If your Invisalign plan mentions IPR, someone has probably told you it means filing between your teeth, and left it there. It deserves a better explanation than that, because it is one of the few parts of aligner treatment that is genuinely irreversible.
What it is and why it is needed
Interproximal reduction is the removal of a very thin layer of enamel from the sides of teeth where they touch, to create space. Crowded teeth need somewhere to go. There are only three ways to make room in a dental arch: widen it, push the front teeth forward, or make the teeth very slightly narrower. IPR is the third.
It is done with a fine abrasive strip or a thin disc, usually takes a couple of minutes, and does not need anaesthetic because enamel has no nerve supply.
How much enamel actually comes off
Typically between 0.2mm and 0.5mm per contact point, and rarely more than 0.5mm. Enamel on the side of a molar is around 1.0 to 1.5mm thick, so the accepted limit stays well inside half of it. For scale, 0.25mm is about three sheets of paper.
Your plan will set out exactly which contacts are reduced and by how much, at which stage. It is worth asking to see that, because a plan requiring heavy IPR at many contacts is telling you something about how crowded the case really is.
Is it safe?
The long-term studies are reassuring. Follow-ups over five years and beyond have not found higher rates of decay or gum problems at reduced contact points compared with untouched ones, provided the surface is polished smooth afterwards. That last part matters: a rough surface holds plaque, and polishing is not an optional finishing step.
What IPR is not is reversible. Enamel does not grow back. That is the honest trade-off, and it is why it should be planned rather than improvised.
IPR or an extraction?
For mild to moderate crowding, IPR across several contacts can create the two or three millimetres needed without removing a tooth. For severe crowding, no amount of safe IPR will substitute for an extraction, and pretending otherwise produces flared front teeth and an unstable result.
A good provider will tell you which of those two situations you are in at the consultation, using the scan rather than a guess.
What to expect on the day
Nothing dramatic. A few minutes per contact, a slightly odd sensation of pressure and vibration, no injection. Some people notice mild sensitivity to cold for a day or two. You go back to normal immediately.
How the amount is worked out
It is not judged by eye. The digital plan measures the space available in the arch against the combined width of the teeth, and the shortfall is the space that has to come from somewhere. That figure is then distributed across several contact points rather than taken from one, which is why a plan might show 0.2mm at six contacts rather than 1.2mm at one.
Spreading it matters. Concentrated reduction at a single contact changes the shape of that tooth visibly and leaves a broader flat contact area that traps plaque. Distributed reduction is invisible.
The alternatives, and when they are better
Expanding the arch pushes the teeth outwards to create room. It works well in the upper jaw where there is genuine narrowness, less predictably in the lower, and pushed too far it produces teeth that sit outside the bone and gums that recede.
Proclination, tipping the front teeth forward, also creates space but changes the profile of the smile and is the least stable of the three.
Extraction removes a whole tooth, usually a premolar, and creates far more space than IPR ever will. For genuinely severe crowding it is the honest answer, and a plan that tries to solve a 10mm shortfall with filing alone is the one to question.