Your orthodontist might be entirely correct. A rapid palatal expander is used in children or teenagers as the body grows and we can use the body's own growth pattern to expand the upper jaw. this can happen only if the median palatal suture is still open. that is the ridge in the center of the palate that runs from step back. as an adult, did suture is closed. if you do attempt to expand the palat on in adults, you might end up pushing it back teeth outside the bony limits of the jaw. Effectively pushing the teeth out of the bone to the side. Unfortunately, the best advice I can give you is to obtain a second opinion from an orthodontist who treats adults. Best of luck, Dr. Zev Kaufman