It's interesting when you're doing research for a story. Sometimes you run across things like this. https://www.youtube.com/watch?v=qvGWM3Lw5RA
Now do I think Ivar was someone who had brittle bone disease? I don't know. While it is plausible I doubt it. Nonetheless I felt the documentary held some rather interesting concepts as to why he was nicknamed "the boneless." Personally I like the one where he probably had gigantism.