By ANDY HO, SENIOR WRITER

MORE women are getting breast implants here. What health risks do such implants pose?

The earliest implants were just thick-walled silicone sacs filled with viscous silicone gel. First made by speciality chemicals manufacturer Dow Corning in 1962, they were larger and firmer than those available today.

Very early on, they were found to induce painful "capsular contractures": Scar tissue that quickly formed around these implants would harden and shrink badly. This caused the breasts to become hard, stiff and deformed.

According to Rudolph Guthrie's The Truth About Breast Implants, Dow Corning then rushed out implants with thinner walls and "more watery silicone gel" to prevent contractures. But these implants were prone to rupture, causing silicone gel "bleeds" into breast tissue.

In the 1970s, competitors tried coating the sacs on the outside with polyurethane foam to prevent contractures. A 2006 study showed that women with such implants were significantly less likely to have developed contractures.

However, polyurethane breaks down into 2-toluenediamine, a chemical known to cause cancer. In 1991, the United States banned polyurethane as a breast implant coating. However, its use continued to be permitted elsewhere.