In 1921 A. George published “A Method for More Accurate Study of Injuries to the Atlas and Axis”. This is a significant work because the only way to make this determination, especially at that time, was to take two films, one at full extension and the other at full flexion, and measure both extremes. The sum of the differences, if exceeding 3.5 mm, indicated excessive stretching of the ligaments. The George rule is that once stretched to that extent the ligaments would remain stretched. There was no going back to a more compressed condition.
George had managed to make this observation concerning the extremes of Motion without being able to observe Motion. It required just two films and significant observational skills to make this determination. It is no wonder that this technique was not as popular as hoped for.
However, now that Motion studies are readily available the ability to observe aberrant motion should be simple and extremely helpful.
Note that the distance starting at C 4 has caused an offset between the two sides of the cervical spine. These two distances sum thus indicating that stretching has occurred. If the stretching sum exceeded 3.5 cm the damage was considered to be permanent.