An apple a day keeps the doctor away, according to the proverb, but North scientists say it's an orange you need if you want to prevent painful visits to the dentist.

Or rather applying pressure equivalent to the weight of an orange for two minutes is the perfect toothbrushing technique.

Scientists at Newcastle University who calculated the formula warn that tooth brushing needs to be precise to keep teeth healthy and sparkling clean.

Brushing longer and with more pressure can be damaging to tooth enamel and gums but not enough brushing may leave you at risk of gingivitis and periodontal disease.

Prof Peter Heasman, of the university's School of Dental Sciences, said: "Although we found that you have to brush your teeth reasonably long and hard to get rid of the harmful plaque which causes dental diseases, our research shows that once you go beyond a certain point you aren't being any more effective. You could actually be harming your gums and possibly your teeth.

"Despite this, anecdotal evidence within the dental profession suggests that the majority of the population still believe that the longer and the harder you brush, the better for your teeth it is.

"The way in which you brush your teeth is just as important, and this goes hand in glove with the time you spend brushing and the pressure you apply to them."

Twelve volunteers took part in the four-week study, which examined 16 combinations of brushing times, from 30 to 180 seconds, and pressures of 75 to 300 grams.

Participants were trained how to use an oscillating electric toothbrush, wired up to a computer.

Levels of plaque - the bacteria-ridden, glue-like substance which forms on teeth and gums - were measured before and after brushing. Plaque removal steadily improved as brushing times and pressure were increased. But when people brushed for longer than two minutes, at a pressure higher than 150 grams, they stopped removing any additional plaque.

The findings are published today in the Journal of Clinical Periodontology. Prof Heasman said that though timing was straight- forward, it was extremely difficult for people to gauge how much pressure to apply with their toothbrush.

"The force applied could be quite light, but when confined to a very small area the pressure could still be high. If you are unsure how to go about brushing your teeth, the best thing to do is to make an appointment with your dentist or dental hygienist."

He gave these tips:

* Take advice from your dentist or dental hygienist on the size and shape of your toothbrush.

* Take advice about your cleaning technique.

* Avoid scrubbing the teeth.

* Floss, or use interdental toothbrushes, to reach inaccessible areas.

* Brush thoroughly at least once a day.