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Os acromiale is an anatomical variant and represents an unfused accessory center of ossification of the acromion of the scapula.
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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.
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.
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:
Meta- and mesoacromial are the most common 5.
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.
The unfused anterior acromial ossification center is best seen on axillary views.
- os acromiale readily identified in patients with pathology (e.g. rotator cuff tears)
- advantage of dynamic imaging and reproducing symptoms 6
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".
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
- 1. Kaplan P. Musculoskeletal MRI. W B Saunders Co. (2001) ISBN:0721690270. Read it at Google Books - Find it at Amazon
- 2. Papatheodorou A, Ellinas P, Takis F et-al. US of the shoulder: rotator cuff and non-rotator cuff disorders. Radiographics. 26 (1): e23. doi:10.1148/rg.e23 - Pubmed citation
- 3. Haaga JR, Boll D. CT and MRI of the whole body. Mosby. (2009) ISBN:0323053750. Read it at Google Books - Find it at Amazon
- 4. Butler P, Mitchell A, Healy JC. Applied Radiological Anatomy. Cambridge University Press. (2012) ISBN:0521766664. Read it at Google Books - Find it at Amazon
- 5. Zlatkin MB. MRI of the shoulder. Lippincott Williams & Wilkins. ISBN:0781715903. Read it at Google Books - Find it at Amazon
- 6. Smith J, Dahm DL, Newcomer-Aney KL. Role of sonography in the evaluation of unstable os acromiale. J Ultrasound Med. 2009;27 (10): 1521-6. Pubmed citation
- 8. Winfeld M,et.al Differentiating Os Acromiale From Normally Developing Acromial Ossification Centers Using Magnetic Resonance Imaging. (2015) Skeletal radiology. doi:10.1007/s00256-015-2098-4 - Pubmed