With saline implants, if the implant breaks, the saline is absorbed by the body (it is the same saline that is used in your IV during surgery so it won't hurt you). Instead of having a full implant, you will simply have a deflated implant until the time that you have it removed/replaced.
The newer silicone gel implants are called cohesive gel. They are very soft but are so sticky and cohesive that you can literally cut into them and they will hold their shape. In fact, I keep one in my office that I cut to show to my patients. This feature makes a ruptured implant easier to remove intact but more difficult to detect using conventional techniques. An MRI may be necessary to fully determine whether or not a cohesive gel implant is actually ruptured. Even with a ruptured implant, you might go for a very long period of time without any problems. The main problem that you might encounter would be a capsular contracture, the formation of thickened scar tissue around the broken implant which can make the breast feel firm or distort the shape of the breast. When this happens, surgery may be required to remove the scar tissue and replace the broken implant.