The modern breast implant was developed by American plastic surgeons Thomas Cronin and Frank Gerow in coordination with the Dow Corning Corporation in 1961. The implants were made with a silicone elastomer (synthetic rubber) shell and filled with silicone gel.
A few years later the saline filled silicone implant was introduced. The choice of silicone was obvious; it is a material that is deformable but always returns to its original shape, it is stable across large variations in temperature and most importantly it is biologically inert, meaning it is not readily reactive with other chemical elements and has no pharmacologic or therapeutic action.
Despite the wonderful properties of silicone over time it was found that the implants could spontaneously rupture. Early studies showed that the older the implant was the more likely it was to rupture. Ruptures could be as small as a pin-sized hole which, if the implant was filled with saline, would result in most if not all of the saline leaking out and the implant deflating. Or they could be large tears in the silicone shell which could, if filled with silicone gel, cause the gel to escape the implant. Despite over 50 years of study of these implants no definitive explanation for these ruptures has been found.
One theory for rupture suggests that micro-fractures in the implant shell develop in areas of the shell that are, for unknown reasons, mechanically weak. It is theorized that as the implant moves around the pocket and the shell gently folds and unfolds weaknesses develop.
Another study which looked at the mechanical properties of ruptured implants found that the shells were mechanically weak in areas of rupture. The thought is that when the shells are stretched, such as during surgical insertion, this weakens the silicone elastomer.
The good news is that over the past 50 years as manufacturing technology has improved substantially so have the quality of breast implants. We are now using 5th generation implants and rupture rates continue to decline. The use of cohesive silicone gel has been a significant leap forward. Because this gel is much more viscous than previous gels there is minimal concern if the implant ruptures.