My OP was flat and I got a bit of a chimp lip with 6 mm advancement.
I've brought this up on a few different posts but didn't mention you specifically since I wasn't sure whether you were okay with sharing that publicly.
I think 'chimp lip' is a risk regardless of the OP steepness. it just tends to look worse on steep OP patients because 1) the pre-op philtrum is typically already one the longer side and 2) philtrum is more likely to be flatter pre-op. It seems to me that patients with steep OPs and various forms of open bite are more likely to suffer from lip incompetence (versus short-face, deep bite patterns) and therefore the upper lip is pulled downward to form the lip seal. This lengthens the philtrum over time, per Gunson.
