@Dr San Agreed ,we can also see that this is a child's arm because of the non-fused/open epiphyseal plates...

Its not a contour line rather its the cortex , we say 'there is a break or discontinuity in the cortex'

What do you mean by ''crack'' ......''upper bone and lower bone ''......''double band'' ? Try to use as much medical terms as you can , that way we can learn from you and contribute to your ideas too.