title: The double-density sign: a radiographic finding suggestive of an os acromiale.
abstract: BACKGROUND
  An os acromiale results from the failure of fusion of the acromial secondary centers of ossification. It is most easily seen radiographically on an axillary lateral view. The purpose of the present study was to describe two simple radiographic findings, the double-density sign on a standard anteroposterior view of the shoulder and a cortical irregularity found on a supraspinatus outlet view, that are highly suggestive of an os acromiale.
  
  
  METHODS
  Anteroposterior, axillary lateral, and supraspinatus outlet radiographs of thirty-four shoulders in thirty adult patients with an os acromiale were reviewed by two independent observers and were compared with those of a control group of thirty-one shoulders in twenty-nine patients without an os acromiale. Statistical analysis was performed with use of a generalized logistic regression model to determine if an os acromiale could be detected on all three radiographic views. A kappa analysis was performed to determine interobserver reliability.
  
  
  RESULTS
  In the group with an os acromiale, twenty-eight shoulders had a meso-acromion and six had a pre-acromion. A double-density sign was noted on the anteroposterior radiograph of 82.4% of the shoulders, an os acromiale was noted on the axillary lateral radiograph of 95.6% of the shoulders, and a cortical irregularity was noted on the supraspinatus outlet radiograph of 75.8% of the shoulders. In the control group, a double-density sign was noted on the anteroposterior radiograph of 4.8% of the shoulders, no os acromiale was seen on the axillary lateral radiograph of any of the shoulders, and a cortical irregularity was noted on the supraspinatus outlet radiograph of one shoulder. These differences between the os acromiale and control groups were significant (p < 0.0001). The overall sensitivities of the anteroposterior, axillary, and supraspinatus outlet views for the detection of an os acromiale were 82.4%, 94.1%, and 73.5%, respectively. The overall specificities of the three views were 95.2%, 100%, and 98.4%, respectively. The interobserver reliabilities of the three views were 0.66, 0.88, and 0.7, respectively (p < 0.0001).
  
  
  CONCLUSIONS
  The double-density sign on a standard anteroposterior radiograph of the shoulder and a cortical irregularity on the supraspinatus outlet view are highly suggestive of an os acromiale. An os acromiale should be suspected in a patient with these radiographic findings.
  
  
  METHODS
  Diagnostic study, Level IV-1 (case-control study). See Instructions to Authors for a complete description of levels of evidence.
authors.affiliation: Division of Orthopedic Surgery, University of Alabama at Birmingham, 905 Faculty Office Tower, 1530 Third Avenue South, Birmingham, AL 35294-3409, USA. donald.lee@ortho.uab.edu
authors.forename: Donald H
authors.initials: DH
authors.lastname: Lee
authors.forename: Kwan H
authors.initials: KH
authors.lastname: Lee
authors.forename: Robert
authors.initials: R
authors.lastname: Lopez-Ben
authors.forename: Edwin L
authors.initials: EL
authors.lastname: Bradley
mesh_terms: D000174:Acromion / Q000002:abnormalities* / Q000000981:diagnostic imaging*; D000328:Adult; D000368:Aged; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011859:Radiography; D012680:Sensitivity and Specificity; D012785:Shoulder Joint / Q000000981:diagnostic imaging