Saline implants can of course end up with some trapped air, no question about that. This can be in part due to air dissolved in solution, coming out of solution post-operatively. This can of course lead to the unfortunate 'sloshing' sound some women experience.
As far as the risk of saline contamination, one prophylactic approach is to add something like 10% Betadine to the saline. I don't recall where I'd first read about it, but when we consulted with Dr Stagnone back in 2007, he said that yes, he gives his patients this option, adding that there's been a study showing that Betadine can degrade implant integrity over time (no mention of whether it was 10% of the solution, or 100% concentration, or what), then went on to say that for his patients who requested it, they've never reported any such issue.
Hang on, I managed to dig up an old email I'd written to someone about this. Here's the relevant snippet:
I had mentioned reading about one of the local surgeons (couldn't remember his name right away) and how he reduces the rate of capsular contracture by liberal usage of antibiotics. Dr Stagnone picked up on this and ran with it. He said that was Dr Adams, and said that his published approach definitely helps. Using injectibles as well as washing out the pocket with a Betadine solution (and rinsing with saline afterwards) is the way he prefers to do things. He mentioned that he prefers to add a small amount of Betadine to the implant saline itself as yet another level of keeping things sterile. He said that there have been studies done showing that Betadine can be damaging to the implant shell, but he pointed out that (for those patients who agree to this technique) for the long-term patients who have been with him, they haven't experienced premature implant failure. He asked what our preference was, and we said to definitely stick with the Betadine, so he wrote that down as well. He said that Dr Adams had come up with an alternative, that it wasn't quite as effective as using Betadine, but that it wasn't bad as alternatives go. His personal preference was still with using Betadine.