The Development of an In-vitro Model for a Cracking Joint

So what creates the cracking sound when joints move? Sometimes this happens unexpectedly with joint motion, and in the case of my profession, we intend to elicit the popping sound to reduce joint pain. Interestingly, we really don’t know why some joints pop and others do not and so the mechanism has always intrigued me.

Background and a Little Personal History

In 1995, I graduated from UBC. At the time, it was exciting to be crowned a bachelors of science degree in Biopsychology. ‘Crowned’ is not the right word because I didn’t know much about anything. However my teachers, in particular, Dr J P Pinel, drilled it into us that we must be sceptical of all facts.

Bachelor degrees don’t lead to big paychecks. They are simply a solid foundation for further study. And when I stepped out from UBC and into the real world, I knew I had to find a career.

I wasn’t too interested in medicine because of pharma influence on the medical education. My Mum and Dad were children of the 50s and 60s and we did not visit the doctor very often. Our medicine cabinet was sparse and usually the place where we would only find a band-aid.

I thought of alternatives to medicine and found myself considering a chiropractic career. But not having been to a chiropractor myself, I thought I’d best visit one and see what it was like to be treated.

After a detailed history, my first chiropractic treatment began in 1996. And what I could tell, was the doctor was quite intent on making my spine go ‘pop’.

So I asked the doctor, ” where is that sound coming from?”

He replied, ” it is coming from a gas. ”

At the time, my Bachelor’s degree was not any use, but I also knew that gases, when formed, do not make that kind of noise. This explanation just did not sit well with me, but I couldn’t argue because I did not have a better idea.

University of Western States

I entered the University of Western States, late in 1996, for a Doctorate Chiropractic Degree…which was a full paced experience. And in there, I was curious to see what they would teach us regarding sounds and synovial joints. In particular, I was still most interested in the sound. Where was that classic audible “pop” coming from, specifically?

It wasn’t until the third year when Dr David Panzer, an instructor of biomechanics, revealed to me that there wasn’t certainty around the sound source. He directed me to an article written by Raymond Brodeur 1 in 1995 that provided an alternative hypothesis that Anthony Unsworth proposed in 1971 2.

Unsworth believed it to be a bubble collapse like seen here but at such smaller scale, whereby an inception and subsequent collapse of a bubble within the synovial joint space occurs within .01 seconds. His work was thought of as ‘the’ mechanism. However, in their study, they did not offer an in-vitro model to show the refractory period. For those that do not know what the refractory period is, it is the period in between ‘popping’ events. In other words, when an audible release is generated from a quick separation of the joint (often referred to as a cracking joint), there is a period that must elapse before the popping can be repeated.

Brodeur wrote his piece in 1995 and believed it to be something a little different. He hypothesised that it was the joint capsule that was drawn inward as the joint surfaces pull apart. He explained that as negative pressure is generated within the joint space, as the two joint surfaces are pulled apart, a tension threshold is reached between the inward pulling of the and the elastic endpoint of the synovial capsule.

audible release, suction cup

Raymond Brodeur in 1995, talked about the synovial capsule recoil. Copyright © 2014 Jerome Fryer DC

Raymond Brodeur’s hypothesis seemed to make more sense to me rather than a collapsing bubble. And it was his theory that I hung on to closely from the day I read his paper up until early 2013.

Building Models for Better Outcomes

For those that do not know, I have developed an anatomical spine model business from the ground up. There was a need for an updated anatomical model and importantly, one that moved like the real spine. It was in the construction of one of the models that got me thinking about an alternative hypothesis to Raymond Brodeur’s.

To give a little background on Dynamic Disc Designs Corp., it was in 2006 when I moved to Nanaimo from Ucluelet to restart practice. With not so many patients and much time on my hands, I started a dynamic spine modelling business. I wanted to connect to patients in an anatomical way.  I knew that educating a patient about their own anatomy was the best way to get great results. But I had a real hard time doing this because the models on the market were quite outdated. Furthermore, they did not move.

I practised for 6 years in these two beautiful coastal communities. Over those precious years I quickly learned that a knowledgeable patient generated the best outcomes. I often used the full spine model I purchased in chiropractic university, but I would have to send it in for repair regularly because the metal rod would snap and break. There was a metal rod that held things together that would eventually fatigue and break. After several repairs, I stopped bending the model because of the cost. But with this, I lost the ability to show patients spine movement and how it related to their pain.

So when I moved, I set out to develop a dynamic spine model and 10 years later, I now know that I wasn’t the only one craving a dependable spine model that is up to date with the current spine research. Over the years I have consulted with the some of the world’s best doctors to be sure I had developed something that all spine professionals can feel confident using. I have over 1000 customers and you can read and see some of the testimonials from all over the world. Now, both patients and doctors have a tool to talk spine mechanics and the respective solutions that goes along with the diagnostics of pain generators.

I try to craft as accurately as I can. And in the development of trying to simulate a synovial joint, a characteristic ‘popping’ sound generated wen I was sculpting the synovial fold. This was an ah-ha moment. And with this knowledge, the Oracle Model was developed, aptly named because of its potential origins in the sound of a joint crack.

All the models I craft start with real cadaveric bone without the hyaline cartilage. So to recreate the joint, I first needed to recreate the hyaline cartilage.


Do you know those shiny crunchy bits found on the end of chicken bones? That is hyaline cartilage. The word ‘ hya’ comes from the latin word ‘glass-like’. It was named this because of its smooth surface.

The bones I use do not have this smooth surface, so I worked to recreate it. Interestingly, once I did that, the synovial fold I crafted generated a popping noise when it was pressed against the smooth surface and quickly pulled away; similar to a suction cup generating a noise when pulled off a piece of glass. The synovial fold (also known as the meniscoid or synovial tag) is a ring-like elastic structure that works to take up the excess space in the joint and it is similar to the meniscus in the knee but a smaller version. This process took me several months and revisions.


in-vitro, model, synovial joint

Development of an in-vitro joint – Dr. Jerome Fryer

What got my curiosity was that this ‘popping’ or ‘cracking’ sound resembled the noise generated from a real joint! At that time, I revisited Brodeur’s work to see if he had talked about the synovial fold. And when I read that he didn’t, I thought that perhaps the sound was generated from the release of the synovial fold.

So, I set out to try and understand what generated the sound of a suction cup release, which seemed to me, a similar physical principle as the ringlike structure I created.

Suction Cup Sound and Video

To investigate this ‘new hypothesis’ I wanted first to see what a slow motion video of a suction cup release looked like. I contacted Alan Teitel of, and he kindly generated some video for me which can be seen below.

I also knew that  I needed someone that could help me explain what makes the noise from a suction cup release. Kevin Donahue’s name came up in a PubMed search. So I contacted him and he kindly did a few things for me including providing an explanation of a negative amplitude shock wave pulse.

For one, he analysed a high definition audio recording of my own knuckle crack as well as my 11yr old son and compared our two fingers. He used the data I acquired as a classroom exercise for his students.

audio, cracking, joint, synovial, mechanism

Dr. Jerome Fryer capturing audio from his knuckle cracking.

Analysis of Two Differing Sized Suction Cup Pops

Kevin Donahue also kindly obtained high definition audio recording of two different suction cups in size and material properties. His results are below.

Suction Cup Experiment

Compiled by Kevin D. Donohue and Ryan Norton

University of Kentucky

For Jerome Fryer

July 27, 2015


An experiment involving the popping noise made from suction cups was performed on July 2.  The goal was to see how suction cup size and softness relate to the sound spectrum made.  Two suction cup sizes (1 and 2 cm) and softness  (hard and soft) were used on two different platforms (glass and plexiglass).  The suction cups were sent from Jerome Fryer.  The recordings were done in a low noise sound booth with a 4 microphone setup in a rectangular pattern 28 cm by 16 cm around the suction cup and 10 cm above the platform.   The suction cup was periodically moistened with tap water during the experiment.

Each recording included at least 30 suction pops at intervals of about 3 seconds.  Software was writing to automatically segment out the pops, compute their spectra and average over all pops over all 4 microphones.  Signals were sampled at 96kHz and a high pass filter (4th order Butterworth at 120 Hz) was applied to reduce low frequency room noise in the chamber.

Figure 1 below shows the spectral difference between the 1 and 2 cm suction cups for the hard and soft rubber.  There are no remarkable difference between these except for slight trend for the hard rubber to have a stronger low frequency component.   This could be due to the cup maintaining its structure during the pull from the hard surface and/or damping the pop sound to make the duration small and giving it a slightly broader band.  But differences are not significant enough to draw a reasonable conclusion.

Figure 1.  Spectra comparison for suction noises between hard and soft rubber. a) 1 cm cup diameter b) 2 cm cup diameter.

suction cup sizes
Figure 2 shows the most dramatic difference between the sizes of the suction cup.  The modal peak of the 1 cm cup is around 11.25 kHz, while the 2 cm cup has a modal peak around 6.4 kHz.  The most dramatic difference appears after 20kHz where there is a whitening (flattening) of the spectrum for the 1 cm, while the 2 cm cup continues to roll off.  This pattern may be worth exploring with addition experiment (a wider range of cup sizes).  The smaller cup sound generally had more power, this may have been more of a construction issue, where the stem connect to the cup and provided support from a greater percentage of the cup structure as compared for the 2 cm cup (stem connecting to the cup was about the same size for both).  We would expect the frequency content for the smaller cup to be higher as the larger structure could support frequencies with longer wavelengths.  However, there is no clear resonance as these are more transient sounds (less than 4 cycles in the pop noise).

No significant differences were found between the glass and Lexan surfaces, these experimental measurement we averaged together in the above plots.

The March to Understand More

Around this time in early 2013, I thought this idea of the synovial fold release should be further investigated. I contacted Greg Kawchuk at the University of Alberta and shared with him my new hypothesis and the model I created.

I am happy he felt it was something worthwhile to investigate, but it did take some convincing.  Greg had thought it, the mechanism, had been determined already. But after I directed him to the Unsworth study, he realised that the mechanism was still unknown.  And so, he worked to set up an MRI study with a colleague. And because I volunteered my knuckle as the test joint, we so aptly named it the “pull my finger study”. I, like many other people, can crack their own knuckles by pulling them. If you want to have a listen to an interview I did with CBC, As It Happens, you can click here.

MRI and My Knuckle Crack

With a special MRI coil to investigate hands and other smaller anatomical specimens, I travelled to the University of Alberta to collect data. Greg pulled my finger while I was laying prone with a long cable attached. We used a finger trap method to get a good hold of it, pulling one finger at a time.

pull my finger, cavitation

Pull My Finger

What we saw changed synovial science forever. Surprising, no-one had used MRI to investigate this phenomenon before. And what we saw challenged over 40 years of conventional thinking. Up until this point, most people believed in Unsworth’s study which believed the noise was coming from a collapsing bubble. We saw cavity formation, not a collapse. I have to thank Greg Kawchuck for taking on this project.

Comparably, in the Unsworth’s study, they used high-speed video recording of an in-vitro model they called the cavitometre. This in-vitro model was constructed of Perspex and was approximately twice the size of a real knuckle joint. Real synovial fluid was used but importantly, the system was open and a refractory period was not shown.

PLOS ONE Study went viral

You can read the full research article here that went viral. Greg described the phenomenon we saw as tribonucleation. However, tribonucleation was not known to generate any noise. I contacted Sander Wildeman, someone that had just published a recent paper on tribonucleation.

My first question to Sander was, “does tribonucleation generate a loud cracking sound?”. He answered,  “no, not in his experiments”.

Tribonucleation Origins

Tribonucleation was originally coined from the generation of bubbles from two surfaces rubbing across one another. ‘Tribo’ word origin comes from latin ‘to rub’. It was originally described by Hayward in 1967 with the rubbing of two structures togetherIkels in 1970, came to write about tribonucleation and included the concept of surfaces coming into contact, separating or rubbing. However, tribonucleation was originally described by Hayward as a rubbing. Ikels included the concepts of separation. In my opinion, tribonucleation is by definition a ‘rubbing’ and other terminology should be used to describe the other physical events that generate cavitation.

While our paper was going through the submission and review process, I continued to expand on the model I had developed.  Introducing fluid changed the mechanics of the event significantly and this is when the pieces started to fall into place.

The Experiment

As you can see in the video above, a bubble formed under the cup but in one case, there was a sudden release and noise generated. This only occurred when the fluid was denucleated. I did this by vacuum.

I bothered Sander for quite some time to see if he could obtain high-speed video of a suction cup release in water. What I had observed initially was a bubble formation under the cup when viewed through a glass wall. There were also events I had noticed that generated a popping sound with the centre of the cup lifting, without full lift. This event was observed and then verified by Sander Wildeman when he conducted some lab testing using denucleated fluid by boiling. We observed the same event and Sander provided me with the insight of a pre-requisite of an absence, or significant decrease of nucleation that enables the audible release and centre cup lift. He also explained to me that this process demonstrated refractory.

I wanted to work with Sander but he was not interested and had other commitments.

At this time, I was advised to publish what I had discovered in a short paper which has now been published in The Journal of the Canadian Chiropractic Association on March 15, 2017.

The investigation continues. And now, I think we have something solid to build from. Understanding the mechanisms of sound should help us reveal the physics of joint movement.

I have to thank several people who have helped me along the way.

My wife Nichole Fryer who has taken care of the children and given me time to explore this passion of mine.

Sander Wildeman for conducting some preliminary testing.

Jeffrey Quon, Francis Smith, and David Panzer for their mentorship. Without them, I would not have the support.

Special thanks to Kevin Donahue, Daniel Russel and Greg Kawchuck for their collaboration.


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