Why has my permanent silicone implant following mastectomy slid underneath my armpit?

Unfortunately I see this problem often in my practice from regular breast augmentations to breast reconstructions. Essentially the pocket that was created is either to large or not reinforced adequately. Both of these problems can be fixed with revisional surgery by using mesh, permanent sutures, and possibly a smaller implant with texture. I hope this helps!

If the expander was migrating, the pocket was too large and open into your armpit.  The pocket would have to be closed down at the time of the exchange or some allograft placed to hold the implant in place.  The pocket can be sutured, heated with cautery and or allograft.  Correction of this problem will require surgery but it can be fixed.

It is difficult to say why your implant has slid to the side without more information on your initial surgery, and on your most recent one.  Usually when this happens, it is because your dermal matrix did not incorporate, or the suture tore through allowing the implant to move out of the desired pocket.  This is often difficult to fix with sutures alone, and may require another piece of dermal matrix.  When your general surgeon removed your breast tissue, they likely made quite a large space.  Your plastic surgeon worked to make the space smaller, to keep your implant in a good position, but sometimes things do not heal as expected.   There are always options for a revision in the future, so do not loose hope!