Os acromiale
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 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
Related Radiopaedia articles
Anatomy: Upper limb
-
skeleton of the upper limb
- clavicle
- scapula
- humerus
- radius
- ulna
- carpal bones (mnemonic)
- metacarpals
- phalanges
- accessory ossicles of the upper limb
- accessory ossicles of the shoulder
- accessory ossicles of the elbow
-
accessory ossicles of the wrist (mnemonic)
- os centrale carpi
- os epilunate
- os epitriquetrum
- os styloideum
- os hamuli proprium
- lunula
- os triangulare
- trapezium secondarium
- os paratrapezium
- os radiostyloideum (persistent radial styloid)
- joints of the upper limb
-
pectoral girdle
-
shoulder joint
- articulations
- associated structures
- joint capsule
- bursae
- ligaments
-
movements
- scapulothoracic joint
- glenohumeral joint
- arm flexion
- arm extension
- arm abduction
- arm adduction
- arm internal rotation (medial rotation)
- arm external rotation (lateral rotation)
- circumduction
- blood supply - scapular anastomosis
- ossification centers
-
shoulder joint
-
elbow joint
- proximal radioulnar joint
- ligaments
- associated structures
- movements
- alignment
- blood supply - elbow anastomosis
- development
-
wrist joint
- articulations
-
ligaments
- intrinsic ligaments
- extrinsic ligaments
- radioscaphoid ligament
- dorsal intercarpal ligament
- dorsal radiotriquetral ligament
- dorsal radioulnar ligament
- volar radioulnar ligament
- radioscaphocapitate ligament
- long radiolunate ligament
- Vickers ligament
- short radiolunate ligament
- ulnolunate ligament
- ulnotriquetral ligament
- ulnocapitate ligament
- ulnar collateral ligament
- associated structures
- extensor retinaculum
- flexor retinaculum
- joint capsule
- movements
- alignment
- ossification centers
-
hand joints
- articulations
- carpometacarpal joint
-
metacarpophalangeal joints
- palmar ligament (plate)
- collateral ligament
-
interphalangeal joints
- palmar ligament (plate)
- collateral ligament
- movements
- ossification centers
- articulations
-
pectoral girdle
- spaces of the upper limb
- muscles of the upper limb
- shoulder girdle
- anterior compartment of the arm
- posterior compartment of the arm
- anterior compartment of the forearm
-
posterior compartment of the forearm (extensors)
- superficial
- deep
- hand (mnemonic)
- accessory muscles
- blood supply to the upper limb
-
arteries
- subclavian artery (mnemonic)
- axillary artery
- brachial artery (proximal portion)
- ulnar artery
- radial artery
- veins
-
arteries
- innervation of the upper limb
- intercostobrachial nerve
-
brachial plexus (mnemonic)
- branches from the roots
- branches from the trunks
- branches from the cords
- lateral cord
- posterior cord
- medial cord
- terminal branches
- lymphatic drainage of the upper limb