The classic “pop” or “snap” or audible noises associated with a synovial joint distraction is a curious sound.
There seems to be a generalized consensus that the structure responsible for the noise is a gas bubble. For a review you can read here and find out that we don’t really know where the sound is coming from.
Dr. Jerome Fryer (Chief Innovation Officer at Dynamic Disc Designs Corp.) believes that the sound is being emitted from the elastic recoil of the synovial fold (meniscoid, synovial tag) rather than the bubble.
As you can see above, the synovial tag can act like a suction cup. As each facet cartilage pulls away from each other, the synovial tag is drawn in either direction and recoils back into original position after it reaches its elongation end range. To understand the mechanics of action, you will need understand how a suction cup works….read here.
As a suction cup is pressed against a surface, air is pushed out. As the cup is pulled away, the now negative pressure under the the cup is similar to the negative pressure found in a healthy synovial joint (approx -3mmHg). When the suction cup is pulled from the surface, the surrounding atmospheric pressure pushes the cup to the surface until the cup reaches its elastic end point. At this point, there is an elastic recoil of the cup and it snaps back into its original shape and the atmospheric pressure surrounding the cup is equilibrated.
We know that a gas bubble presents itself after the “pop” event of distracting a joint. But there has been no definitive research that actually proves that the sound generator is the bubble. One of the references is 27 years old now and used phonoarthrography. In this 1986 study, Meal and Scott saw two sounds with the pop from a synovial joint. In Dr. Fryer’s opinion, if the sound was coming from a bubble, we should have seen one sound, not two. Their two sounds could be explained with this new hypothesis of the synovial fold being the noise generator. This anatomy is not a point source but rather a ring of tissue able to generate two sounds from a single microphone, which is what these researchers used in their methods.
As we begin to unravel the precise definitions of audible joint sounds, Dr. Fryer’s hope is we become better diagnosticians for synovial joints. And as a result of his new hypothesis, research is now moving forward with the University of Alberta using MRI.
(March 29, 2014) As the ongoing investigation of this hypothesis develops, Dr. Fryer decided to write a brief hypothesis article on this topic. This manuscript was submitted to Chiropractic and Manual Therapies on March 2, 2014. The result was a rejection. A revision was then submitted to JCCA on March 18, 2014. A rejection was the initial result and the Editor suggested a resubmission as a commentary. Jerome Fryer decided to upload the hypothesis manuscript. If you want to read more, click Is the sound of manipulation from the synovial fold?