Vacuum sign is a common radiological finding. It is also referred to as a vacuum phenomenon and often associated with degenerative spinal discs, knee joints, hip joints, and shoulder joints. Degenerative spondylolisthesis is a spinal condition whereby one vertebra slips on another. However, it is sometimes difficult to ascertain whether this slippage is stable or not. A more than 4mm movement defines instability, but some have indicated only 2mm as clinically significant.
In a research paper published in World Surgery, 1 a group of authors looked at the vacuum sign in the facet joint as an indication of degenerative instability of the spine. They wanted to investigate the relationship between the vacuum facet phenomenon and lumbar instability. Why, you may ask? More and more research is directing spine researchers to the cause of pain and disability to the imbalance of motion of the individual vertebral segments of the spine. Some have coined this motion sharing.
Each vertebral motion segment consists of two vertebrae, and a disc should have a certain stiffness level. That is, it should move similar to its adjacent segment above and below in the spinal column. For this study, they looked at L4 on L5 (which is a prevalent spinal level to degenerate with age) and used flexion/ extension X-rays in both the fully bent forward (flexion) and the fully bending backwards (extension) with degenerative spondylolisthesis. Additionally, when available, they also looked at CAT scans of these same patients. To determine the slippage degree, they used a dynamic motion index to measure the degree of slippage.
In a total of 67 patients examined, 35 patients had vacuum signs on their CAT scan, and 32 patients did not. The degree of slippage appeared to correlate with the vacuum sign as well. That is, the more the vertebrae had slipped forward, the more likelihood of the presence of the vacuum sign. With this, the authors concluded a linear correlation between the degree of slippage and the presence of vacuum sign.
Commentary by Jerome Fryer
Vacuum sign or vacuum phenomenon is often considered an incidental finding. However, based on the modelling research I’ve done, I believe that the vacuum sign can be a clue into joint mechanics’ stiffness. In 2017 I published an article related to the cracking event we are familiar with, and in there, I believe in having revealed the vacuum phenomenon. In the presence of cavitation, a joint will have less stiffness, and in time I hope we can collectively use these radiographic findings to help us determine which joint requires more stability in the treatment of them. JF