The technique involves placing tissue expanders (interim implants) under the skin on top of a woman's pectoralis muscles after her breast tissue has been removed. (sciencedaily.com)
You may have already noticed some boys starting to get chest muscles (called the pectoralis muscles or pecs for short). (kidshealth.org)
When a plastic surgeon performs a breast augmentation, they place the inert implant either beneath the milk producing tissues or deep to the pectoralis muscle. (realself.com)