The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 86, Issue 12

Scientific Articles | December 01, 2004

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

Donald H. Lee, MD¹; Kwan H. Lee, MD²; Robert Lopez-Ben, MD³; 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@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

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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.

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

The Journal of Bone and Joint Surgery, Incorporated

J Bone Joint Surg Am, 2004 Dec 01;86(12):2666-2670

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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.

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

Figures in this Article

The os acromiale results from the failure of fusion of the scapular acromial secondary centers of ossification. The reported prevalence of this condition has varied from 1.4% to 15% of the general population^1-7. Failure of fusion of the most anterior ossification center results in a preacromion, failure of fusion of the middle ossification center produces a meso-acromion, and failure of fusion of the center located at the angle between the scapular spine and the acromion creates a meta-acromion (Fig. 1). The rate of bilateral involvement has been reported to range from 33% to 62%^2,4,5. Clinically, an os acromiale has been associated with rotator cuff disease^1,8-14. In most cases, an os acromiale is best detected with use of an axillary lateral radiograph of the shoulder^12-14. Other studies, including oblique radiographs, magnetic resonance imaging, and computed tomography, also have been used to demonstrate an os acromiale^15-17.

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Fig. 1: Diagram showing the various types of os acromiale. PA = pre-acromion; MSA = meso-acromion; MTA = meta-acromion.

The purpose of the present report was to describe two simple radiographic findings, the double-density sign on a standard anteroposterior view of the shoulder and a cortical irregularity on the supraspinatus outlet view, that are highly suggestive of the presence of an os acromiale. The double-density sign is produced by the overlap of two transversely elongated radiographic cortical densities. One density represents the circumferential cortical margin of the os acromiale fragment, and the second density represents the anterior cortical margin of the remaining portion of the acromial process. On the supraspinatus outlet view, a cortical irregularity or a disruption of the normally smooth and continuous acromial profile is also frequently noted. However, the actual unfused symphysis usually is not seen.

An irregularity on the anteroposterior radiographic view has been previously reported² but has not been identified as a double-density sign. To our knowledge, the cortical irregularity noted on the supraspinatus outlet view has not been described. In the present study, we sought to determine the specificity and sensitivity of the anteroposterior, axillary, and supraspinatus outlet radiographic views for the identification of an os acromiale.

Materials and Methods

Materials and Methods | Results | Discussion | References

Following institutional review board approval, a retrospective review of the radiographs of thirty-four shoulders in thirty patients with an os acromiale and a control group of thirty-one shoulders in twenty-nine patients without an os acromiale was performed by two independent, nonblinded observers. One observer was an orthopaedic surgeon (D.H.L.), and the other was a musculoskeletal radiologist (R.L.-B.). All shoulders were evaluated with use of three radiographic views, including a standard anteroposterior view of the shoulder in the plane of the thorax, an axillary lateral view, and a supraspinatus outlet view. A supraspinatus outlet radiograph was not available for one shoulder in the os acromiale group. All patients in the os acromiale group had an unfused acromial physis as seen on plain radiographs, magnetic resonance images, or computed tomographic scans.

The average age of the patients with an os acromiale was 44.5 years (range, twenty to seventy-four years). This group included fifteen women and fifteen men, with eighteen right and sixteen left shoulders. Bilateral radiographs of the shoulder were available for five patients. The average age of the patients in the control group was fifty-six years (range, twenty-six to seventy-seven years). This group included fourteen women and fifteen men, with twelve right and nineteen left shoulders.

Statistical analysis was performed with use of a generalized logistic regression model to determine if an os acromiale could be detected by means of radiographic imaging with use of all three radiographic views (p < 0.05), and a kappa analysis was performed to determine interobserver reliability (p < 0.05).

Results

Materials and Methods | Results | Discussion | References

In the os acromiale group, twenty-eight shoulders had a meso-acromion and six had a pre-acromion. Four of the five patients with bilateral radiographs were noted to have an os acromiale bilaterally: three had a meso-acromion bilaterally, and one had a meso-acromion on one side and a pre-acromion on the other.

An os acromiale was noted on the axillary lateral radiograph (Fig. 2) of all shoulders by one observer and on that of thirty-one of the thirty-four shoulders by the second observer; thus, it was noted in an average of 95.6% of the shoulders as assessed by both observers. However, in four shoulders, the os acromiale was either partially or fully obscured by the posterior cortical margin of the humerus. All four of these shoulders had a meso-acromion.

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Fig. 2: Axillary lateral radiograph demonstrating a meso-acromion (arrow).

The double-density sign was noted on the anteroposterior radiograph of twenty-eight (82.4%) of the thirty-four shoulders as assessed by both observers (Figs. 3-A and 3-B). In most shoulders, the cortical margins appeared smooth, sharp, and well circumscribed or occasionally irregular and poorly defined (Figs. 4-A and 4-B).

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Fig. 3-A: Anteroposterior radiograph demonstrating the double-density sign, with the cortical margin of a meso-acromion (OS) superimposed over the cortical margin of the base of the acromion (Ac) at the nonunion site. The margins of this os acromiale appear smooth, sharp, and well circumscribed. DC = distal part of the clavicle. Fig. 3-B Diagrammatic representation of the radiograph shown in Figure 3-A.

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Fig. 4-A: Anteroposterior radiograph showing a meso-acromion (OS) with irregular and poorly defined cortical margins overlying the acromial base (Ac), which also has irregular cortical margins. Fig. 4-B Diagrammatic representation of the radiograph shown in Figure 4-A.

An os acromiale was detected as a cortical irregularity on the supraspinatus outlet radiograph of twenty-four of thirty-three shoulders by one observer and that of twenty-six of thirty-three shoulders by the second observer (Figs. 5-A and 5-B).

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Fig. 5-A: Supraspinatus outlet radiograph showing an os acromiale (OS) producing a cortical irregularity overlying the remaining portion of the acromion (Ac). Fig. 5-B Diagrammatic representation of the radiograph shown in Figure 5-A.

In the control group, the double-density sign was noted on the anteroposterior radiographs of two shoulders by one observer and on that of one shoulder by the second observer; thus, it was noted in an average of 4.8% of the shoulders as assessed by both observers. Neither observer noted an os acromiale on the axillary lateral radiograph of any shoulder. One observer noted an irregularity on the supraspinatus outlet view of one shoulder.

For both observers, the sensitivity of the anteroposterior view for the detection of an os acromiale was 82.4%, the sensitivity of the axillary lateral view for the detection of an os acromiale was 94.1%, and the sensitivity of the supraspinatus outlet view for the detection of a cortical irregularity was 73.5%. The specificities for these three views were 95.2%, 100%, and 98.4%, respectively. These differences between the os acromiale and control groups were significant (p < 0.0001). The interobserver reliabilities for the detection of the double-density sign on the anteroposterior view, the os acromiale on the axillary lateral view, and a cortical irregularity on the supraspinatus outlet view were 0.66, 0.88, and 0.7, respectively (p < 0.0001).

Discussion

Materials and Methods | Results | Discussion | References

An os acromiale has been implicated as a cause of rotator cuff tears and impingement-type symptoms associated with rotator cuff disease^1,8-14. On the basis of the belief that the os acromiale fragment produces pain and contributes to rotator cuff pathology, surgeons have employed several techniques to deal with the fragment, including simple excision^1,11, arthroscopic anterior acromioplasty^9,10,14, and fusion of the fragment to the remaining portion of the acromion^1,8,12,14.

An os acromiale usually is best detected radiographically on an axillary lateral view^12-14. However, this view may not be available or the os acromiale may be missed on this view if the unfused physis is located parallel to the posterior cortical margin of the humeral head or neck. In our series, the os acromiale was either partially or completely obscured on the axillary lateral radiographs of four shoulders. Thus, the double-density sign on the anteroposterior radiograph or cortical irregularity on the supraspinatus outlet radiograph, both of which are highly suggestive of an os acromiale, can facilitate the diagnosis.

The double-density sign is produced by the overlap of two cortical densities that are elongated transversely. The first density represents the circumferential cortical margin of the bone fragment, either a pre-acromion or a meso-acromion. The second radiographic density is created at this site by the anterior cortical margin of the remaining portion of the acromial process. These radiographic densities have the appearance of two elongated, separate, and completely circumscribed cortical rings. The cortical margins of the os acromiale fragment and the acromial process can be smooth and well circumscribed (Figs. 3-A and 3-B) or irregular and less well defined (Figs. 4-A and 4-B). The supraspinatus outlet view shows a profile of the acromion, and cortical irregularity of the normally smooth acromion is frequently noted in patients with this condition (Figs. 5-A and 5-B). The presence of either radiographic finding should alert the clinician to the possibility of an os acromiale.

This concept of an abnormality on an anteroposterior radiograph of the shoulder has been previously described. Liberson² described four types or shapes of os acromiale: quadrangular, oval, triangular, and falciform. On the basis of anteroposterior and oblique radiographs of the shoulder, fifteen different os acromiale sizes or forms were described. The radiographic appearance of the os acromiale will vary with its type (pre-acromion or meso-acromion), shape, size, and position.

In the case of a shoulder in which an os acromiale is suspected on the basis of the double-density sign on the anteroposterior radiograph or a cortical irregularity on the supraspinatus outlet radiograph but is absent or questionably present on the axillary lateral radiograph, we recommend repeating the axillary lateral radiograph at a slightly different angle or performing either magnetic resonance imaging or computed tomography. In this situation, the radiologist should be made aware of the possibility of an os acromiale so that complete imaging of the entire acromion is performed.

In summary, although the os acromiale usually can be seen on an axillary lateral radiograph, it can be obscured by the proximal part of the humerus and be missed. Recognizing the double-density sign on an anteroposterior radiograph or detecting a cortical irregularity on a supraspinatus outlet radiograph should alert the clinician to the possibility of the presence of an os acromiale.

References

Materials and Methods | Results | Discussion | References

Edelson JG, Zuckerman J, Hershkovitz I. Os acromiale: anatomy and surgical implications. J Bone Joint Surg Br.1993;74: 551-5.74551 1993

Liberson F. Os acromiale—a contested anomaly. J Bone Joint Surg Am.1937; 19: 683-9.19683 1937

Macalister A. Notes on the acromion. J Anat Physiol.1893;27: 245-51.27245 1893

Nicholson GP, Goodman DA, Flatow EL, Bigliani LU. The acromion: morphologic condition and age-related changes. A study of 420 scapulas. J Shoulder Elbow Surg.1996;5: 1-11.51 1996 [PubMed][CrossRef]

Sammarco VJ. Os acromiale: frequency, anatomy, and clinical implications. J Bone Joint Surg Am.2000;82: 394-400.82394 2000

Symington J. On separate acromion process. J Anat Physiol.1900;34: 287-94.34287 1900 [PubMed]

Wood VE, Marchinski L. Congenital anomalies of the shoulder. In: Rockwood CA Jr, Matsen FA 3rd, editors. The shoulder. 2nd ed. Philadelphia: WB Saunders; 1998. p 99-163.99 1998

Hertel R, Windisch W, Schuster A, Ballmer FT. Transacromial approach to obtain fusion of unstable os acromiale. J Shoulder Elbow Surg.1998;7: 606-9.7606 1998 [PubMed][CrossRef]

Hutchinson MR, Veenstra MA. Arthroscopic decompression of shoulder impingement secondary to os acromiale. Arthroscopy.1993;9: 28-32.928 1993 [PubMed][CrossRef]

Jerosch J, Hepp R, Castro WH. An unfused acromial epiphysis. A reason for chronic shoulder pain. Acta Orthop Belg.1991;57: 309-12.57309 1991 [PubMed]

Mudge MK, Wood VE, Frykman GK. Rotator cuff tears associated with os acromiale. J Bone Joint Surg Am.1984;66: 427-9.66427 1984

Ryu RK, Fan RS, Dunbar WH 5th. The treatment of symptomatic os acromiale. Orthopedics.1999;22: 325-8.22325 1999 [PubMed]

Warner JJ, Beim GM, Higgins L. The treatment of symptomatic os acromiale. J Bone Joint Surg Am.1998;80: 1320-6.801320 1998

Wright RW, Heller MA, Quick DC, Buss DD. Arthroscopic decompression for impingement syndrome secondary to an unstable os acromiale. Arthroscopy.2000;16: 595-9.16595 2000 [PubMed][CrossRef]

Andrews JR, Byrd JW, Kupferman SP, Angelo RL. The profile view of the acromion. Clin Orthop.1991;263: 142-6.263142 1991

Park JG, Lee JK, Phelps CT. Os acromiale associated with rotator cuff impingement: MR imaging of the shoulder. Radiology.1994;193: 255-7.193255 1994 [PubMed]

Uri DS, Kneeland JB, Herzog R. Os acromiale: evaluation of markers for identification on sagittal and coronal oblique images. Skeletal Radiol.1997; 26: 31-4.2631 1997 [PubMed][CrossRef]

Topics

axilla ; osteogenesis ; shoulder region ; acromion ; supraspinatus ; antimicrobial susceptibility ; surgical margins ; radiographic views ; shoulder radiography

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Donald H. Lee, Kwan H. Lee, Robert Lopez-Ben, Edwin L. Bradley; The Double-Density Sign: A Radiographic Finding Suggestive of an Os Acromiale. The Journal of Bone & Joint Surgery. 2004 Dec;86(12):2666-2670.

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