Studies indicate that rupture or deflation occurs in roughly 10 - 25 percent of silicone gel and saline implants over 10 years. Blunt force trauma hardly ever ruptures a breast implant. Usually, a sharp object would have to cause this. At some point, the implant shell weakens through the years and is more capable of being ruptured. If there is very little breast tissue and the patient is not depending upon her upper body strength for a career such as weightlifting, boxing, bodybuilding, etc., submuscular placement is a better choice. Going above the muscle is not unreasonable in your situation. Discuss these options with your board certified plastic surgeon.
Repeated trauma may increase the chance of implant rupture, but the rate of this is probably very small. As long as my patient understands this, I think it is safe to proceed with surgery and continue boxing after you are well healed.