Dynamic Radio logic Evaluation of Horizontal Instability in Acute Acromioclavicular Joint Dislocations

Background: Biplane radiologic evaluation is indispensable for the correct diagnosis of acute acromioclavicular (AC) joint injuries. Thus far, no functional radiographic techniques have been quantified to evaluate horizontal instability in acute AC joint dislocations. Hypothesis: Supine dynamic axillary lateral shoulder views detect horizontal instability of the distal clavicle in patients with acute AC joint dislocations. Study Design: Cohort study (Diagnosis); Level of evidence, 2. Methods: Twenty-five consecutive patients with a mean age of 39 14 years with acute AC joint injury underwent biplane radiologic evaluation, including a conventional Zanca view and an axillary lateral view in a sitting position. In addition, supine axillary lateral views with the arm in 900 of abduction and 60 of flexion and extension were taken to evaluate the horizontal dynamics of the distal clavicle. The gleno-acromio-clavicular angle (GACA) was introduced and used to quantify the horizontal clavicular dynamics in terms of angle differences. The unaffected shoulders served as the control group. Results: Superior dislocation of the lateral clavicle in the Zanca view was classified as Rockwood type II in 7 patients, type III in 15, and type V in 3. The axillary lateral view in a sitting position showed posterior dislocation of the distal clavicle in 8 patients (Rockwood type IV injury). Dynamic radiologic evaluation revealed an average GAGA difference between the neutral and anterior position of the arm of 7.10 +/- 5.5 for the unaffected shoulder. In the injured AC joints, 11 patients showed no radiologic evidence of horizontal instability (group A) with a GACA difference of 7.1 degrees +/- 4.8 degrees. Increased anteroposterior translation was evident in 14 patients (group B) with a GACA difference of 30.3 degrees +/- 14.3 (P < .001). Conclusion: Functional axillary radiologic evaluation seems to represent a simple imaging tool to reveal dynamic horizontal instability. Clinical Relevance: Horizontal instability of the distal clavicle in acute AC joint injuries represents an indication for surgical treatment. Dynamic axillary radiologic evaluation may detect previously missed unstable injuries. This evaluation might be relevant when deciding on surgical AC joint stabilization.