Breast augmentation is a very common procedure in the United States, and there are different anatomical areas where breast implants can be placed. You can place the implant under the muscle or above the muscle. I prefer putting the implant under the muscle because it is the best anatomical place for a foreign body like a silicone breast implant; it provides good coverage and helps prevent aesthetic complications.
If your implant is in a partial submuscular plane and you have a certain amount of animation deformity, it may be a result of inferior attachment of the pectoralis major muscle because a capsule has been formed around the implant; sometimes, you can fix this issue by cutting the scar and releasing the implant from the surrounding scar formation.
There are different grades of animation deformities. For the most part, they do not bother the patients most of the time. The best way to prevent an animation deformity is to place the implant above the muscle. However, in my opinion, the benefits of having the implant under the muscle, including improved upper-pole soft tissue coverage, camouflage in thin patients, less visible rippling, are worth the risk of animation deformity.
Breast augmentation can do wonders for a patient, but in reality, you also need to accept the risk of rare complications and their consequences. If you develop an animation deformity, you might need a revision surgery if it bothers you. Most of the time, if you are not compressing the pectorals major muscle, the breast will have a normal shape.