A recent article in Skeletal Radiology by Resnik [3] implies that effusions caused or contributed to inferior glenohumeral subluxation in three cases of septic arthritis of the shoulder. The author states that "the volume of fluid necessary to cause a drooping shoulder is unknown... Other causative factors, such as the capsular necrosis evident in patient 2, must also contribute to the development of drooping shoulder."
The commonly held belief that glenohumeral effusions cause inferior subluxation of the shoulder, like the belief that hip effusions cause lateral displacement of the femoral head, is tenable but unproven. It is true that subluxation is frequently associated with effusion, as is described with fractures, hemophilia, and septic arthritis. However, I believe that effusions play a minor etiological role in this entity and that subluxation is usually caused by lack of tone in the periarticular muscles of the shoulder. Diminution in muscle tone might be secondary to neuromuscular causes such as hemiplegia [2], or be a nonspecific reflex reaction to pain or injury.
Basmajian et al. [1] have demonstrated that, contrary to expectations, it is not the vertically running muscles of the shoulder but the horizontally oriented supraspinatus muscle, posterior deltoid muscle, and superior joint capsule that apparently prevent infcrior subluxation of the shoulder in the hanging arm. These authors suggest