With respect to the shell design, while most breast implants are single lumen (just the shell), some breast implants are double lumen (one shell inside another shell). With respect to the filler, some breast implants are manufactured with a fixed volume of filler, some are filled during the operation, and some allow for adjustments of the filler volume after the operation.

It should be noted that tissue expanders, which are silicone shells filled with saline, are regulated by FDA in a different way than breast implants. This is because tissue expanders are intended for general tissue expansion for a maximum of 6 months, after which, they are to be removed. Because of this, the design specifications (e.g., thinner shell) and preclinical testing recommendations are different for tissue expanders than for breast implants. Tissue expanders are not to be confused with the third type of double lumen silicone gel-filled brea