Os acromiale is an accessory bone which results from an unfused center of ossification of the acromion of the scapula bone.
Os acromiale is seen in about 8% of the population and may be bilateral in 60%. Os acromiale can be mistaken for normal acromioclavicular joint, degenerative ossicles and fracture of the acromion.
Ossification of Scapula
At birth, the body and spine of scapula form one ossified mass, with the coracoid, acromion, glenoid, and inferior angle all being cartilaginous.
At 3 to 18 months, a center of ossification appears at the mid-coracoid.
At 7 to 10 years, the coracoid base, including the upper third of the glenoid, appears. Sometimes called a subcoracoid bone and it joins the rest of the coracoid at 14 to 16 years.
An ossification center at the tip and shell-like center of the medial apex of the coracoid may appear at the same time and go on to fuse between 18 and 25 years.
Two or three acromial centers form at the age of 14 to 16 years, coalesce at 19 years, and fuse to the spine between 20 and 25 years of age.
Failure of fusion with persistence of one ossification center past 25 years of age creates what is known as an os acromiale. [see below]
The glenoid fossa ossifies from four sources:
- The coracoid base (including the upper third of the glenoid)
- The deep portion of the coracoid process,
- The body,
- The lower pole
Between 8 and 13 years of age, the glenoid border may show evidence of irregular ossification.
At the inferior angle of the scapula, an ossification center appears at 15 and fuses at 20 years of age; the vertebral border center appears from 16 to 18 and fuses by 25 years of age.
The ossification centers may be asymmetric, and thus comparison films may not be helpful.
Types of Os Acromiale
There are four types of os acromiale depending on fusion pattern of the three acromial ossification centres (preacromion, mesoacromion and metacromion and os acromiales are classifed on their pattern of articulation with the acromion
Meta- and meso-acromial are the most common types.
Presentation of Os Acromiale
Os acromiale is usually not symptomatic but rotator cuff deformity has been noted more in people with step-off deformity of the os acromiale.
The unfused anterior acromial ossification center is best seen on axillary views. Factors favoring the diagnosis of os acromiale over fracture are
- Bilateral occurrence
- Rounded borders with uniform space
- Bony ossification center is even with or above the posterior acromion on the anteroposterior view.
Other Scapular Anomalies
Hypoplasia of the glenoid may resemble and thus be misinterpreted as an impaction fracture of the glenoid.
Marked glenoid retroversion is also seen with this condition. It usually takes a benign course.
There could be times when doubts arise in the interpretation of history and x-rays. Following entities may be considered in differentials of scapular fractures