Vacuum phenomenon in the shoulder refers to the presence of intra-articular gas in the shoulder joint. It is a very common occurrence, particularly in external rotation. This can cause circular or linear areas of low signal intensity on GRE MR images of the shoulder obtained with external rotation of the arm and represents small foci of intra-articular gas. Awareness of this potential pitfall and an understanding of the characteristic appearance and location of this collection of intra-articular gas may help prevent misdiagnosis of intra-articular loose bodies or chondrocalcinosis. This is a joint-specific example of the vacuum phenomenon.
The presence of gas in a joint can occur in almost any joint, such as the sternoclavicular joint, where it may outline the intra-articular disc (see case 2).
Pathology / physiology
Possible mechanisms for gas entry include development of negative pressure within the joint cavity. As a result of a sudden contraction of the surrounding muscles, there is a distraction of the joint, resulting in an increase in joint volume creating a negative pressure (i.e. development of pressure gradient across the joint cavity).
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