Here is a link to a study with some very interesting numbers ...
I always knew that following LIS there is a dramatic decrease in anal resting pressure, followed by a gradual increase over the course of the first year. What I didn't know is that even at its lowest point following surgery the anal pressure is still higher than that of healthy controls, and further is significantly higher than those same healthy controls in the data one year later. While the pressure doesn't seem to ever reach pre surgery levels, it does rise quite high. The take home message in that is even after LIS it is a wise thing to keep the diet safe and sane permanently and to keep practicing whatever works for you to keep those stools soft, easy to pass, etc., and it would seem to be even more important to do so the longer the period of time you are from your surgery. It would be interesting to see data even further out, like 3 years, 5 years, etc. It also makes me wonder about rates of fissure recurrence following LIS, particularly years later. It seems logical to think that LIS works really well for healing the fissure you've got now, but doesn't do that much to your physiology to prevent it from happening again.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1357726/