It was carried out by the Cardiff University SchoolofDentistry and Public Health Wales and was published by the Welsh Oral Health Information Unit which is run from the university.
Why it is so tough has always lacked a good explanation but now researchers at the University of Southern California's SchoolofDentistry are closing in on the secret.
Professor Ian MacKenzie, a researcher in stem cell biology from the Queen Mary Schoolof Medicine and Dentistry in London, said that it was still likely that stem cells played a significant role in the development of cancer.
Professor Stuart Morganstein, dean ofdentistry, said theschool will need volunteers to receive treatment from final year students and Leicester was the perfect size and demographic to draw potential patients.
Dr Berkman, a medical student at the University of Medicine and Dentistryof New Jersey, conducted the research while studying for his MA at the John Hopkins Bloomberg Schoolof Public Health.