The majority of the scapula undergoes membranous ossification. Some of the outer parts of the scapula are cartilaginous at birth, and therefore undergo endochondral ossification. The scapula is ossified from 7 or more centers: one for the body, two for the coracoid process, two for the acromion, one for the vertebral border, and one for the inferior angle.
At birth, a large part of the scapula is osseous, but the glenoid cavity, the coracoid process, the acromion, the vertebral border, and the inferior angle are cartilaginous.
From the 15th to the 18th month after birth, ossification takes place in the middle of the coracoid process, which as a rule becomes joined about the 15th year.
Between the 14th and 20th years, the remaining parts ossify in quick succession, and usually in this order; first, in the root of the coracoid process, in the form of a broad scale; secondly, near the base of the acromion; thirdly, in the inferior angle and contiguous part of the vertebral border; fourthly, near the outer end of the acromion; fifthly, in the vertebral border.
Failure of bony union between the acromion and spine sometimes occurs ie os acromiale, the junction being effected by fibrous tissue. It seems that a similar process can occur on the inferior angle of the scapula as in this case.