While I can't address a complicated fistula as mine was rather mundane, I do know that muscle involvement comes into play even with a seton. I was told my seton was in a non-muscular area so I would have less risk of incontinence and I was not having surgery beyond that. I think it is great you are going to see a well versed specialist. Make sure you address this concern with them. I only hope for you that the seton can fix your problem without surgery so you are as lucky as I was!