Mentor Corp., which hopes to soon win Food and Drug Administration approval to sell its silicone-gel breast implants for general cosmetic use, faced a problem last year as it prepared to distribute to doctors the demonstration models that prospective customers would try on for size: The implants sometimes left behind an unsettling slick of silicone oil.

A former senior engineer with the company told the FDA last month that he and others were asked to solve that problem and came up with a fix -- a less permeable material for the one-inch patch that seals a hole left by the manufacturing process. But despite urging from its staff, the company never made the same modification to the devices destined to be actually implanted in women who have their breasts enlarged or restored after surgery, the engineer said in a letter to the agency.

''I am very concerned about the safety of women using these breast implants if they were to become widely available as an FDA-approved product,'' wrote the engineer, who provided a copy of his letter to The Washington Post on condition he not be identified because of family and job concerns. He also wrote that he believed it was misleading to show patients the modified models but implant the others.

The former Mentor engineer's allegations are before the agency and several congressional oversight committees, adding an unpredictable new factor to the long-running debate over wider use of silicone breast implants.

Possible health complications from silicone gel ''bleeds'' are among the issues being assessed by the FDA as it moves toward a final decision on two applications to allow unrestricted sale of silicone-gel implants for breast enlargement for the first time since 1992. Both Mentor and Inamed Corp. have received letters from the FDA saying their devices are ''approvable'' if certain conditions are met, and both companies have undergone final plant inspections by the agency.

Controversy lingers, however, over the safety of the implants. Several officials inside and outside the FDA, as well as some members of Congress, including Rep. Rosa DeLauro, D-Conn., have urged agency officials to investigate the issues raised by the Mentor whistleblower. Susan Wood, who recently resigned as head of the FDA's Office of Women's Health, said she contacted top officials in August and was assured the agency would investigate.

Schultz said the FDA's device center has spent more time reviewing breast implants than any other product during his 11 years as a manager there. If they are ultimately approved for wider use, he said, ''all the good, bad and indifferent'' information on issues such as gel bleed will be fully disclosed so women can make informed decisions.

More than 264,000 breast implants were performed last year for cosmetic reasons, according to the American Society of Plastic Surgeons. Another 63,000 were done for women who had had mastectomies. Since the 1992 FDA decision to restrict silicone gel implants because of health concerns, they have been available only to mastectomy patients and women willing to participate in a clinical trial. Most women seeking breast enlargement have had to use saline-filled models, which many say produce a less natural-seeming result.

The former senior engineer left Mentor this spring for another company. In his letter to the FDA, he said that after it became known that he was raising concerns about the Mentor implants, the company made accusations against him that resulted in an indictment involving disputed company property.

Small amounts of silicone oil are known to seep out of all implants. Improvements to the shells, or outer envelopes, have significantly reduced the leakage since the devices were taken out of general use and studies have shown that most silicone that leaks out remains in the cavity created by scar tissue around the implant.

Nonetheless, the issue remains scientifically contentious and emotionally raw. While studies generally show that silicone in the body does not cause ill effects, some women have reported disturbing consequences. Several told the FDA advisory panel that silicone oil had seeped out of their foreheads, eyes and elsewhere.

During the panel's hearings, the FDA reported Mentor's patch is not made of the same low-bleed material as the rest of the shell. Inamed's competing device has a low-bleed shell and patch.

The former Mentor engineer first took his concerns to a women's health advocate, Diana Zuckerman of the Washington-based National Research Center for Women & Families, who has been critical of the lack of long-term safety studies of breast implants. Zuckerman helped him contact the FDA to talk about his experiences at Mentor.

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