The patient wears a clear plastic "aligner" that fits tightly over the teeth, rather like a sports guard, and gently but firmly moves them back into line. Every fortnight, the aligner is discarded and replaced with a new, slightly modified one, until the desired effect is achieved.According to Les Joffe, Invisalign can move teeth by a third to a quarter of a millimetre every fortnight. "The orthodontist takes an impression, and then sends it off to Invisalign, where they use a computer program to make a virtual replica of your teeth and gums," he explains. The virtual mouth can then be manipulated on-screen to show how the treatment will work, and then each small movement needed is translated into an aligner, one for each shift. "If the problem is simple, you might need 10 aligners; for a more severe problem, as many as 28," says Joffe.The aligners have to be slipped out when eating, but apart from that, to be effective, they must be worn for at least 20 hours a day.

"There's no danger of swallowing them while you're asleep," says Joffe reassuringly. "They clip tightly over the teeth, which is why they are effective and comfortable; they don't move around." Clients, he says, are most attracted to the invisibility factor. "They add a very faint glaze to the teeth, but from 2ft away, you can't see them, unless you know exactly what you're looking for." Some patients, he says, can develop a slight lisp while adapting to their aligners; but it takes just a few days to adapt.Gail Breen has been wearing her aligners for three months now, and claims she has a bit of a lisp, though in fact she sounds normal; she found it hardest to adapt to taking them in and out. "When I'd just had them done, I met a girlfriend for coffee, and said I just had to take my braces out; half an hour later, I was still fiddling. The first couple of weeks I broke all my fingernails getting them in and out, but now I can discreetly slip them out at the dinner table and no one notices." Nail damage apart, she is enthusiastic.

She works as an interior designer, and has to look the part; having good teeth, she says, is the equivalent of "good lighting in a show flat".Invisalign is not for everyone, says Les Joffe. Around 40,000 patients in the US and Canada have so far been treated; but, he says, it's not suitable if there is a jaw discrepancy or severe crowding. At the moment, Invisalign can only be fitted by specially trained orthodontists; there are currently 18 practices in the UK that can offer the service, all part of the OrthoWorld plc group. Invisalign can only be used when all permanent teeth are there, so it's for adults rather than children. And it is expensive; between £3,500 and £4,000 for one jaw, and £4,500 to £5,000 for both Gail Breen sees the cost as an investment. "It's like everything else you have done; your hair, waxing, manicure.

I pay by direct debit, over 15 months, so it's like paying out for gym membership."It seems that having a neat set of teeth is becoming as important to us as to the Americans, long known for their obsession with oral perfection. Invisalign is just part of a wider trend towards bettering the state of the nation's teeth. British gnashers have long been the subject of some hilarity, particularly in the US. "Snaggletoothed Brits" are a long-running joke in The Simpsons. And when Mike Myers went into make-up for the film Austin Powers, he came out with crooked, stained teeth to make the lead character unmistakably British. A recent NOP survey showed that 21 per cent of Brits are self-conscious when being photographed, 19 per cent feel uncomfortable when smiling, and 11 per cent try to cover their mouths while talking.