The misery of repeat trips to the dentist to replace lost fillings could soon be consigned to history.

Currently, if taken care of, resin fillings typically last for seven to ten years at most before they crack and fall out.

But now scientists have worked out how to make fillings stay in the teeth for much longer.

The breakthrough was made possible by an extract from the bark of pine tree roots, which scientists discovered alters the chemical structure of teeth to make them stronger and allow fillings to bind to them better.

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Dr Guido Pauli, of the University of Illinois at Chicago, said compounds in the pine tree extract work by strengthening dentine - the tissue layer beneath the enamel in teeth.

He said: "The weak link in the chain of bonding is the interface between the resin and the dentine.

"We have clear evidence that our intervention materials change this interface much to the better."

Tests show the tree extracts can increase the strength of dentine by up to ten times.

Dr Pauli said it would be reasonable to expect the lifespan of fillings to increase to more than a decade.

With his colleague Dr Ana Bedran Russo, Dr Pauli tested extracts taken from the root bark of Chinese red pine, called pinus massoniana. They applied these to human molars before testing how they deformed when a force was applied to them.

The results, published in the Journal of Organic Chemistry, showed that the pine bark extracts strengthened the dentine in the teeth, which remained even a year after application.

The researchers also found the dentine was changed so that it was more compatible with resin fillings when they were applied to a tooth, binding to them more tightly.

Dr Pauli said they hoped to produce a refined extract that dentists could apply to a tooth after drilling to help strengthen it and ensure a better bond to the filling.

He said: "Extract-treated dentine show superior binding to resin fillings."

About 80 per cent of us have at least one filling. They are used to repair damage caused by decay which creates holes in the teeth.

Usually the dentist drills out the decayed part of the tooth and fills the hole with a metal or resin. While metal fillings were once common, they are increasingly being replaced with resin fillings which match the natural colour of teeth.

Resin, however, tends to be less resilient and so can become cracked and fall out.

Dr Nigel Carter, head of the Oral Health Foundation, said: "Fillings are susceptible to wear-and-tear and can occasionally be in need of repair and replacement.

"Having fillings which have a longer lifespan could be more comforting to the patient, as the procedure will have to be carried out less often."