Water flossers clean effectively in some situations and poorly in others. The distinction follows from what a stream of water can and cannot do to an attached bacterial film.
Plaque is attached, not loose
Bacteria adhere to the tooth through a protein coating and bind themselves together with material they secrete, forming a structured layer rather than a loose deposit.
This structure resists shear force, which is why vigorous rinsing with a mouthwash does not remove it and why physical disruption is needed.
A water jet applies shear, and it removes the outer and less organised portion of the film. Mature layers adhering directly to a flat surface are not simply washed off, however forceful the stream.
The contact area is the hard part
Two adjacent teeth touch across a small flat area, and directly beneath it the surfaces are close together and nearly parallel.
A stream entering that region passes through rather than pressing against either wall, so the surfaces receive flow but not the wiping contact that removes attached film.
Floss works precisely because it is pressed against one surface and moved along it, which is a mechanically different action. Pressure and contact are supplied by the hand rather than by the medium itself.
Where the stream performs well
Around orthodontic brackets and wires, under bridges and around implant restorations, floss is difficult to place and a stream reaches areas it cannot.
Deeper gum pockets can be irrigated by a stream reaching further below the margin than a strand comfortably does.
Debris removal is also genuinely effective, and for people with dexterity limitations the practical difference between using something and using nothing is substantial.
Studies measure different things
Bleeding scores often improve with irrigation, because reducing inflammation responds to disturbing the film and flushing the pocket environment.
Plaque scores measured on accessible surfaces improve less, since those are the surfaces where mechanical wiping matters most.
Two products can therefore both appear supported by evidence while being better at different things, and comparing headline claims obscures that.
Using both is the usual conclusion
Most clinicians treat irrigation as an addition to interdental cleaning rather than a substitute, with the split depending on what a particular mouth contains.
Someone with wide spaces, appliances or restorations spanning teeth has more to gain from a stream than someone with tight healthy contacts.
A hygienist can identify which surfaces a routine is missing, since deposits accumulate in a pattern that indicates where the method is falling short.