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The government is considering lifting a ban on most silicone-gel breast implants (search), but federal scientists question whether there's enough evidence indicating how long the devices will last inside a woman's body — or what health risks may result if they break.

The Food and Drug Administration will consider the issue in a three-day meeting with its scientific advisers next week. The meeting is a key hurdle in determining whether the implants can re-enter the market. Since 1992, they have been available only to women enrolled in strictly controlled research studies because of fears they could cause major health problems.

The implants have been largely exonerated of causing such serious illnesses as cancer or lupus. But painful scar tissue that can form around the implants, breaks that require surgery to remove or replace implants, and other complications remain contentious. Just 15 months ago, the FDA told manufacturers that it wouldn't lift restrictions on the implants' sale until questions about breakage in particular are settled.

The FDA posted documents on the controversy on its Web site Wednesday.

Competitors Inamed Corp. and Mentor Corp. believe they've met that requirement, arguing that breast implants, just like pacemakers or metal hips, don't last forever but that newer generations are acceptably durable. They're better than salt water-filled implants that U.S. women can buy today, Inamed Vice President Dan Cohen said.

Broken implants don't always cause immediate symptoms, so the FDA asked manufacturers to use MRI scans (search) to track durability. Mentor cited one study suggesting three-year breakage rates ranging from fewer than 1 percent to 5 percent of patients. Inamed data showed that over four years, implants ruptured in 3.4 percent of women who had received them for breast enlargement patients — and in 20.5 percent of breast cancer patients who received implants after a mastectomy. The company attributed that higher rate to a particular implant model that is being redesigned.

But those studies tracked small numbers of women for a short time, FDA scientists cautioned in preliminary analyses posted on the Web site.

"These data are of limited value" in determining how many years a woman can expect her implant to last — and what pain or other complications she might experience if it breaks and silicone gel oozes out, one FDA document concludes.

In a second document, FDA scientists estimated that up to three-quarters of the devices might rupture within 10 years of implantation, requiring women to undergo additional surgery to remove or replace the implants.

Moreover, additional documents suggest that if the FDA approved widespread implant sales, implant recipients would be encouraged to get regular MRI exams, perhaps every year or two, to check for breakage. Those exams cost hundreds of dollars, and there's no way to ensure women would get them, said Diana Zuckerman of the National Research Center for Women and Families.

It's the second time in less than two years that the FDA is debating the silicone-gel implants. In October 2003, its advisers narrowly recommended allowing the implants to sell again, advice the agency declined to follow.

Next week, the debate again will pit woman against woman, patients who say the implants caused lasting scars versus others who say the silicone-gel versions look and feel more natural than saline implants. The FDA has set aside an entire extended day — from 8 a.m. to 11 p.m. on Monday — to hear from the public.

"I'm 23 years old and I'm in bed most days almost all day," says Shannon Scott of Lakeside, Calif., who developed severely painful scar tissue a year after receiving her 2002 breast implants. Uninsured and on disability, she says she cannot afford to have them removed — and she will tell the FDA committee that her surgeon never reported her complaints so that researchers could properly count side effects in implant studies.

On the other side is Kerri Branson of North Aurora, Ill., who first received silicone-gel implants in 1990 and was so pleased with them that she got a second set last summer to firm up breasts sagging from breastfeeding the four children she had in the interim.

"I've never ever regretted my choice," Branson said. "I wish that other women could feel as great as I do."