Forgot to mention that when Haytham writes that Stephane and Deborah's knees were cut, he heard it from Connor who was actually referring to certain key muscles and tendons. They aren't amputees, but key tendons and muscles were sliced through in such a way that they're unable to get the full use of a particular leg (hence the limping, though it's actually limping with the help of a crutch).

I remember reading a HP fanfic a long time ago where Voldemort slit the tendons in Harry's ankles or knees (don't recall which). It stuck with me.