When Michelle Brady developed her adult front teeth, she had an embarrassingly large gap between them. And when the rest of her milk teeth fell out, her dentist told her that she would have many more gaps.

Dental X-rays showed that 12 of Michelle's adult teeth would be missing, and so when she was 13, she was referred to the Eastman Dental Hospital in London, a specialist clinic. There, she underwent a complex treatment plan to move existing teeth, reshape others, and build up milk teeth. She was also provided with a fixed brace, then a bridge. Now, at 17, she is delighted with her recently finished teeth.

'At 13, I didn't fully realise what it would mean not to have adult teeth,' she says. 'It is a family problem. My grandma had no adult teeth at all. But I hated my teeth, I really did, and if I looked now the way I did at 13, I would never smile at all.'

The Eastman has the largest hypodontia (developmentally missing teeth) clinic in the world. It is run by a team of experts in prosthodontics (removable replacement teeth), orthodontics (which deals with prevention and correction of irregular teeth), children's dentistry and conservative dentistry.

Hypodontia is surprisingly common; up to two and a half million people fail to develop at least one adult tooth (excluding wisdom teeth), while nearly a quarter of a million lack six or more. A third of the clinic's patients are missing between six and 10 teeth; over a fifth 11-15, and 14 per cent are minus more than 15. Most of the patients are children, and more than a third attend by the age of 11. A few patients come for the first time in their thirties.

The condition affects three girls for every two boys, but it is not known why. Hypodontia is a common feature of Down's syndrome, and of ectodermal dysplasia, whose sufferers have thin hair, deformed fingernails and dry skin. In severe cases the jawbone fails to develop, giving a sunken appearance to young faces. Even if just a few teeth are missing, their appearance may be unsightly because of bad spacing and existing teeth being spikey or small.

Jane Goodman, a consultant in paediatric dentistry at the Eastman, says there is a genetic component to the condition, although the pattern is difficult to determine. 'Sometimes it skips a generation; sometimes a mother is missing one tooth, one of her children has three missing and another six. But often the condition occurs out of the blue, with no apparent family connection.'

About half the parents whose children are referred to the clinic don't realise anything is wrong. It is very rare for milk teeth to be missing, or for the first adult teeth, the molars, not to come through around the age of six. The first hint of a problem may not appear until a child is eight or nine, when some of the upper incisors - the front teeth - fail to appear. The family dentist is usually the first to notice this and the diagnosis is confirmed by X-ray.

John Hobkirk, professor of prosthetic dentistry at the Institute of Dental Surgery, which is attached to the Eastman, says that parents are often anxious about their child's condition. 'Nowadays we expect to keep our teeth for life. To discover that your child has a large number of permanent teeth missing is a severe social blow. It can have marked psychological effects on many children.'

By the time a child reaches the age of 10 or 11, hypodontia experts can be fairly sure how many teeth he or she will develop. More problematic is judging whether the milk teeth will remain. 'Usually the milk teeth stay for a long time if there are no teeth pushing up beneath them. But we don't know for sure in each case,' says Jane Goodman. 'Also, if the jaw bone is developing normally, it may look as if the milk teeth are sinking into the bone, and they appear very small.'

Researchers at the Eastman are attempting to predict the life of milk teeth, and how to extend and improve them. (All teeth are saved, where possible, as this helps to prevent bone shrinkage.) Some 'sinking' milk teeth can be built up with white composite materials, to create biting contact with the opposite jaw again. This may prevent them from breaking down.

Once the dentists begin to move teeth around the mouth, it is necessary to hold them in place. 'If we start treatment at eight or nine, a child will have to wear dentures or some fixed appliance for the whole of his or her childhood to keep teeth in their new position,' says Ms Goodman. 'These children have a higher risk of gum disease and dental decay. So we are wary of embarking on such a long treatment plan if the child is not concerned about their teeth.'

Children's attitudes vary enormously. One young boy of 10 who had only three teeth was immensely proud of his dentures: another, the same age and with the same number of teeth, couldn't be bothered to wear his. One teenage boy was regularly in tears over having few teeth and keen for all the treatment he could get. Another quickly disposed of his bridge and growled, 'If anyone says anything about my teeth, I just thump them.'

Often it is the parents who pressurise for treatment, believing it is best for their child's problems to be sorted out before significant life changes such as starting secondary school, college or a job. 'But if children don't want treatment, they will manage to sabotage it somehow,' says Professor Hobkirk.

The treatment plan is 'a complicated mixture of crystal ball experience and clinical observation,' he says. Usually it entails repositioning teeth (orthodontics), filling out smallish gaps by adding veneers (glued-on white materials) to existing teeth and/or replacing what is missing with metal-based dentures or bridges. A big breakthrough has been the ability to attach bridges or dentures directly to retaining devices implanted surgically into the jawbone. But this cannot be done before the jaw stops growing, sometime between the ages of 19 and 21, and treatment is not available on the NHS.

Treatment can entail years of monthly visits, but is worth it, according to Michelle Brady: 'I am so much happier and more confident now. A small gap between your front teeth can be nice but a really big gap like mine was absolutely awful.'

(Photographs omitted)