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Os Acromiale

os acromiale

The os acromiale is a failure of fusion of the acromial secondary ossification centre, present in approximately 8 percent of the population and bilateral in up to 60 percent of cases. The unfused acromial segment is mobile rather than rigidly attached to the scapular spine, and this mobility produces subacromial impingement from inferior displacement of the os acromiale during deltoid contraction, which narrows the subacromial space dynamically.

Anatomical Data

Structure & Landmarks

Articulations	Scapular spine at the meso-acromion junction (fibrous or partially ossified synchondrosis)
Landmarks	Palpable step deformity at the acromion when present, a sulcus felt across the acromion at the unfused junction On the axillary lateral radiograph. The unfused junction is visible at the pre-acromion or meso-acromion level
Ossification	The acromion develops from three to four secondary ossification centres (pre-acromion, meso-acromion, meta-acromion, basi-acromion) that normally fuse between ages 18 and 25. The pre-acromion or meso-acromion fails to fuse in 8 percent of people.

Clinical Relevance

Clinical Notes

Os acromiale can produce impingement symptoms indistinguishable from rotator cuff impingement from conventional subacromial causes. MRI may show oedema at the synchondrosis (unstable os acromiale syndrome) or simply the ossicle without oedema. Treatment ranges from conservative management to acromioplasty (resection of the os acromiale) or open reduction and internal fixation (ORIF) of the mobile segment for young patients.

Pathology

Common Injuries & Conditions

Symptomatic Os Acromiale

Motion at the unfused acromial ossification centre under deltoid pull can cause shoulder pain and contribute to subacromial impingement and rotator cuff disease.

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