The ideal image for identifying a posterior dislocation is a axillary film, with the patients arm abducted at 70-90 degrees and the image taken from an angle of 45 degrees through the axilla. This should be done as part of a standard set of imaging after shoulder injury, along with an AP and lateral view of the shoulder.
Typically the humeral head is forced posteriorly in an internal rotation while the arm is abducted 1,3. In adults, convulsive disorders are the most common cause. Electrocution is a classic but uncommon cause of posterior shoulder dislocation.
Because posterior shoulder dislocations are uncommon (< 4% of shoulder dislocations), and often diagnosed late, obtain orthopedic consultation prior to reduction. Findings on anteroposterior x-rays may be subtle. A scapular Y view or an axillary view x-ray should be done. A CT scan should be done if the dislocation is unclear on x-ray.