Posterior shoulder dislocation is most commonly due to which mechanism?

• A. Seizures or trauma
• B. Athletic overuse
• C. Improper lifting
• D. Degenerative disease

Answer: (A)

Posterior dislocation occurs when the humeral head is forcibly pushed backward out of the glenoid by a forceful internal rotation and adduction of the arm. This is classically produced by a seizure or electric shock, where intense, unopposed contractions of the internal rotator muscles (subscapularis, pectoralis major, latissimus dorsi, teres major) overpower the external rotators, driving the head posteriorly. Trauma can cause it as well, but seizures or trauma as a mechanism best explain the typical posterior dislocation. The other options don’t provide the sudden, strong inward rotational force needed to displace the head posteriorly.