THURSDAY, July 28 (HealthDay News) -- The U.S. Food and Drug Administration issued an "approvable letter" Thursday to Mentor Corp. for its application to bring silicone gel-filled breast implants back to the American market.

The agency said federal law prohibited it from disclosing the specific contents of the letter. An approvable letter is one of several intermediate steps in the FDA review process of new products, and is often a sign a product is nearing final regulatory clearance.

Mentor said the letter spells out certain conditions the company must meet to receive FDA approval to market the implants in the United States, the Associated Press reported.

In April, the FDA's General and Plastic Surgery Devices Advisory Committee voted 7-2 that Santa Barbara, Calif.-based Mentor's application to bring back silicone-gel implants was "approvable with conditions."

The FDA stressed that Thursday's letter doesn't mean the devices have been approved for marketing in the United States.

During the review process in April, the advisory panel rejected a similar request by silicone-gel implant maker Inamed Corp.

Inamed officials testified that the new generation of implants were less prone to rupture and leakage. But the FDA advisers were skeptical, saying significant questions still persisted on how long the implants last inside a woman's body -- and the health consequences when they break.

Silicone gel implants went on sale in 1962 in the United States. Thirty years later, the FDA banned all but those used for breast cancer reconstruction, amid fears that leaks might be responsible for autoimmune diseases and even cancer. Other problems included the fact that many women needed repeat operations to deal with painful scar tissue.

By 1998, more than 170,000 lawsuits had been filed against implant manufacturers by women claiming their silicone implants had caused immune system disorders and chronic diseases such as rheumatoid arthritis and systemic lupus. Four companies paid out millions of dollars in settlements to the women, and one of them, Dow Corning, filed for bankruptcy to pay $3.2 billion to settle its claims.

However, studies by the Mayo Clinic, Harvard Medical School and the Institute of Medicine (IOM) panel from the National Academy of Sciences subsequently found no evidence that leaked silicone from gel implants caused systemic disease.

More recent research has focused on the problems of scar tissue developing at the site of the implant and on the rate of rupture of the devices.

The rupture rate was a prime reason the FDA refused last January to allow the devices back on the market.

More information

The FDA has more on breast implants.