Acromion orientation

Acromion orientation

Normally, the acromion has no slope either anteriorly, laterally or inferiorly. A lateral or anterior down-sloping acromion and a low-lying acromion are thought to play a role in the development of subacromial impingement[21].

A low-lying acromion is diagnosed when the lower acromial surface is below the lower surface of the clavicle at the AC joint level on the anterior coronal oblique MR image (Figure ​(Figure2A).2A). On the same image showing the AC joint, an infero-lateral slope is detected by measuring the angle between the acromion axis and the clavicle (Figure ​(Figure2B);2B); an angle more than 10 degrees is abnormal[27]. An anterior slope is diagnosed on sagittal oblique images when the anterior part of the acromion is closer to the related part of the humeral convexity than its posterior part.

Acromion orientation. A: Coronal oblique T1-weighted image shows low lying acromion (a) in relation to the clavicle (c) at the Acromio-clavicular joint level; B: Coronal oblique T1-weighted image in another patient shows infero-lateral slope of the acromion. The angle between the acromion and the clavicle is tilted by 20 degrees.