A US safety authority says that silicone breast implants weaken with age and rupture more frequently than the manufacturers and many doctors had suspected until now.

In a study published in the Lancet medical journal, Dr Lori Brown of the Food and Drug Administration (FDA) says: "There is emerging consensus that both the incidence and prevalence of breast-implant rupture are much higher than previously suspected."

But the agency's doctors are still not sure if the leakage of the gel causes disease in other organs of the body.

Dr Brown and two colleagues who studied the existing published evidence on silicone implants found the risk of breakage increased with age. They found that rupture rates were much higher than the 0.2% to 1.1% reported by manufacturers.

One study they reviewed showed that 63.6% of breast implants which had been in place for between one and 25 years had ruptured or were leaking.

Implants have restored the body's natural form for patients who have undergone mastectomies following breast cancer - and have supplemented what nature had skimped on for those who considered themselves less well endowed.

Of the estimated two million American women who have had implants, 80% were for cosmetic reasons.

In some cases, breast implants ruptured during mammography

But 30 years after silicone implants were first introduced, women with a range of medical problems - from joint pains to rheumatoid arthritis and lupus - claimed leakage was the cause of their ills.

Legal cases against manufacturers for failing to properly test the product are pending in American courts.

According to Dr Brown, ruptures can have many causes: falls, accidents, compression during mammography, and gunshot wounds. However, she points out that "most implant ruptures have no obvious traumatic cause."

Silicone leaking from implants has been found in women's lymph nodes, chest, ribs, upper arms, elbow, hands and liver.

After researchers expressed concern about health risks, the United States and other countries limited or banned the use of silicone implants in the early 1990s.

Dow Corning, one of the largest manufacturers, stopped making silicone gel implants in 1992.

Nowadays, saline and soy solutions are used in the latest implants.

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