Breast implants are medical devices that are implanted either under breast tissue or under the chest muscle for breast augmentation or reconstruction. There are two major types: saline-filled and silicone gel-filled. Saline-filled breast implants are silicone shells that are either prefilled or filled with saline during surgery, and some of these allow for adjustments of the filler volume after surgery. Silicone gel-filled breast implants are silicone shells prefilled with silicone gel. Breast implants vary in profile, size, and shell surface (smooth or textured).
Source: Center for Devices and Radiological Health