Although extremely rare, it is possible to bleed post-operatively resulting in another surgery to control and drain the collected blood. You could develop a post-operative infection and need to have the implant removed, the infection dealt with and still have to wait for several months before an additional surgery can be performed to re-implant. Loss of sensitivity is common, although temporary. Permanent sensation loss in the areola (nipple) area or breasts, in general, can and may happen. There is also the possibility of developing a Seroma which is a mass caused by the accumulation of serum fluid within a tissue or organ. Or a Hematoma which is a localized mass of blood that is typically confined within an organ, tissue, space, or potential space and may be a result from a broken blood vessel.
There is a risk of Capsular Contracture (the evil scar tissue encapsulating the implant, hardening around and squeezing the implant). This rarely ever goes away on its own. Nor does it tend to lay dormant after a revision surgery is performed. It may happen due to bacteria on the implant, surgical implements or airborne and the body attempting to place the foreign body as far away from itself as possible. Or it may develop after injury. If this happens, you can develop pains, hardening, deformity and deflation of the implant. It sometimes even happens again after the surgery to remove the scar tissue has be performed.
There is a chance of rippling (wrinkling or indentations from the implant) being apparent, especially when one has no breast tissue and chooses to go over the muscle. It is possible that the implant can shift and push through layers of tissue, showing through the skin. The implant can deflate or rupture from an injury or from wear and tear from an improperly under filled implant (even your breathing motions can cause creasing in the implant causing it to weaken at these creases). Even an overzealous doctor performing a mammogram can rupture your implant. You can have a complete deflation within several hours if it is an un-encapsulated saline-filled implant. If it is a silicone gel-filled implant, you may not know for months or years. Of course either way, they will have to be replaced. Then there is always a risk of hematoma and scarring. Also, difficulty in early breast tumor detection is possible when you have either silicone gel (more pronounced) or saline-filled silicone shelled implants.
There is also the risk of disappointment in size. A lot of women wish they would have gone bigger. Realize that when you are doing the rice test that they will have to add a little more to make up for the tissues and/or muscle flattening the implant a little if you choose the submuscular placements. When you pre-operatively try on the larger bras and fill them out a bit, they are on top of your body, probably lifted, as well, by an under wire. Take this into account and communicate with your doctor, the results you really want.
There is also the disappointment in the implants not lifting the breasts as you would like. This is not a breast lift this is an augmentation. If it is lift you want as well as augmentation, get them both. After your augmentation surgery, the breasts will be heavier than what you are accustomed to. The heavier weight will speed up the sagging process especially if you go around braless all of the time.
There is the possibility of extrusion of the implant, breast tissue atrophy from the force exerted by the implant. This is according to the F.D.A. and you can read the info on their site by visiting: Breast Implants: An Informational Update.