The Audible Release – Investigating the Cracking Joint

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A cracking joint (audible release) has been at the chiropractic profession’s foundation for over a century. It is performed on patients daily across the world in the management of spinal pain. However, what is the deal with the mechanism? Where is the sound coming from?

As a chiropractic student between 1996 and 2000, I was curious about the phenomenon.  It was at the heart of manipulation, but the explanations regarding the mechanism were not convincing. It made sense that a joint separated due to the event as I could understand that and see it with my own knuckles, but why could some people crack their knuckles and others could not? Do all low back patients benefit from spinal manipulation? So many questions I had and still have to this day.

Reading the research back then pointed me to Unsworth’s paper [i], which was the one citation that most people reference. He described the phenomenon as cavitation but explained the noise as a collapsing cavity. This is a process where a cavity forms first, followed by a subsequent collapse. This event is believed to occur within .01seconds. This hypothesis challenged Roston’s 1947 paper [ii], which proposed the sound from a cavity formation. There was also another interesting paper that perked my interest, written by Brodeur[iii]. He believed the sound had something to do with the outward snapping of the capsule.

What I knew we needed to understand the mechanism of joint cracking was a bench-top model we could test; an in-vitro model.  And it was by pure serendipity that I stumbled upon a possible mechanism. In 2006 I began designing and building dynamic disc models (which I named Dynamic Disc Designs…a bit of a play on acronyms: DDD for Degenerative Disc Disease) when years later, as I was increasing the detail of the synovial joint, a ‘crack’ occurred spontaneously. It happened after polishing the simulated hyaline cartilage and sculpting an elastomeric synovial fold. The fold generated a cracking sound when it pulled off the shiny surface…similar to a suction cup release. I developed a lumbar model and, from that experience and called it The Audible Release Dynamic Disc Model, which also has been named by one of my mentors, The Oracle.

This basic observation led to the paper conducted in 2015 at The University of Alberta, using myself as the lone subject. Our PLOS Study[iv], led by Greg Kawchuk and co-authored by myself, challenged Unsworth’s collapsing cavity hypothesis. You may have seen this popular video of Greg pulling my finger.  It has been used to create many cracking joint videos throughout Youtube. It was after this MRI study that I decided to introduce fluid into the Audible Release Model, which led to a 2017 publication in JCCA[v].

Currently, I am leading the charge on a study at the University of British Columbia, having found some like-minded people to work with through the Mechanical Engineering Department. Data collection is currently underway. However, it was a challenge to find the funding, but I did find it with help from the collective help from the British Columbia Chiropractic College, patients and colleagues.  The main goals are to settle the debate about whether the sound is generated from a cavity collapse or a cavity formation. To settle the debate, a temporal resolution of 1200 fps[vi] is required so one can observe the event under the high-speed video. I believe we have the set-up.

In the meantime, I am happy to have developed a model we can use to show patients how manipulation (with audible release) generates facet space and likely disc height gains to improve the spacing for the nerves often. Often patients are nervous about the physical event as it requires some force to elicit. With a dynamic disc model, a practitioner can now easily show the patient what this event looks like and how side posture manipulation rotates a motion segment to allow separation of the facet joint. The models also help explain disc herniation, for example, as most of the popular models I craft have both an annulus and nucleus.  With a controlled and specific squeeze (flexion and compression) of the model, a patient can quickly understand that there may be an underlying herniation if that is clinically suspicious. The models are all handcrafted and designed with the patient in mind. Having been a chiropractor for 18 years, I understand those questions that come up or those questions that patients are afraid to ask. These models bridge and strengthen bonds between chiropractors and patients as it enhances the  “therapeutic alliance.” They are engagingly dynamic and can show the spinal problems as well as the proposed solutions. They are perfect for showing what we do to help patients with back and neck pain.

[i] Unsworth A, Dowson D, Wright V. ‘Cracking joints’ A bioengineering study of cavitation in the metacarpophalangeal joint. Ann Rheum Dis. 1971:348–358. [PMC free article] [PubMed]

[ii] Roston JB, Haines RW. Cracking in the metacarpophalangeal joint. J Anat. 1947;81:165–173.[PMC free article] [PubMed]

[iii] Brodeur R. The audible release associated with joint manipulation. JMPT. 1995;18: 155–164. pmid:7790795

[iv] Kawchuk GN, Fryer J, Jaremko JL, Zeng H, Rowe L, Thompson R. Real-time visualization of joint cavitation. PLOS ONE. 2015;10(4):e0119470. [PMC free article] [PubMed]

[v] Fryer JC, Quon JA, Vann RD. A proposed in vitro model for investigating the mechanisms of “joint cracking”: a short report of preliminary techniques and observations. The Journal of the Canadian Chiropractic Association. 2017;61(1):32-39.

[vi] Chandran Suja V, Barakat AI. A Mathematical Model for the Sounds Produced by Knuckle Cracking. Scientific Reports. 2018;8:4600. doi:10.1038/s41598-018-22664-4.

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