Clearly, you are mistaken what a internal bra actually is; think along the lines of a shelf, like a sling or hammock, fastened on either side, medial and lateral. It is not bra shaped; there are no cups. It hold the base of the implant in place to avoid the gradual, and inevitable, effects of gravity. It is constructed from pigskin, which grows into the under-breast skin, adding extreme strength. It was taken from the strategy used in difficult abdominal hernia repairs. The internal bra allows for continued implant expansion because it is not attached to the implant nor does it in any way constrain the implant; it only creates a set height for the inferior aspect of the implant.
Today, a slightly older picture.