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 head of the bone almost always comes out through the anterior portion of the capsule and slips beneath the coracoid process. From this point it may shift its position either a little farther inward, when it is called a subclavicular luxation, or a little farther downward and outward, when it receives the name of subglenoid.
As a matter of fact the head usually comes to rest beneath the coracoid process and permanent fixation of the bone either in the subclavicular or subglenoid positions is very rare. As the symptoms and methods of treatment are identical they will all be included under the one head of subcoracoid luxations. What are commonly regarded as subglenoid luxations are really subcoracoid.
Anterior luxations are produced by the arm being hyperabducted, rotated outward, and the head of the bone pushed or pulled in toward the body. Rotation may not be essential, but it is largely responsible for the wide detachment of the capsule which is present in these injuries. When the arm is raised from the body much beyond a right angle the greater tuberosity strikes the acromion process. If the hyperabduction is continued the acromion process acts as a fulcrum and the head of the bone is lifted-from its socket, tearing away the capsule of the joint in front of and below the glenoid cavity.
Fig. 245. - Dislocation of the shoulder; action of the bones; by extreme abduction of the humerus over the acromion process as a fulcrum the head is levered out of the socket.
If now the arm rotates, the capsule is still farther detached and if the force continues to act, as in those cases in which a person is thrown forward and alights on the outstretched arm, or if the axillary muscles contract, the head is thrust from its socket. After once leaving the socket, subsequent movements may cause the head to assume various positions around the glenoid cavity; as a matter of fact it is almost always below the coracoid process.
When the luxation occurs the arm is hyperabducted and, owing to the acromion process being somewhat posterior to the glenoid cavity, pointing backward, this places it up almost or quite alongside of the head. The force which thrusts the bone out acts downward toward the axilla and inward toward the body. The posterior border of the scapula is prevented from descending by the levator scapulae and rhomboid muscles, hence it is the joint which descends and tears loose the capsule already stretched tightly over the head of the humerus.
This is the reason why the lower portion of the capsule is torn; it is the longitudinally acting force that does it. When the transverse force acts it is expended on the anterior portion of the joint because the joint is at the anterior portion of the scapula. Posterior to the joint the scapula rests on the chest, so it is its anterior portion which is forced inward, thus rupturing the capsule at this point. The fulcrum, or acromion process, is also posterior to the midline of the joint.
By a combination of these two forces (longitudinal and transverse) the capsule is ruptured at its lower and especially its anterior portion. Its tearing is favored by a twisting or external rotation of the humerus. The attachment of the capsule is torn from the rim of the glenoid cavity, not from the humerus, and a fragment of the bony rim frequently comes with it. The opening is large and embraces nearly or quite half the circumference of the joint. It is limited above by the coracoid process. The coracohumeral and superior glenohumeral ligaments lying in front of the long tendon of the biceps also limit the tear upwards. If the tear does not extend so high it is because the subscapularis muscle, instead of being torn, is wedged in between the head and the coracoid process. Below, the tear is limited by the insertion of the long head of the triceps. As the head luxates it cannot pierce the triceps tendon, so it slips behind it in a posterior luxation and in front of it in an anterior luxation. The supraspinatus, infraspinatus, and teres minor muscles are all posterior; they blend more or less with the capsule and as the head luxates they are stretched with it over the glenoid cavity.
Fig. 246. - Surface view: subcoracoid dislocation of the humerus, showing the elevation of the shoulder abduction of the arm, prominence of the displaced head below the coracoid process, flattening of the shoulder, and tense fibres of the deltoid muscle.
The long tendon of the biceps, while it may sometimes be torn loose from the bicipital groove after rupture of the transverse ligament, is usually so loose that it follows the head without being detached from its connections. The subscapularis muscle not infrequently has its lower edge torn.
Fig. 247. - Subcoracoid dislocation of the humerus, showing the position of the bones in relation to each other and to the soft parts.
As the circumflex nerve winds around the surgical neck of the humerus, it may be ruptured or tightly stretched over the head of the bone. Hyperabduction stretches the vessels and nerves so forcibly over the head just prior to its leaving the socket as sometimes to produce serious injury to them.