For me it broke the rules of really good body-horror, in that it was just too much, too fast. The same thing could have been done in a much more subtle way; say, she tries to walk forwards, but trips with a horrible noise. We see the camera on her face, she does a pain/shock
face then we pan back to reveal one leg twisted as if the kneecap has been smashed, not moving except perhaps twitching, just a really good latex piece. It's not showing the backwards knee that's bad (unlike ghost stories, body horror has to actually show us something), it's showing the actual reversal of the knee. Body horror (in my opinion) rests upon our memories of our own fear and confusion during puberty and sexual awakening, and also our anxiety about medicine etc. The key element is the sudden realisation that you've changed, the slow inexorability of it, the act of removing a bandage or piece of clothing and suddenly there is something under it that's not
you. For the viewer, tension builds in the space between the character realising that something is wrong with them and the audience seeing what is wrong.