Interesting,
AC joints get degenerative change early in life so I think the age of the patient makes a difference after trauma.
If they are young, for example 25 and you are seeing these changes they are likely related to the trauma but in an older patient you are seeing normal for age AC point degenerative changes.
I use forward flexion and expect the acromion and the clavicle to squeeze together a bit in a normal patient. If the clavicle rises superiorly relative to the acromion this is evidence of AC joint instability. Everything about the AC joint is designed to hold the clavicle down. The joint itself as well as the conoid and trapezoid coraco-clavicular ligaments hold the clavicle down. If the clavicle rides up the joint is unstable.
Steve