MRI Web Clinic — August 2007

Os Acromiale
Mark H. Awh, M.D.

Clinical History: A 28 year-old male with persistent shoulder pain and limited range of motion presents for MR imaging. (1a) T1-weighted coronal and (1b) fast spin echo T2-weighted sagittal images are provided. What are the findings? What is your diagnosis?

1a
1b
Figure 1:

(1a) T1-weighted coronal and (1b) fast spin echo T2-weighted sagittal images

Findings

2a
2b
Figure 2:

Failure of fusion of an anterior acromial ossification center is demonstrated on both images. The persistent apophysis is visible within the lateral acromion (arrows) on (2a) the T1 weighted coronal view and within the posterior acromion (arrow) on (2b) the T2-weighted sagittal image.

Diagnosis

Os acromiale.

MR Diagnosis

Admittedly, the diagnosis in this case based on only coronal and sagittal images is challenging. Indeed, the os acromiale, an un-united acromial ossification center in adults, is one of the most frequently missed abnormalities by physicians who interpret MR. The reason is two-fold. For one, the os acromiale, when viewed in the coronal or sagittal planes, bears a strong resemblance to a normal acromioclavicular (AC) joint (3a,4a). Secondly, the os acromiale is fairly common, being seen in approximately 8% of shoulder examinations.1 This combination of a not-uncommon, yet challenging finding, leads to the high frequency of missed diagnoses of the os acromiale.

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Figure 3:

(3a) The original T1 coronal image of the os acromiale (arrow), on the left, is now seen next to a T1-weighted coronal image on the right that depicts the normal AC joint (arrow). Note the similar configurations, but the fact that the os acromiale lies at a point lateral and posterior to the AC joint.

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Figure 4:

(4a) Similarly, the os acromiale (arrow on the left), though it resembles an AC joint, is seen to lie at a point both posterior and lateral to the AC joint (arrow on the right) on T2-weighted sagittal views. Note that the os acromiale is visible on lateral images where the rotator cuff tendons are continuous (red arrowheads), whereas the AC joint lies at the level of the musculotendinous junction of the cuff (blue arrowheads).

It is essential in routine MR imaging of the shoulder to obtain axials that extend superior to a point above the level of the acromion. The key to the simple and reliable diagnosis of the os acromiale lies in these upper axial images. On such images, the acromion is completely visualized in the axial plane, and the diagnosis of an os acromiale becomes much simpler. Only in the axial plane is one able to reliably visualize both the AC joint and the os acromiale on a single slice (5a).

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Figure 5:

(5a) A fat-suppressed proton density-weighted axial image through the upper shoulder demonstrated the un-united anterior acromial ossification center compatible with an os acromiale. The synchondrosis of the os (arrow) is easily distinguished from the more medial and anterior AC joint (arrowhead).

Discussion

One to three ossification centers of the acromion appear by age 15-18 years, and they normally are fused no later than 25 years of age.2 Failure of any of these ossification centers to fuse results in an os acromiale. The three potential ossification centers are referred to as the preacromion, mesoacromion, and meta-acromion, from anterior to posterior (6a). The adjacent ossification center for the lateral scapular spine is known as the basi-acromion. Failure of fusion can occur at the junction of any of these ossification centers, involving a single junction or in combination. As a result, there are 7 potential types of os acromiale that may arise (7a).2

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Figure 6:

(6a) A view of the scapula from above illustrates the three ossification centers of the acromion, the preacromion (PA), the mesoacromion (MSA), and the meta-acromion (MTA). The adjacent basi-acromion (BA) of the lateral scapula is also depicted. Illustration by Michael E. Stadnick, M.D.

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Figure 7:

(7a) The seven potential types of os acromiale, caused by failure of fusion at any of the three potential synchondroses of the acromion, are represented. The Type A, or mesoacromion variety, is most common. Illustration by Michael E. Stadnick, M.D.

By far the most common site of non-union is posterior to the mesoacromion, which is referred to as the mesoacromion variety or a Type A os acromiale. The next most common site of non-union is posterior to the preacromion, or the preacromion variety. In one of the original studies of os acromiale by Liberson in 1937,3 21 of 25 os acromiales were found to be of the mesoacromion variety.

The os acromiale has been implicated as a risk factor for the development of impingement syndrome.4 Hypertrophic osteophytes may arise at the synchondrosis of an os acromiale (8a), and the os acromiale is thought to increase the incidence of osteoarthritis at the AC joint,5 both of which may predispose the patient to impingement. In addition, when an os acromiale is unstable, the downward pull of the deltoid muscle reduces the subacromial space, causing mass effect upon the rotator cuff (9a).

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Figure 8:

(8a) Inferior osteophyte formation (arrow) at the synchondrosis of a mesoacromion type os acromiale is depicted in this 3-dimensional representation of the lateral shoulder. Illustration courtesy of Michael E. Stadnick, M.D.

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Figure 9:

(9a) A T1-weighted coronal image in a patient with an os acromiale (arrow at synchondrosis) demonstrates the lateral deltoid attachment to the os (arrowhead). With deltoid contraction, the downward pull upon an unstable os acromiale narrows the subacromial space (red area), increasing the patient's risk for impingement syndrome.

In addition to the role that an os acromiale plays in increasing the risk for impingement syndrome, it is important to realize that recognition of an os acromiale is necessary because the os itself may be a primary source of patient symptoms.6 Such patients typically have point tenderness over the os acromiale and pain with forward elevation of the shoulder. On MR images, edema and/or fluid may be noted along the synchondrosis of the os acromiale (10a,11a), and as is true with other body regions, edema on MR is a reliable indicator of a site of patient pain.

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Figure 10:

(10a) A fat-suppressed T2-weighted sagittal view in a patient with a symptomatic os acromiale reveals edema and fluid at and adjacent to the synchondrosis of the os (arrow). The more anteriorly located AC joint is also visible on this sagittal slice (arrowhead). This configuration has been referred to as the "double-joint" appearance2 of an os acromiale.

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Figure 11:

(11a) A T2*-weighted axial image confirms the edema and fluid (arrowheads) in association with an os acromiale.

Treatment

The clinical importance of an os acromiale in the development of shoulder pain has not been fully established. The os acromiale can be found in asymptomatic patients, but as described above, it has been implicated as both a risk factor for impingement syndrome and as a primary cause of patient pain. In patients with os acromiale and symptoms of impingement syndrome without rotator cuff tear, treatment is generally conservative, utilizing rest, ice, and NSAIDS to reduce inflammation. Steroid injections may also be of benefit.

If conservative measures fail over a period of 6 weeks to 6 months, operative therapy may be warranted.7 Preoperative recognition of an os acromiale is important in patients with impingement syndrome or rotator cuff tear, as an unstable os acromiale may render a typical anterior acromioplasty impossible. It is generally accepted that in patients with both an os acromiale and a tear of the rotator cuff, that the surgeon should correct both abnormalities.8 Small os acromiales such as the preacromion or small mesoacromion variants are usually resected, and it is now possible to accomplish this procedure completely arthroscopically. Neer reported that large os acromiales should be stabilized rather than resected at the time of rotator cuff repair,9 as resection of large fragments may lead to unacceptable weakness. Though not without controversy, such an approach remains popular with many orthopaedic surgeons.

Conclusion

The os acromiale is a not uncommon abnormality that is frequently missed on routine MR examinations of the shoulder. Proper MR scanning technique and the careful evaluation of axial images through the acromion make the correct diagnosis relatively simple. In patients with impingement syndrome, recognition of an os acromiale is important, as the lesion not only may be an important source of patient symptoms, but awareness of its presence can significantly alter the planned operative approach.

References

1 Sammarco VJ. Os Acromiale: frequency, anatomy, and clinical implications. The Journal of Bone and Joint Surgery 2000; 82:394-400.

2 Park GP, Lee JK, Phelps CT. Os acromiale associated with rotator cuff impingement: MR imaging of the shoulder. Radiology 1994; 193:255-257.

3 Liberson F. Os acromiale: a contested anomaly. Journal of Bone and Joint Surgery 1937; 19:683-389.

4 Davlin CD, Fluker C. Bilateral os acromiale in a Division I basketball player. Journal of Sports Science and Medicine 2003; 2:175-179.

5 Grass A. The incidence and role of the os acromiale in the acromiohumeral impingement syndrome. Radiol Med (Torino) 1992; 84:567-570.

6 Warner JJP, Beim GM, Higgins L. The treatment of symptomatic os acromiale. The Journal of Bone and Joint Surgery 1998; 80:1320-1326.

7 Swain, R., Wilson, F. and Harsha, D. (1996) The os acromiale: another cause of impingement. Medicine and Science in Sports and Exercise, 28, 1459-1462.

8 Boehm TD, Matzer M, Brazda D, Gohlke FE. Os acromiale associated with tear of the rotator cuff treated operatively: review of 33 patients. Journal of Bone and Joint Surgery 2003; 85B:545-549.

9 Neer, C. (1972) Anterior acromioplasty for the chronic impingement syndrome in the shoulder. The Journal of Bone and Joint Surgery, 54-A, 41-50.

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