Well, breaking your legs itself shouldn't have any risk.
Having contact with the materials the fixators were made of shouldn't, since they wouldn't have been made of these materials. And if there was any risk, then it would be because it's still unknown that the material is dangerous. So leaning on a fence of the same material should be dangerous, too (well, ignoring the fact there would be a skin between the fence and your bone, but it'd still cause you other problems, if not in the bones, directly). However, it should be safe unless you get fixators made of plumb or (edit: dunno the exact translation. Absesto(s), maybe?) (yeah I'm joking, those don't exist since it's been a while we're aware of their effects, lol)
So the only risk is in substances. Painkillers are used for many other situations and the ones that are used don't have any big risk so far (I'd rather worry about heart issues than bone cancer). I don't know about the bone growth/fast regeneration substances, but if there's some risk in the whole operation, it resides in there so far.
I'll do some research soon. Thanks for the question.