joint cracking

Exploring the Sound of Synovial Joint Distraction: Is it a Gas Bubble or Something Else?

When we experience a “pop” or “snap” sound during the distraction of a synovial joint, most people wonder what exactly causes this audible noise. While there is a prevailing belief that it’s a gas bubble that produces sound, a new, yet simple, hypothesis challenges this notion.  

Dr. Jerome Fryer, the Chief Innovation Officer at Dynamic Disc Designs, proposes an alternative explanation. He suggests that the sound originates from the elastic recoil of the synovial fold rather than a bubble, similar to how a suction cup makes a sound. Below, he summarizes the details of this theory and its potential implications for diagnosing synovial joint conditions.

The Synovial Tag and its Role

Dr. Fryer introduces the concept of the synovial tag, a structure within the synovial joint that behaves like a suction cup for joint surface congruity and stability. During joint distraction, as the facet cartilage separates from each other, the synovial tag is drawn in either direction and then recoils back into its original position after reaching its elongation end range. To understand the mechanics behind this hypothesis, we need first to understand how a suction cup functions.

Suction cup release


Synovial Fold Tag with Joint Cavitation and Cracking

Suction Cups and Negative Pressure

When you press a suction cup against a surface, it pushes out the air between the cup and the surface. When the cup is pulled away, a negative pressure arises underneath it, like the negative pressure found in a healthy synovial joint (approximately -3mmHg). 

As the suction cup continues to be pulled from the surface, the atmospheric pressure surrounding the cup eventually pushes it back to the surface, reaching its elastic endpoint. At this point, the cup has an elastic recoil, snapping back into its original shape while the atmospheric pressure goes back to its natural tissue state.

Reevaluating the Sound Source

Previous research has attributed the audible joint sound to a gas bubble that appears after the “pop” event during joint distraction. Dr. Fryer challenges this assumption. In his article, he highlights a study from 1986 by Meal and Scott, where they observed two sounds accompanying the joint “pop.” 

According to Dr. Fryer, if the sound really comes from a bubble, we would expect to hear only one sound. But the occurrence of two sounds aligns more closely with his hypothesis of the synovial fold acting as the noise generator. Unlike a single-point source, the synovial fold forms a ring of tissue that can make two distinct sounds. These two sounds were recorded by the researchers who used a single microphone in their methods.

Advancing Research and Diagnostic Precision

By going deeper into the mechanisms and definitions of audible joint sounds, Dr. Fryer hopes to improve our diagnostic abilities on synovial joints and respective treatments. His hypothesis opens up new avenues of investigation. At the University of Alberta, MRI technology was used to explore this phenomenon further. This research tries to unravel the true nature of the sound produced during synovial joint distraction. This could, in turn, improve our understanding of joint pathologies and refine diagnostic techniques. 


ABOVE IS Dr. Jerome Fryer’s finger the PLOS ONE study as one of the authors


Moving Forward

The longstanding belief that a gas bubble causes the “pop” sound during synovial joint distraction may now be challenged by a different hypothesis proposed by Dr. Jerome Fryer. Suggesting that the sound originates from the elastic recoil of the synovial fold, similar to Richard Brodeur’s hypothesis, as this alternative explanation presents an intriguing perspective on the mysterious auditory phenomenon. In 2017, Fryer led a paper published  in JCCA1 whereby he developed a bench-top model using the modelling skills from Dynamic Disc Designs, revealing a possible mechanism to test. This model demonstrates similar characteristics when compared to a real human joint.

  1. joint gapping
  2. audible release
  3. force to elicit
  4. refractory period