How can I keep my implants from sliding into my armpit?

Hi.  It sounds like your implant pocket has enlarged, opening up towards the side.  This is called "lateral pocket expansion", and yes, it can be fixed.  I usually use a internal suture technique to repair the opened-up area at the side, together with a capsulotomy to re-open the pocket in the cleavage area.  This gets the implant "front and center", where it should be.  But it would have to be done through a different incision that your previous armpit (axillary) approach.Sometimes, particularly in very thin patients or ladies that have had a recurrence, an additional step of placing a reinforcing material (strattice or others) in the area of the sutures is necessary.Best to see an experienced board-certified plastic surgeon in your area.

  I am sorry to hear about the problem you are experiencing, with lateral breast implant displacement. The lateral breast implant displacement can be corrected using an internal suture technique, decreasing the size of the pockets and moving the implants toward the midline.   The use of acellular dermal matrix is an option ( although not usually necessary)  especially if significant implant rippling/palpability is present.
I hope this (and the attached link) helps.

YES! Revision and closure of the lateral pocket can be done in another operation, most likely through the infra mammary approach...