Native Coronal Acromioclavicular Joint Orientation Is Associated with Rockwood Grade in Acute Acromioclavicular Joint Dislocations

Background: The Rockwood classification is the most widely used radiographic framework for grading acromioclavicular (AC) joint dislocations. However, native anatomical variation may be associated with differences in the radiographic appearance of AC joint dislocations. Purpose: To evaluate the association between the acromioclavicular joint coronal orientation angle (AJCO) and Rockwood grade and to characterize the relationship between native coronal orientation and radiographic grade. Methods: This retrospective observational radiographic study included 198 adults with acute AC joint dislocations classified as Rockwood II, III, or V on standardized bilateral Zanca radiographs. AJCO was measured on the contralateral, uninjured side as the angle between the AC joint orientation line and a vertical reference. Two blinded orthopedic surgeons performed measurements independently, with reliability assessed using intraclass correlation coefficients. AJCO was compared across Rockwood groups, its monotonic association with grade was evaluated using Spearman correlation, and exploratory receiver operating characteristic analysis was performed for the tested binary contrast. Results: AJCO differed significantly across Rockwood injury grades, decreasing progressively from type II to type V injuries (median [IQR]: type II, 18.3° [14.6–26.0°]; type III, 15.3° [12.0–21.3°]; type V, 14.0° [8.6–18.0°]; p < 0.001). Consistent with this finding, AJCO demonstrated an inverse weak-to-moderate association with Rockwood grade (Spearman ρ = −0.338, 95% CI −0.453 to −0.214; p < 0.001). AJCO showed limited ability to discriminate higher-grade injuries (Rockwood ≥ III), with an AUC of 0.694 (95% CI, 0.612–0.777; p < 0.001), characterized by high specificity but low sensitivity at the optimal threshold. Furthermore, lower AJCO values remained significantly associated with higher Rockwood grades after adjustment for age and sex (OR = 0.899 per degree increase, 95% CI, 0.864–0.935; p < 0.001). Conclusions: Lower AJCO values were associated with higher Rockwood grades in this cohort, but the magnitude of association and discriminatory performance were limited. These findings support AJCO as a morphology-related contextual variable associated with Rockwood grade rather than as a stand-alone diagnostic or treatment-guiding measure.