Valve replacement and subsequent scar tissue after 2nd replacement

My wife has had 2 valve replacements, she is 60 years old and her istory started with some other surgery when she was nine years old.  She had a pig valve done in May of 2011, she now has scar tissue mass behind the wire used to close her chest.  Is it exceedingly more dangerous when going for a 3rd replacement verses a 2nd one. I was told it was almost impossible!  How long does it take for scar tissue from the 2nd replacement to form and how serious is it.  

[CCF HeartMD6]

Any re-do surgery that involves an open heart procedure is more complicated than the first one. A third re-operation carries more difficulty in having to go through the scar tissue from previous surgeries, but is not uncommon. These re-do open heart surgeries are often performed by skilled surgeons at most major academic centers.