The Air Inside Joint: A Sign of Disease Pathology or a Benign Condition?

'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.