This is a frequent concern for women. Ruptures are always possible with implants. That said, the odds are against such a rupture happening during sexual activity. If you do experience breast pain, then there is a slight potential of a rupture occurring.
Ruptures most often occur in older implants due to normal aging. Chest trauma, such as a motor vehicle accident, can result in rupture. Saline leaks out quickly, so the rupture is immediately apparent. Because of its gel form, it takes silicone longer to leak out, and the free silicone can get trapped in the scar tissue surrounding the implant. The leakage may have been going on for a while before it becomes obvious. For this reason, imaging tests are recommended every few years to ensure the integrity of silicone implants.