In the United States, placing breast implants partially below the muscle is very common. When the implants are situated partially below the muscle, it means that the upper 2/3 of the breast implant is covered by your pectoral muscle (red line in front of implant in photo). The the bottom of the breast implant is supported by your breast tissue (yellow area in front of implant in image).
Advantages of Partial Submuscular Breast Implant Placement
Partial submuscular breast implant placement allows for better views during mammography screenings. The pectoral muscle actually helps push the breast tissue forward. The breast implant itself and the breast tissue are not in much contact with one another except for at the lower portion of the breast.
Women with very little breast tissue gain quite a few advantages with partial submuscular placement. In many cases, breast implant rippling can be camouflaged by pectoral muscle.
Partial unders can offer a much more natural slope to the upper pole of the breast. That's not to say a "fake" look can't be achieved with partial unders. You can achieve a fake look, but implant size, profile, and the way the pocket is created all play a role in this, so be sure to discuss this with your surgeon.
Capsular contracture rates tend to be lower in women with partial unders as opposed to women with their breast implants placed above the muscle.
Disadvantages of Partial Submuscular Breast Implant Placement
When breast implants are placed behind the muscle, more discomfort is to be expected in the early post-operative period. Imagine how a pulled muscle feels. Under the muscle breast implants give a similar feeling during recovery except that it is more exaggerated than your average pulled muscle. This is because the muscle has to stretch to accommodate the implant. Recovery with partial unders will take a bit longer.
Breast implant size may be limited when going under the muscle, as the muscle can only stretch so much. So, if, for example, you wanted 500cc breast implants, it may be that that much won't fit underneath the muscle during your first breast augmentation. This is why some women opt for implants over the muscle. However, keep in mind that going submuscular does not mean that you have to get a very small implant. It simply means that you likely won't be able to get extremely large breast implants during your initial surgery.
Muscle distortion is seen with implants placed beneath the muscle. 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. 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.
The pectoral muscles can be released to a certain degree, but over-dissection means a much bigger risk of symmastia, a difficult complication to repair, not to mention costly.