Does having implants wider than your breast base width pose any increased long term risks?

Thanks for your inquiry.  Every doctor has different ways to assess
their patients and choose the right implant sizes.  It is not uncommon to have 3 different
measurements from 3 different plastic surgeons. 
Some doctors are very diligent about sticking to the numbers of the base
width diameter and others a little bit more liberal with it.  So, I would not focus on the actual numbers,
I would focus more on the results you are looking for and whether or not your
doctor can achieve those results.  One
way to do this is to look at the doctor’s before and after photos and find
photos that are similar to your build. 
Then, look for results that look similar to what you want.  I hope this helps. 

I appreciate your question
After performing hundreds of breast augmentations, I have found that the size of implant best for you is dictated by your chest wall measurements.  Once we determine that we can choose the profile based on what you want or need to achieve.  If you are seeking a natural look, then the diameter of the implant should be equal to or, more ideally, smaller than the width of your breast. The breast width is a measurement of how wide your breast is at the base, which should be measured at the level of the nipple. Choosing an implant that is smaller in diameter than your breast width will avoid the "side breast" fullness that is often associated with a more artificial appearance. Implants that are wider than this can be more palpable, fall more laterally overtime and also be more palpable. After that we will then choose the implant volume based on the measurements and also looking at any asymmetries you might have. You should choose a board certified plastic surgeon that you trust to help guide you in this decision. Bringing photos as well can help us to finalize, but please understand that these are of other peoples bodies and your results will probably be different.
Silicone will give you a fullness at the top (upper pole fullness). Silicone implants come pre-filled with a silicone gel and are the softest implant available. They feel more natural, which makes them a good option for women with less natural breast tissue; but they require a larger incision. It may be more difficult to realize if this type of implant has ruptured, so it is important to monitor them with annual follow-up visits. Additionally, because this implant contains a more softer silicone (less cross-linked), if this implant should rupture, it may leak but only into the scar capsule formed. However, the newer silicone in these implants may not leak even when the implant shell is ruptured. 
Anatomic gummy bear implants might be a good choice to give you a more natural look. These highly-sought-after, anatomic implants offer a look that more closely resembles the natural silhouette of a breast, and, therefore, are a very attractive option for individuals seeking a natural-looking, aesthetic primary breast augmentation. Additionally, these implants are an especially excellent option for patients undergoing restorative or corrective breast surgery because they provide more stability, shape, and reduced incidence of capsular contracture. Compared to other types of silicone gel implants, the silicone in the cohesive gel implant is more cross-linked; therefore, should the implant shell “rupture,” it maintains its shape and silicone does not leak.
During your breast augmentation consultation, you should feel the different types of implants available, and try on various implant sizers in front of a mirror to help you to get an idea of how you will look following the surgery. You should also bring pictures of the look you would like to achieve, as well as a favorite top to wear when trying on implant sizers. I also use a 3D imaging system to allow you to try on a variety of shapes and sizes. I prefer this method over adding volume due to implants being under the muscle.
We will also discuss the possibility of needing a lift along with an implant. This will depend on the amount of ptosis (nipple position) as it relates to your breast, inframammary crease and chest wall. Needing a lift or not will also determine where to place the scar for the incision.  In some cases if the the breast and nipple is really ptotic (low) it might be best to stage the procedure by performing a lift first and then doing an implant at a second stage. This will also allow you to have a better understanding of the true size and shape of your breast.
The best way to assess and give true advice would be an in-person exam. Please see a board-certified plastic surgeon that specializes in aesthetic and restorative breast surgery.
best of luck!
Dr Schwartz
Board Certified Plastic Surgeon 
#RealSelf100Surgeon
#RealSelfCORESurgeon

The goal in Breast Augmentation today is to respect the natural diameter of your breast.  When I advise patients about this. I want to stay within one centimeter of your starting point.  Sometimes a very narrow or unusually shaped breast needs a more aggressive approach.  If you stretch the base diameter more than this, there is increased risk of tissue damage as you asked.  The results would include a more visible implant, an implant edge that would be easier to feel, wrinkling, rippling, loss of nipple sensation, and even implant position problems.  Many of these are not correctable and must be avoided.  
Please be sure to consult with a Board Certified Plastic Surgeon to discuss your options.
Best of luck in your research
Dr. Schwartz