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 Shoulder/Elbow Test 6: Topics About the Shoulder
CME 4: October, November, December 2004
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The Journal of Bone and Joint Surgery (American). 2004;86:2666-2670
© 2004 The Journal of Bone and Joint Surgery, Inc.

The Double-Density Sign: A Radiographic Finding Suggestive of an Os Acromiale

Donald H. Lee, MD1, Kwan H. Lee, MD2, Robert Lopez-Ben, MD3 and Edwin L. Bradley, PhD4

1 Division of Orthopedic Surgery, University of Alabama at Birmingham, 905 Faculty Office Tower, 1530 Third Avenue South, Birmingham, AL 35294-3409. E-mail address: donald.lee{at}ortho.uab.edu
2 899 Alford Avenue, Birmingham, AL 35226
3 Department of Radiology, University of Alabama at Birmingham, JTN 368, 619 19th Street South, Birmingham, AL 35294
4 Department of Biostatistics, University of Alabama at Birmingham, 1665 University Boulevard, RPHB Room 327, Birmingham, AL 35294-0022

Investigation performed at the Division of Orthopedic Surgery, University of Alabama at Birmingham, Birmingham, Alabama

The authors did not receive grants or outside funding in support of their research or preparation of this manuscript. They did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

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.

Level of Evidence: Diagnostic study, Level IV-1 (case-control study). See Instructions to Authors for a complete description of levels of evidence.


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