Abstract
‘Vacuum phenomenon’ or ‘pneumoarthrosis’ term is used when there is air within a joint space. It has been described commonly in the spine and occasionally in the peripheral articulations. It is usually seen following trauma, and sometimes spontaneously in joints showing degenerative changes. Although it has been mainly described with a conventional radiograph, other diagnostic modalities such as ultrasonography, CT scan, and MRI have also been used for its delineation. We present three cases of vacuum phenomenon observed in the shoulder joints and the hip joint. These radiolucent shadows were visible in the radiograph and vanished subsequently. The ‘air inside the joint’ may be a benign condition and does not necessitate further workup unless the patient is symptomatic.
Highlights
Pneumoarthrosis, i.e. air within a joint cavity, can be seen in the radiograph of a joint
We present three cases of vacuum phenomenon observed in the shoulder joints and the hip joint
Because of the development of pressure gradient across the joint cavity, these gases rapidly diffuse into the joint cavity and outline the synovial layer or the articular cartilage not covered by the synovium, and form a curvilinear lucency in the radiograph
Summary
Pneumoarthrosis, i.e. air within a joint cavity, can be seen in the radiograph of a joint. Apart from left perihilar opacities, bilateral glenohumeral joints showed lucencies due to the vacuum phenomenon with arms of the infant kept above the head. In this case, it might have resulted from overhead abduction of the shoulder joint resulting in joint distraction. The third radiograph in an adult showed a thin crescent of air in the left glenohumeral joint probably due to a slight external rotation of the arm, as seen with the reduced overlap of glenoid rim and humeral head In all these three instances, follow-up clinical and radiological examination showed clinically normal joints (Figure 5)
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