Anterior or posterior shoulder dislocations can be occult on AP films, making the axillary view necessary. For example, posterior shoulder dislocations, which make up only 2-3% of shoulder dislocations, place the humeral head posterior to the glenoid fossa. On an AP film the humeral head appears to be in its normal position because it is still superimposed over the glenoid fossa. It is impossible to tell if it is actually posterior to the glenoid fossa.

In this scenario the axillary view is needed. The x-ray beam is directed either up into the axilla (inferior to superior), or from the superior aspect of the shoulder down through the axilla (superior to inferior). We can now visualize whether the humeral head is within, or posterior to, the glenoid fossa (in this film it is within the fossa).

You should be able to identify the clavicle, coracoid process, head of the humerus and glenoid fossa.