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?
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.
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).
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
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).
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.
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.
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.
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.