Acute injuries of the AC joint are currently treated based on the grade of instability according to Rock- wood’s classification. This classification was presented by Rockwood in 1998 and is still widely accepted. The higher the Rockwood grade, the higher the severity and extent of injury of surrounding anatomical structures (Table 1).
Successful management of AC joint instability is challenging for many reasons. A variety of non-anatomical and anatomical operative techniques is described, but there is no consensus which should be preferred. Moreover, the distinction between type III and IV injuries is still controversial. A solution to clarify this distinction would streamline treatment choices and may thus lead to more favorable clinical outcomes. As such, it is necessary to determine both vertical and horizontal instability with precise and reproducible methods. Radiographs are commonly used as a routine imaging tool in the assessment of shoulder injuries as they are widely available and provide convincing results.
However, the literature suggests a broad range of imaging modalities and techniques that can be used to elucidate the extent of the AC joint disruption, including special X-ray images: alternative planes (e.g. bilateral Zanca, axillary view, dynamic axillary view), stress imaging, dynamic measures, indices, or additional modalities like ultrasound or MRI. Currently, a standardized protocol to image acute AC joint separations does not exist, rendering proficient and unanimous diagnosis difficult. The purpose of this review was to summarize all relevant available studies dealing with diagnostic imaging of acute AC joint injuries. A focus was set on three questions: 1) Assessment of vertical instability; 2) Assessment of horizontal instability; 3) Importance of weighted panoramic views.