Should Implants Be Placed Under or over the Muscle for a Slim Frame?

It's a great question. I strongly prefer under the muscle implants because the muscle provides another layer of camouflage for your implants. The muscle contours the top of the implant to give it a more natural slope and hide rippling of the implant. Implants under the muscle have a decreased risk of scar tissue forming around the implant (capsular contracture).  Also, breast cancer screening with mammography gives better images of breast tissue when implants are placed under the muscle. Although an implant on top of the muscle can be a good choice for some women, for very thin women there is not enough camouflage for the implant to look natural. When considering breast augmentation make sure you find a board certified plastic surgeon who will take the time to answer all of your questions and explain all of your options, so that you can decide if breast augmentation is right for you.

If there is thin skin and soft tissue, it is best to place implants in the submuscular plane. The more soft tissue between the implant and the outside world, the better the result. There will be less visibility of the implant edges, less chance of rippling, and a more aesthetic contour i the upper portion of the breast. If the nipple position is above or slightly below the fold of the breast this is the best approach.

If you are very thin, then you are better off having as much soft tissue coverage as possible. The muscle adds another layer especially in the upper pole.

If you have thin tissue coverage, then the best placement for the breast implant is under the chest muscle. It provides more coverage, better shape, better for mammograms and has been documented to have lower capsular contracture rate. The implants will move when you flex the chest muscle so patients who are body builders may have concerns about that but otherwise most patients who are thin will do better with under the muscle placement.

I generally prefer to place the implant under the muscle especially with a thin patient. There are many options in breast augmentation. Educate yourself about the different choices. The more soft over the implant the more natural the result. If you are thin and you place the implant above the muscle any degree of capsular contracture wiill give you a very rounded, implanted appearance. If your goal is a more natural result then below the muscle may be the best approach.

ANNAF:
When I discuss this issue with patients, I recommend subpectoral (under the muscle) for women unless there is an unusual reason to do otherwise. Discuss with each of your surgeon choices why they advise the method they do.
Subpectoral placement generally improves mammograms, lessens capsular contracture risks, and contours smoothly over the upper portion of the implant. Thinner patients benefit more from the latter consideration.
Silicone gel filled implants have recently become more popular, since the FDA permitted their marketing by manufacturers, although saline are still very effective and safe.

In general, the thinner you are, the more we prefer to put implants under the muscle in order to give you a better "cushion" and better tissue on top of the implant. It is difficult to say that it is wrong to put the implant above the muscle. These are very individual decisions. I would say that most would recommend below the muscle for a slim individual with thin breast tissue. Good luck.

You have been given some good advice from the previous answers. I would recommend that you go under the muscle. With a small pinch test, the right answer is under the muscle. Even if you had a thicker pinch test the long term issues favor an under muscle pocket.
The silicone implant is always a good choice.

You have to decide what is important to you and what risk you are willing to assume. The reason why there are different opinions and techniques is that there are different advantages and disadvantages. Both can give acceptable results depending on what you want.
Rolls Royces are no better or worse than Ferraris. If you want a fast car, get a Ferrari. If you want comfort, get a Rolls. But personally, I drive a Mercedes and Honda. You get the picture.
Under the muscle technique is used mainly because it reduces the risk and incidence of capsular contracture or hardening, unfortunately, not to zero, though.
Theoretically, over the muscle technique is the most natural because that is where the breast normal lives. Nothing lives under the muscle except for the ribs! However, depending on the size and dimension of your implant, an under muscle implant can look very natural while an over the muscle implant can look very unnatural.
In the end, it is a combination of multiple factors that are interrelated, but not directly comparable, that you need to assess. There is no such thing as best, only choices. Your surgeons should discuss all factors with you. Having all the facts and option, you cannot make a bad choice. You will make the best choice at the time you make it.
Usually, the thinner you are, the smaller the implant you can tolerate above the muscle and the more the under muscle technique makes sense as the size of the implant increases. Of course,there comes a point when the size of the implant will make either technique more unnatural and riskier.

Hard to say with out examining you but it sounds like you should go under the muscle.  Better coverage of the implant, less chance of encapsulation most of us feel. If you are that thin then under the muscle will be more natural.
Good luck!