This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The epiphyses that are liable to separation are those of the coracoid process, the acromion process, and the upper end of the humerus.
The coracoid process has three separate centres of ossification which fuse with the body of the bone from the fifteenth to the twentieth year. Therefore displacements occurring before the latter age may be separations of the epiphysis and not true fractures, particularly if the line of separation runs through the base of the coracoid.
The acromion process is cartilaginous up to the fifteenth year. Then two centres appear and the epiphysis unites with the rest of the spine of the scapula about the twentieth year or later. The epiphyseal line runs posterior to the acromioclavicular joint, just behind the angle of the spine of the scapula. It has been suggested that many cases diagnosed as sprains and contusions of the shoulder are really epiphyseal separations of the acromion process.
Separation of the Epiphysis of the Upper End of the Humerus- The upper end of the humerus has three centres of ossification, one for the head and one each for the greater and lesser tuberosities. These three centres are blended by the seventh year, and the whole epiphysis unites with the shaft at about the age of twenty-five years.
The epiphyseal line follows the lower half of the anatomical neck and then passes outward to the insertion of the teres minor muscle. This brings the outer end of the epiphyseal line some distance away from the joint, while the inner portion of the line is within the joint. Disease of this region may therefore follow the epiphyseal cartilage into the joint. A separation of the epiphysis from injury will implicate the joint.
The surgical neck of the humerus lies a short distance below the epiphyseal line and farther away on the outer side than on the inner. The line of the epiphysis rises higher in the centre of the bone than on the surface, making a sort of cap for the end of the diaphysis. The symptoms of epiphyseal separation are almost exactly the same as those of fracture of the surgical neck (see page 245).
The supraspinatus is the main agent in tilting the upper fragment outward, while the muscles inserted into the bicipital ridges, - the pectoralis major into the outer ridge and the latissimus dorsi and teres major into the inner, - draw the lower fragment inward. The relative position of the fragments when the lower is displaced outward is seen in Fig. 262.