A Novel Radiographic Index for the Diagnosis of Posterior Acromioclavicular Joint Dislocations

<jats:sec><jats:title>Background:</jats:title><jats:p>Posterior acromioclavicular (AC) joint dislocations are frequently misclassified because posterior translation of the clavicle is difficult to evaluate in Zanca radiograph views. A novel radiographic index was used in this study to accurately diagnose posterior dislocations of the AC joint.</jats:p></jats:sec><jats:sec><jats:title>Hypothesis:</jats:title><jats:p>This novel index has a high degree of accuracy for the diagnosis of posterior AC joint dislocations.</jats:p></jats:sec><jats:sec><jats:title>Study Design:</jats:title><jats:p>Cohort study (diagnosis); Level of evidence, 2.</jats:p></jats:sec><jats:sec><jats:title>Methods:</jats:title><jats:p>This was an analytic, descriptive study of 150 patients with different grades of AC injuries according to the Rockwood classification (30 patients for each grade of injury: I, II, III, IV, and V). The diagnosis of an AC injury was made both clinically and radiographically by using comparative Zanca and axillary views. Two measurements were performed in Zanca views: the coracoclavicular distance and the AC width distance. A width index was calculated for each patient. The Student t test, Bonferroni test, logistic regression, linear regression, and receiver operating characteristic (ROC) curves were used for statistical analysis. Forty cases were impartially selected to obtain a κ concordance value.</jats:p></jats:sec><jats:sec><jats:title>Results:</jats:title><jats:p>The average value of the AC width index per group (according to the Rockwood classification) was as follows: type I, 2.1% (range, –12% to 25%); type II, 4.2% (range, –19% to 29%); type III, 19.1% (range, –59% to 91%); type IV, 110.3% (range, 47% to 181%); and type V, –3.8% (range, –71% to 62%). There was a significant difference between the average width index in the patients with type IV injuries and those in the remaining groups ( P < .05). The ROC curve showed that a width index of 60% has a sensitivity of 95.7% and specificity of 97.5%, with a positive predictive value of 96.7% and negative predictive value of 95.6% to predict a type IV injury. Intraobserver reliability was rated as substantial agreement for each of 3 observers; the interobserver reliability of the 3 independent raters was almost perfect.</jats:p></jats:sec><jats:sec><jats:title>Conclusion:</jats:title><jats:p>An AC width index of ≥60% is highly accurate for the diagnosis of a posterior AC joint dislocation, with high intraobserver and interobserver concordance.</jats:p></jats:sec>