The pearly king behind Kate Middleton's smile?

The French-born orthodontist specialises in "invisible" braces and is credited with treating the future queen. If the diary columns can be believed, Prince William's bride-to-be has visited Fillion's Wimpole Street consulting rooms for a teeth makeover.

Her smile certainly looks more Hollywood than Home Counties in recent photographs. But Dr Fillion is disappointingly discreet on the subject of Kate's dental secrets. All my enquiries are met with a polite "no comment".

Instead, we discuss his latest techniques for achieving the perfect teeth. "We can always improve how people look," says the dentist, who shares the London Lingual Orthodontic Clinic with Dr Asif Chatoo.

"The goal is to create the most beautiful smile possible. Society judges us by appearance. If it's a choice between two people, one with a nice smile and the other with an ugly one, well... "

Achieving perfection is Fillion's skill. He was the man who introduced lingual braces - invisible braces that fit behind the teeth - to London. Both Kelly Brook and Sadie Frost have benefited from his techniques.

Another first was his use of 3D digital technology, which he ­developed with a ­laboratory in South Korea. More precise than existing methods, 3D technology enables patients to "design" their own smiles.

With the help of his laptop, the orthodontist, 60, shows me how it works.

"With 3D, we can place the brackets (of the brace) in the exact position they need to be on each tooth. Their position is the most important thing. It cuts at least three months off the treatment time."

Once the client is satisfied, the design is despatched to Korea where technicians turn it into a template which is used to make the brace.

A smile designed by Fillion can cost up to £11,000, which includes two years of straightening and realigning crooked teeth. But many of the people he treats are not wealthy.

"People will save up if they want a nice smile - they find a way."

It was back in the 1970s that Fillion first read about the benefits of plastic braces worn behind the teeth instead of metal "railway tracks".

Seeing the potential for transforming teeth, he opened a clinic in Paris which was the first in the world dedicated to orthodontics. "Braces for adults - they were not nice. And I like new challenges," he says.

Such is the demand for his services that Fillion, who is president of the British Lingual Orthodontic Society, spends two days a week in London, then takes the Eurostar back to Paris where he still has a clinic.

Then there are his trips abroad to speak at conferences: "I'd describe myself as an internationalist. My wife complains that she never sees me."

More than nine in 10 of his patients come for treatment because they want to look better, not for medical reasons such as an overbite: "Sometimes they have perfect teeth but just want a tiny discrepancy corrected."

The "wide" smile, where all the teeth are clearly visible, is considered very desirable. Fillion whips open his laptop again to show me what he means. In the "before" picture, the woman looks as though her back teeth are missing. This is known as a "black corridor" in dentistry jargon and is corrected by using a brace to realign the teeth so they follow the lip line. Gaps between the front teeth are also popular, a fad which
Fillion dismisses as "crazy". "They [women] want to look like Vanessa Paradis. For me, it's ridiculous wanting a gap."

The London Lingual Orthodontic Clinic also offers teeth whitening, or "lightening" as Fillion calls it. We are straying into Kate Middleton territory again here. There was speculation that her teeth had undergone a bleaching treatment. Avoiding any direct references, I ask Fillion what is a good look - dazzling white or a more natural shade? In his view, natural looks best, "not too white".

A perfect smile is not just about straight teeth, according to Fillion. The profile of the face is important too and this limits what can be achieved.

Fillion has based his reputation on giving clients the "smile of their dreams. A treatment is fully accomplished only when the patient himself is totally satisfied with the final result," he says.

Judging by the public reaction to Kate's smile, he has done a good job.

The crucial test, though, will be the "kiss" photograph. The moment Wills and Kate emerge post ceremony and embrace on the royal balcony will be the most memorable image of the whole day. Let's hope Kate's teeth will be dazzling.