Yes. Implants can slip out from under the muscle especially since only the top half of the implant is placed beneath the muscle. This is of particular risk if you have very large, heavy implants. Hope this helps.
Yes, over time, especially with large heavy implants, your breasts may stretch and sag so that the implants are no longer under the muscle, or much less of the implant remains under the muscle. The pocket may stretch over time to let the implants drop. The pocket could be tightened. If you have large heavy implants, you may need to change to a smaller implant that will not be so heavy on your tissues.