The way small children get cavities fixed in America is a machine most people never see the inside of.

A toddler cannot hold still for a drill, so treating serious decay in a two-year-old routinely means an operating room, general anaesthesia, and a bill that runs into thousands - if the family has coverage and lives near a hospital that does it. Families with neither wait, and the decay spreads.

Which is the context for a result published in JAMA Pediatrics on July 29.

A University of Michigan-led Phase III trial - the pivotal kind, the kind regulators read - tested 38 percent silver diamine fluoride: a liquid brushed onto a cavity with a sponge-tipped applicator in about a minute, reapplied every six months [1].

In 830 children under six, it arrested decay in more than half of treated baby teeth. No drill. No injection. No sedation [1].

The trial's design is half the story. It recruited through dental offices, pediatric medical practices, and Head Start and Early Head Start programmes across Michigan, New York and Iowa [1] - meaning it went looking for the children who actually carry the burden of untreated decay, not the ones already sitting in dental chairs.

The National Institute of Dental and Craniofacial Research put more than $12 million behind it [1].

Margherita Fontana, the Michigan professor who led it: 'This is a very effective and safe treatment -- even in children as young as 1' [1].

The honest catches, because there are three.

The silver permanently darkens the treated spot - a real cosmetic cost, though for a baby tooth that will fall out on its own, one many families would trade for skipping anaesthesia [1].

It needs reapplication every six months, so it is management rather than cure.

In the United States, meanwhile, SDF for arresting decay is still off-label: cleared as a desensitiser, used against cavities on a dentist's judgement [1].

That last one is the actionable item. Amr Moursi, an NYU pediatric dentistry professor, named it: 'Removing SDF from off-label status would be an important innovation which could lead to increased utilization by providers' [1]. A pivotal, NIH-funded trial is precisely the evidence that moves a treatment from tolerated to labelled.

The gap this closes is not subtle. Children with insurance get fillings; children without get extractions or operating rooms or nothing.

A one-minute liquid that a pediatrician's office or a Head Start screening can apply is dentistry finally shaped like the problem.