Even if there was a precise and accurate representation available of bone density vs fall damage risks, it's still only one factor in the equation. What about your body weight or muscle mass? What about the speed at which you're moving when you fall? How about the angles at which your body decides to twist or the exact placement of your feet on the pedals or even whether you're riding seated or standing? You're better off doing the best you can to prepare yourself for a fall or crash and avoid situations beyond your skill level, than stress about something that's not really even in your control. Just get out there and ride, man!