Why were Polypropylene breast implants not approved by the FDA?

I've been steadily losing mass in my breasts ever since I had my first child (I have three and am now fixed) I was a DD and now I'm barely an A. When I first heard of it my figuring was "grown back what I want then take the string out." I then researched more and read that theyaren't aproved here. Is it due to the massive growth that can happen pros there more harmful things I'm not finding?