Complete submuscular placement does not literally mean "completely under the muscle." While the vast majority of the breast implant is covered by muscle, there is a small portion of the implant that isn't covered by the muscle. The pectoral muscles (also called "pecs") are fan-shaped. As with partial unders, the pecs cover about 2/3 of the upper portion of the breast implant. However, with complete unders, the breast implant is also behind the serratus anterior muscles (see illustration below). The lower pole of the implant is supported by the fascia (note the white portion at the bottom of the implant in the image to the left). Fascia is fibrous connective tissue composed mainly of collagen fibers. It is very strong, and stretches slowly, but it offers wonderful internal support for breast implants. Some surgeons equate this support to an "internal bra."
Advantages of Partial Submuscular Breast Implant Placement
In women with very little breast tissue, breast implant rippling can be camouflaged the most by having the implants placed in the complete submuscular plane. Rippling is more commonly noticeable on the outer sides of the breasts. Since the outer side of the implants are covered by both the serratus and pectoral muscles, visible or palpable rippling is not as much of a risk.
Complete unders can offer a more natural slope to the top of the breast. However, if you want complete unders and a fake look, you can get both. (You must let your surgeon know that you are looking for upper pole roundness.)
Bottoming out is less of a risk since the bottom of the breast implant is supported by the fascia. Capsular contracture rates with complete submuscular placement are also lower.
Disadvantages of Partial Submuscular Breast Implant Placement
While most surgeons do both over the muscle and under the muscle breast implant placement, not all of them do complete unders. More surgeons are offering this placement than they were a decade ago, so it isn't quite as hard to find a surgeon experienced with this placement.
Completely submuscular breast implants can often take longer to settle than partial unders. Not only do the pectoral muscles need to stretch, the fascia also needs to stretch, as the fascia is what supports the lower pole of the breast implant. Because the fascia stretches somewhat slowly, the implants will likely take a bit longer to settle ("drop") than in women with partial submuscular placement.
Recovering with totally submuscular breast implants results in more soreness and discomfort post-operatively due to the muscle stretching to accommodate the breast implant.
As with partial unders, size is more limited with complete unders versus implants over the muscle. This is because the muscle will only stretch so much, particularly if it is a primary (first) breast augmentation surgery. However, this does not mean that it is impossible to get anything other than a very small breast implant simply because nothing other than a small implant will fit underneath the muscle. Larger implants can be placed underneath the muscle, even in first-time breast augmentation patients; however, each individual is unique. Just because woman A gets 550cc underneath the muscle the first time does not mean that woman B can accommodate that large of a size the first time. The point is to note that size is always more limited when going underneath the pectoral muscle, whether it's partial or complete unders.
Muscle distortion occurs when the implants are placed underneath the muscle. When the pectoral muscles are flexed, the implants "jump." It is important to remember that this occurs with all submusclar implant placement. Some cases are worse than others. If you are a bodybuilder and work your chest muscles a great deal, muscle distortion may be more noticeable. Bodybuilders often notice that they have a wide space between the implants as well. This is due to the well-developed pectoral muscles. If you're not a bodybuilder, you will probably still notice that the implants move when your chest muscles are flexed, but not to the same extent as someone that regularly works their pectoral (chest) muscles. While anyone with "unders" knows that it is possible to make their implants move by flexing their chest muscles, it is not noticeable to other people unless you want it to be.
While the pectoral muscles can be released to a degree to make muscle distortion less noticeable, it should be noted that over-dissection of the chest muscles can result in symmastia, which is complicated to repair, not to mention expensive.