Imaging description

The os acromiale is an accessory bone that is found in 7–15% healthy subjects. This acromial bone is formed when there is a non-union of ossification centers during development. Os acromiale is easy to detect on an axillary view of the shoulder on plain radiographs but is difficult to see on a standard anteroposterior view.

Importance

The os acromiale (Figure 11.1) – optimally seen on axial MR images – is connected to the basiacromion via a diarthrosis or synchondrosis and should not be diagnosed as fracture. The acromion is normally formed by the fusion of several ossification centers, which is generally complete by the age of 25. The discovery of os acromiale on plain radiographs in a symptomatic patient should prompt a search for rotator cuff tendon disease. Its recognition in patients with rotator cuff disease may help the surgeon select an appropriate therapy.

Typical clinical scenario

Presence of os acromiale may be associated with rotator cuff impingement.

Differential diagnosis

The presence of an os acromiale may be confused with a fracture of the acromion distally.

Teaching point

Os acromiale, which can predispose to rotator cuff impingement, can be detected on routine shoulder MR imaging.