The shoulder is stabilized mainly by negative intraarticular pressure when the arm is at the side of the body with all the muscles relaxed. During arm motion in the midrange of motion, the contraction force of the muscles provides dynamic stability. In shoulders with atraumatic instability, the joint capsule is thin and enlarged, making it more difficult to maintain the negative pressure. Decreased joint volume by capsular imbrication results in creating the negative intraarticular pressure. Imbalance of muscle forces may cause decreased abduction of the scapula during arm elevation or decreased concavity compression (or both), either of which may result in instability. Muscle exercises are effective in most cases of atraumatic instability. Congenital hypoplasia of the glenoid also contributes to decreased concavity compression. Glenoid osteotomy is the treatment of choice in such cases. One of these factors may play a role in the occurrence of atraumatic instability by itself or in combination. Better understanding of the pathophysiology of atraumatic shoulder instability is useful when selecting the best treatment option in each case.
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