What types of evidence or reasons for non union can be obtained by using ct or xray can it how soft tissue interposition for instance?

Look for faint calcs: Ct would be better than radiographs (x-rays) to look for faint calcifications around the fracture as a sign of healing.

Evidence not reason: Ct or xray cannot show reasons for nonunion, but they can be read as evidence for nonunion. The type of evidence seen is brighter white bone (sclerotic appearance) around the edges of a fracture) or lack of fluffy gray cloud (callus formation) between fracture ends. If this is the same nondisplaced fracture, it's unlikely there is soft tissue interposition.