Breast implants over or under the muscle.
The appearance of your breasts depends on several factors and has no simple yes or no answer. Let me give you an oversimplified analogy to explain.
If you take a baseball and cover it with a silk sheet, you will see all the details of the ball and it will be "obvious". If you cover it with a comforter, it may look like a small "natural" bump. Do you understand? Same baseball but two different degrees of coverage. Same goes with your breast implant and the amount of tissue you have to cover it.
I am not aware of any study which states subglandular placement has a higher risk of losing nipple sensation but it does make sense.
The decision as to which pocket to use for implant placement is based on one main issue - tissue coverage. If you are very thin, going under the muscle is the preferred method. The muscle is only released in a certain segment and retains its function. If you do not wish to have the implant placed under the muscle, then there are some trade-offs. With thinner tissue coverage you will have more implant visibility and rippling showing through. I would highly recommend use of silicone gel implants above the muscle. You also do have a higher rate of capsular contracture above the muscle. These are all issues to discuss with your board certified plastic surgeon before undergoing the procedure. If you don't like the trade-offs with either placement, then having no surgery may be the answer. Good luck.
Sub Glandular Breast Implants
I believe that Sub Glandular Breast Implants are the most reasonable choice for people like you for many reasons. Some are the ones you stated. When you move the arm, the implant under the muscle will deform. This is more readily apparent in people that are small. There is also a higher incidence of implant rupture in some studies when placed under the muscle. Additionally, the recovery when the implant is placed over the muscle is significantly faster (6-48 hours). There is a higher risk of tight scar around the implant making it feel firm when a smooth implant is placed over the muscle. For that reason I use a textured implant that tends to stay soft. Most people will have enough tissue to cover the implant. However, if you wish to achieve the most natural feel, you should use a silicone implant. When done properly, a textured silicone implant above the muscle can look and feel quite natural. Nipple sensitivity is the same, no matter where the implant is placed. Good luck. Don’t get talked into something with which you are uncomfortable.
Breast augmentation is technically rather simply but very complex in terms of decision-making. There is no easy answer as a paitent and surgeon has to consider a mukltitude of factors, some of which you have mentioned. In general, you are better off going smaller than large. If you have minimal breast tissue, you get better coverage of the implant goingh submuscular. There are always going to be tradeoffs that you nee to discuss and accept. Sometimes the most important thing is not deciding on what to do based on what you get but deciding on what not to do because of what you don't want.
Above vs below the muscle in a thin woman
You will find a couple of opinions in your case from different plastic surgeons. I personally am a strong advocate of partially under the muscle breast augmentation to provide a better coverage of the implant in the upper and inner breast. I have minimal complaints from my patients about movement of their breasts with flexion. The tradeoff os the increased potential for visibility of the implants in the forms of visible ripples or palpable folds. Your best bet is to meet with a board-certified plastic surgeon and after your exam have them review the option(s) they think will work best for you. I wish you well.
Subpectoral v. Subglandular
Because you have a small build I would recommend going under the muscle. Most of the breast augmentations we perform at my practice are done sub-pectoral. Another important factor to consider is whether you are going to use saline or silicone implants. Saline is more likely to show rippling and if you choose a subglandular placement that rippling has very little camouflage and will be more evident. Silicone implants are less likely to ripple, but I usually still recommend going under the muscle. Every woman’s body is different so talk with your surgeon and ask to see pictures.
Subglandular or "above the muscle" breast implants
If you are a think woman, a breast implant in the subglandular (above the muscle) location may be difficult to conceal. This may lead to an unnatural appearance, or even visible rippling, compared to placing the implant in a subpectoral (under the muscle) location. Also, implants in the subglandular location have a higher incidence of capsular contracture (hard scar tissue). The recovery is more involved with the subpectoral procedure, however the benefits in the long run usually outweigh the additional pain associated with the surgery.
You should consult with several board certified plastic surgeons in your area until you are comfortable with a surgical plan, including incision, location of breast implant, and type of breast implant.
Breast Implant "Over" or "Under" the muscle
The most important decision on breast implant placement is the amount of soft tissue coverage that you have. The more soft tissue that you have the less you will need to place the implant "under" the muscle.
There are pros and cons with both subglandular and subpectoral placement of the implants. You mentioned motion deformity, nipple sensation and "natural look" are all issues that needs thoughtful discussion with your plastic surgeon.
Typically, on a petite build with small breast that have mild sagging, the choice generally favors under the muscle placement even with the possibility of motion deformity.
The final choice may also dependent on your desired size and whether the need for breast lift exists.
Subglandular vs. Subpectoral
I would not place the implants subglandular just for the animations deformity that you can get, unless you are a bodybuilder that competes. In that case, I think it would be justified to place the implants in a subglandular position.
Other than the the risks and benefits are not in your favor. You will have more rippling, higher chance of capsular contracture and more of an unnatural look. Since you are thin and dont have much overlying tissue, then you are even more at risk for rippling and more of the unnatural look.
Breast Implants Above or Below the Muscle
There are pros and cons to breast implant placement above or below the chest wall muscle. In a woman with a thin build with a lack of breast tissue, placing the implant above the muscle may make it difficulty to camouflage the implant. The upper portion of the breast may appear rounder and you may be more likely to see visible rippling. To lessen the chances of this, and to provide a more natural appearance, you almost certainly should choose a silicone implant. With the older generation of silicone gel implants, capsular contractures (hard, painful scar capsules around the implant) were more frequent with implant placement above the muscle; it's not clear if this is the case for the silicone breast implants currently utilized.
Placing the implants below the muscle may actually provide you with your best chancefor a natural appearance; it's impossible to know without actually examining you. You should definitely see a board-certified plastic surgeon who can examine you and engage you in a detailed conversation about implant types and placement above and below the muscle.
Hope this helps. Best of luck.