Pocket Stretched 6 Months After Breast Implants

This is a difficult problem to predict and therefore, prevent. Some patients are more prine to this problem and certainly larger implants can exacerbate this tendency. A revision may be required as well as long term use of nocturnal support garments to prevent recurrence. Textured implants could help but have their drawbacks.

Since the muscle covers the top and inner portion of the implant but not the outer part or bottom, occasionally the muscle essentially pushes the implant down and out. It is usually repaired by sewing up the pocket surgically. However, if your tissues are thin around the implant, the stitching may not be enough to prevent it from recurring. In that case a reinforcing graft with a material called Strattice will be helpful: www.renewingyou.com

From the description, it sounds like you dislike the position of your submuscular implant when you are lying flat- the implant slides off your chest into your armpit region. It could be the case the outside portion of your implant pocket was made too large, allowing the implant to displace too far to the side. Without a photo, this is all conjecture. If the problem really bothers you, I'd recommend waiting about 12 months for the implant capsule to thicken, and then consider a lateral capsular repair (capsulorraphy) and a medial capsulotomy with some additional muscle release to centralize the implants. 

Check out the weblink below for an example of this problem.