothing. The patient feels they have cleaned, the action is comfortable, and the biofilm stays where it was.

It is the most insidious failure because it produces no signal: no resistance, no bleeding, no discomfort. Only the absence of result at the next review.

An oversized brush, conversely, will not enter or enters by force, and traumatises the papilla. The patient notices and stops using it, often without saying so.

The correct size is the one that enters with slight resistance and touches both walls. The ISO scale runs from 0 for the narrowest spaces up through larger sizes for wider ones.

Measuring probes allow the size to be determined for each space rather than estimated. They are inexpensive and turn an approximate prescription into a precise one.

The point that gets overlooked is that a single patient's spaces have different sizes. Prescribing one size for the whole mouth means getting it wrong in half of them.

A useful prescription specifies two or three sizes, stating which to use where. It is harder to explain but it is the only approach that actually works.

Shape affects adaptation. A cylindrical brush fills parallel spaces uniformly; a tapered one adapts to triangular spaces and to posterior areas where the opening widens towards the back.

For spaces that widen inwards, typical after periodontal attachment loss, the tapered shape cleans a larger surface for the same entry dimension.

The wire core must be coated where the brush is used around implants or restorations, because exposed wire scratches titanium and ceramic. Models with plastic cores exist for these indications.

Replacement is due when the filaments splay, typically after one or two weeks of daily use. A worn brush no longer fills the space and falls back into the first error.

In summary: size is determined with a probe rather than by eye, one patient needs several sizes, the tapered shape suits widened spaces, the core must be coated around implants, and a worn brush becomes undersized again.