Os acromiale and shoulder pain

Clinical History

Presented to casualty department following a road traffic accident and injury to the left shoulder. Examination revealed local tenderness and a bony prominence with mild deformity, which got spontaneously better.

Imaging Findings

Presented to casualty department following a road traffic accident and injury to the left shoulder. Examination revealed local tenderness and a bony prominence with mild deformity, which got spontaneously better. Plain radiographs showed suspicion of Os acromiale (figure 1a & b) and was confirmed by CT (figure 2b). A MR shoulder arthrogram (figure 2c & 3) at a later date for persistent pain further confirmed the presence of Os acromiale with rotator cuff disease.

Discussion

Os acromiale is a developmental anomaly leading to formation of accessory ossicles in close relation to the acromion. The incidence of Os acromiale has been estimated at 8%, is said to be more common in black males, and is often bilateral. In the mid teenage years, one, two or three separate ossification centres form, and fuse with themselves and the base of the acromion between 22-25years. The anterior ossification centre is termed the pre-acromion, the middle is the meso-acromion and the posterior fragment is known as the meta-acromion. The basi-acromion forms the point of attachment of these three ossification centres to the scapula. A number of variants of Os acromiale have been described, arising from the failure of fusion of the three separate ossification centres of the acromion. The commonest variant is nonfusion between the meso and meta-acromion. The Os acromiale may be joined to the basi-acromion by cartilage, fibrous tissue, periosteum or a synovial joint. The Os acromiale may be symptomatic and predisposes towards subacromial impingement and rotator cuff rupture. It is thought that contraction of the deltoid muscle pulls the Os acromiale downward causing it to impinge on the rotator cuff. Some variants of Os acromiale form an osteophytic lip at the margins of the acromial gap and this may also impinge on the rotator cuff. Pre-operative recognition of Os acromiale is essential when surgery is being contemplated for impingement or rotator cuff tear. This allows the surgeon to choose the best approach to decompression, since some types of Os acromiale can be excised whilst others require fusion. There are clearly consent implications involved. In addition, the joint between the meso and basi-acromion may resemble a fracture at arthroscopy. If the surgeon has not been forewarned of the presence of an Os acromiale, the surgeon may assume that his technique has been too forceful and subject the patient to unnecessary immobilisation.
The diagnosis of Os acromiale is radiological. The easiest method is an axial radiograph of the shoulder (figure2a), and this is particularly important in institutions where impingement or rotator cuff tears are assessed with ultrasound alone. The Os acromiale may also be recognised on the AP shoulder view (figure 1a) as it overlaps the acromion producing a bilobed appearance. Axial CT sections (figure 2b) can be employed for the diagnosis of Os acromiale, and three dimensional reconstructions have been shown to be of use.
However, many institutions examine the painful shoulder with MR, and it is therefore vital that the radiologist is able to recognise the MR features of Os acromiale.
The unfused ossification centres are demonstrated easily in the axial (figure 2c) plane. However, the routine axial shoulder MR may not cover the level of the AC joint. Os acromiale can be recognised on the oblique sagittal (figure 3) and coronal images but the abnormalities can be subtle, and may mimic the AC joint. Uri found that correct identification was most reliably achieved by noting the site of insertion of the coraco-acromial (CA) ligament. The CA ligament does not insert into the clavicle and can thus be used to distinguish a pseudo-joint from the AC joint. Park has described a “double joint “ sign representing the junction of the Os acromiale posteriorly and the clavicle anteriorly, although Uri found this sign was present in only 18% of cases. Uri recommended the use of a vertical line bisecting the sagittal image of humeral head, where the true AC joint is located at or just anterior to the line, and the “pseudo-AC joint” of an Os acromiale lies posterior to the line. However, this finding was insensitive. The signal returned from the site of non-fusion varies between patients, perhaps reflecting the fact that the synovium, cartilage, fibrous tissue or periosteum can bridge the gap between the Os acromiale and the acromion itself. Hypertrophic spurring and bone marrow oedema are also helpful, but non-specific signs. In conclusion,
Os acromiale is a relatively common developmental anomaly, which may be associated with rotator cuff dysfunction;
Pre-operative diagnosis is important since it may have significant implications for surgical planning;
The diagnosis is easily made on axial plain radiographs;
Axial MR should include the level of the AC joint since this is the most reliable plain for diagnosis of Os acromiale.

Differential Diagnosis List

Final Diagnosis

Os Acromiale

URL:	https://eurorad.org/case/2477
DOI:	10.1594/EURORAD/CASE.2477
ISSN:	1563-4086