Sarah I always put implants on top of the muscle regardless of how thin you are and I'll give you my reasoning why. I hope I can explain my reasoning so as you will be able to have a good understanding, and at least then judge for yourself. However you will hear many differing opinions on this, and every plastic surgeon does things in a subtly different way that makes their surgery work for them and their patient.The two main reasons implants were put under the muscle many years ago were: firstly to disguise them, and secondly to reduce capsule formation. I'll talk about capsule first. Capsule is the body's reaction to the implant. Everybody gets a capsule to an extent, as all implants are "walled off" from the surrounding tissues. This "wall" around the implant is the capsule. Normally you wouldn't even know it was there, as it's very thin and flimsy, like cling film. However if the capsule gets "thick" or "tight" it squeezes the implant which then adopts a firmer, rounder shape. The implant can then be felt, and even seen. We know now how to reduce the incidence of this thick capsule. There are several techniques we use to help prevent it, so capsule is now nowhere near as common as it once was. Great news. But putting the implants under the muscle is not one of these techniques and does nothing to prevent capsule (the same goes for texturing or different shapes).Now let's talk about putting implants under the muscle. It was thought that with more "cover" over the implants (i.e. skin plus breast PLUS muscle), the edge of the implant above would be less visible. However even in a very thin woman with small breasts, an appropriately sized implant on top of the muscle will look just as good, with minimal if any visibility of the upper pole of the implant. This is true with any implant, including round ones. If a thick or tight capsule forms this changes, and the implants become visible or palpable, whether under or over the muscle! It doesn't matter.So the first thing is to ask the surgeon what steps he or she takes to prevent capsule. That is the most important thing in breast implant surgery. Get them to show you typical results at two or three years, not just a few months (when all implants look good because capsule hasn't had time to develop).Putting implants under the muscle has other problems. Firstly the operation is painful, as opposed to on top which has minimal pain. Secondly the implants move when the muscles move. Thirdly a band can develop across the breast after a year or so. This band is the detached muscle pulling on the breast tissue and distorting it. Finally if a problematic capsule does develop, the capsule itself cannot be removed entirely (at least not easily), because the capsule is attached to the ribs which are under the muscle.Now many implants put under the muscle look fine if the above problems don't occur. But they do occur occasionally. However these problems are less of an issue if implants are placed on top of the muscle.What I can say is that this work very well for my patients. I follow nearly all of them for five years if I can.All the best.