title: A novel radiographic index for the diagnosis of posterior acromioclavicular joint dislocations.
abstract: BACKGROUND
  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.
  
  
  OBJECTIVE
  This novel index has a high degree of accuracy for the diagnosis of posterior AC joint dislocations.
  
  
  METHODS
  Cohort study (diagnosis); Level of evidence, 2.
  
  
  METHODS
  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.
  
  
  RESULTS
  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.
  
  
  CONCLUSIONS
  An AC width index of ≥60% is highly accurate for the diagnosis of a posterior AC joint dislocation, with high intraobserver and interobserver concordance.
authors.affiliation: Alex Vaisman, Facultad de Medicina Clínica Alemana Universidad del Desarrollo, Francisco Bulnes Correa 3737, dep B-31, Las Condes, Santiago, Chile. drvaisman@gmail.com.
authors.forename: Alex
authors.initials: A
authors.lastname: Vaisman
authors.forename: Ignacio Eduardo
authors.initials: IE
authors.lastname: Villalón Montenegro
authors.forename: María Jesús
authors.initials: MJ
authors.lastname: Tuca De Diego
authors.forename: Juanjose
authors.initials: J
authors.lastname: Valderrama Ronco
mesh_terms: D000173:Acromioclavicular Joint / Q000000981:diagnostic imaging / Q000293:injuries*; D000328:Adult; D003937:Diagnosis, Differential; D005260:Female; D006801:Humans; D008297:Male; D011859:Radiography; D015203:Reproducibility of Results; D012189:Retrospective Studies; D012680:Sensitivity and Specificity; D012783:Shoulder Dislocation / Q000000981:diagnostic imaging*
keywords: acromioclavicular joint dislocation; diagnosis; instability; radiograph; radiographic index; shoulder; sport-specific injuries