The acromion is the flat bony projection of the scapular spine that forms the roof of the shoulder joint and the lateral shoulder bony landmark. It articulates with the lateral clavicle at the acromioclavicular joint and with the coracoacromial ligament medially to form the coracoacromial arch, the structural arch under which the rotator cuff and subacromial bursa reside. Its shape (flat, curved, or hooked) determines subacromial space dimensions and impingement risk.
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| Ossification | The acromion ossifies from three to four secondary centres (pre-acromion, meso-acromion, meta-acromion, basi-acromion) fusing between ages 18 and 25. Failure of the pre- or meso-acromion to fuse produces the os acromiale in 8 percent of people. |
Bigliani type I (flat), II (curved), and III (hooked) acromial morphology determines subacromial space dimensions. Type III hooked acromia are associated with higher rates of rotator cuff tears from direct supraspinatus contact with the acromion undersurface during elevation. Subacromial decompression (acromioplasty) removes the anterior inferior acromion hook to increase subacromial space.
Trace the clavicle outward to the small step of the acromioclavicular joint, then move just beyond it onto the flat bony plate at the top of the shoulder; this is the acromion. Follow its edges forward, laterally and backward. From behind, trace the spine of the scapula laterally to the sharp corner of the acromial angle.
Narrowing of the space under the acromion, often with a hooked acromion or spur, compresses the rotator cuff and bursa during overhead activity.
Usually from a direct blow; stress fractures can also occur after reverse shoulder replacement.
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