Os acromiale is an anatomical variant and represents an unfused accessory center of ossification of the acromion of the scapula.
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Epidemiology
Os acromiale is relatively common, seen in ~8% (range 1-15%) of the population 1,2 and can be bilateral in 60% of individuals 5.
Clinical presentation
They are usually asymptomatic. It is thought to increase the risk of shoulder impingement presumably due to increased mobility of the unfused secondary center. There is, however, no statistical difference in the incidence of supraspinatus and infraspinatus tears between patients with os acromiale and a normal control group 3.
A step-off deformity of the os acromiale is associated with a greater incidence of rotator cuff tears than those without such deformity 3.
Gross anatomy
Subtypes
The subtypes develop due to the fusion pattern of the three acromial ossification centers (preacromion, mesoacromion and meta-acromion) 4 and are classified on their pattern of articulation with the acromion (from proximal to distal) 5:
basiacromial
meta-acromial
mesoacromial
preacromial
Meta- and mesoacromial are the most common 5.
Pathology
The acromion normally has a secondary center of ossification which usually fuses to the rest of the acromion by the age of 25. An os acromiale merely represents the persistence of this center without bony fusion.
It may cause shoulder impingement, rotator cuff tear or degenerative acromioclavicular joint disease 5.
Acromial apophysiolysis in young patients is considered a risk factor for the development of an os acromiale.
Radiographic features
Plain radiograph
The unfused anterior acromial ossification center is best seen on axillary views.
Ultrasound
os acromiale readily identified in patients with pathology (e.g. rotator cuff tears)
advantage of dynamic imaging and reproducing symptoms 6
MRI
An os acromiale is readily identified if care is taken to examine the acromion in the superior axial plane as it can easily be mistaken for the normal acromioclavicular joint.
History and etymology
Os acromiale is from Ancient Greek, "os" is bone, and "acromiale" is a possessive form of "acromion", a compound word: akros (άκρος) "highest", ōmos (ώμος), "shoulder".
Differential diagnosis
Possible considerations include
normal acromioclavicular joint: note that acromion fusion may not occur till around 18-25 years of age 6.
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