Os acromiale

Os acromiale is an anatomical variant and represents an unfused accessory center of ossification of the acromion of the scapula.

Epidemiology

Os acromiale are 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.
• degenerative ossicles
• fracture of the acromion
• calcific tendinosis of the supraspinatus