Interdental brushes and floss both clean between teeth, but they work on different geometry. Which one suits a given space depends on the shape of that space.
The space between teeth is not flat
Below the point where two teeth touch, their surfaces curve apart and the gum fills the resulting triangular area. That area is small where gums are healthy and larger where tissue has receded.
Root surfaces are also frequently grooved rather than round, with concavities running lengthways that a flat strand cannot press into.
A brush with bristles radiating outward fills the space in every direction at once, contacting surfaces that a strand passes over.
Fit determines whether cleaning happens
A brush narrower than the space touches nothing on either side and gives an impression of cleaning without removing much film.
One too wide will not enter without force, and forcing it compresses gum tissue and can cause trauma over repeated use.
Sizes are therefore selected space by space, and many people need more than one size across the mouth. That selection is usually made with a clinician.
Wire and coating shape the choice
Most designs use a twisted wire core carrying nylon filaments, and the core is what provides the stiffness needed to enter a tight space.
Coated wires exist for use around implants and orthodontic appliances, where a bare metal core could contact surfaces it should not.
Bent or splayed brushes have lost their working shape and no longer contact the surfaces evenly, which is the practical reason for replacing them.
Where floss still holds an advantage
In tight healthy contacts with no visible gap, no brush small enough will pass, and attempting to force one is counterproductive.
Floss slides through such contacts because it is thin and flexible, then adapts against each surface when curved around the tooth.
Many mouths need both, using brushes in the wider posterior spaces and floss where the front teeth remain tightly contacted.
Bleeding and what it indicates
Sites that bleed when first cleaned are usually inflamed, and bleeding often reduces over subsequent weeks as the tissue responds to regular disruption of the film.
Bleeding that persists beyond that, or that comes with recession, looseness or a persistent taste, points to something requiring assessment.
A dentist or hygienist can also identify whether spaces are being missed entirely, since deposits accumulate in a pattern that reveals the gaps in a routine.