Why were Polypropylene breast implants not approved by the FDA?

The FDA looked into the "string" polyporpylene breast implants but they were concerned about asymmetrical growth and development.  They don't necessarily grow in unison even with the same technique.

 Polypropylene, as far as I know, is used in suture material and therefore I do not think that the problem was anything to do with the substance. The issue for the FDA was it was not controllable what the end size of the breast augmentation would be in some people got very large from this procedure. A finite size silicone gel implant is certainly better in my opinion. Good luck.