Why Are Polyurethane-covered Silicone Implants Not Legal in USA?

The surgeons who used these implants liked them. But subsequent to the silicone implant moratorium in 91, they ceased to be available.
At this point in time the costs of submitting these implants to the FDA is not justified by the percieved market. Therefore, no manufacturer has an interest in resurrecting this implant. Despite, their popularity, many questions about the fate and consequences of the polyurethrane remain unanswered.

Thanks for your question -
In addition to the comments posted regarding the uncertainty of the safety of polyurethane implants, significant steps have been made since they're availability to reduce capsular contracture in the currently available implants including placement, triple antibiotic irrigation, minimizing handling, etc.
With improvements in the capsular contracture rate that has dis-incentivized these implants coming back on the market.

Polyurethane covered breast implants became available about 1980 in the US and in our practice at the time we used quite a number of them. The structure of the implant was similar to a regular silicone gel implant, though on the cover a coating of polyurethane was applied much like a textured surface. Capsule contracture had been, and still is, such a problem with subglandular breast augmentation with a silicone gel implant that the polyurethane coating was a very attractive advantage, and the implants worked quite well.
The implant we used at the time was called the Meme implant, and all silicone gel implants including the Meme were withdrawn with the silicone "controversy" in the 1990's. One drawback to the polyurethane implant was that the texture required a larger incision to properly place the implant, usually inframammary, however we no longer were plagued by firm breasts and needed closed capsulotomy which was so common at that time. With the silicone moratorium all gels were no longer available until replacements were allowed, but still no polyurethane.
Polyurethane was a wonderful improvement with implants that stayed soft and natural, until of course the silicone rubber cover leaked, and than capsules begin to form and the implant is in need of replacement. There are two hurdles to overcome before we may again see this implant in the US. The first thing we learned was that shear or stress on the cover can cause the polyurethane to separate from the surface, not what is intended, though the implant still performed well. The second issue is that as we removed and replaced the implants we found the polyurethane disappears over time. Where it goes and what effects it might cause will take many years of study to answer.
The fact that we are again noticing an interest in using polyurethane implants should remind us of the real problem of capsule contracture with silicone gel breast implants.
Best of luck,
peterejohnsonmd