Thousand of women who received faulty silicone breast implants made by a French firm PIP are being barraged by confusing and conflicting numbers on the risk of the devices rupturing. The implants, made by Poly Implant Protheses and sold over the past two decades primarily in Europe and Central and South America, have been found by the French government to fail and leak their contents more frequently than the average implant, leading to potential irritation, inflammation and the need to remove the implant. Yet U.K. authorities and the European Commission say there is no convincing evidence of abnormally high failure rates.

What makes the PIP breast implant particularly difficult to assess is that their contents may vary by country, possibly leading to different rates of rupture. French authorities have found some of the implants contained industrial-grade silicone gel instead of surgical grade. The health implications are unclear. Tests have found no evidence the silicone causes cancer, but it could produce more irritation than other implants.

Researchers believe until it is clear how the PIP implants differed, and what risks are posed, there is no reason to scare all  women who have implants. Also clouding the calculation of risk is uncertainty over the number of women who received any implant. Estimates of PIP implants in each country are based on the number sold to plastic surgeon, but no one knows how many were actually inserted.

U.S. regulators never approved the PIP implants for plastic surgery in US, but the safety of some approved medical devices similarly is unclear. Not all devices have registries, and most that do are maintained by manufacturers, a Food and Drug Administration spokeswoman said.