It never fails—somebody always asks about what's going on when knuckles crack when we get to the topic of articulations.
Recently, researchers used modern MRI techniques to demonstrate exactly what is happening—a process first described in the 1940s. As joint surfaces separate, the changing tension in the synovial fluid causes a vapor pocket or "air bubble" to form in a process called cavitation.
The recent research proved that the popping sound typical of cracking a knuckle is produced as the vapor pocket forms. It also disproved the theory that the popping occurs when the bubble collapses.
What can we use from this in teaching undergraduate A&P?
- You'll now always be ready for that inevitable "what causes knuckles to crack?" question when discussing joints in your A&P course.
- Consider having a slide containing the media offered below to show students.
- Discussing joint-cracking mechanisms engages students in a subject that may not immediately interest them.
- Joint-cracking allows us to reinforce concepts of fluid dynamics in the context of synovial joints.
- Consider stimulating some critical thinking by asking "what could cause a cracking sound in a cartilaginous or fibrous joint?" (Hint: think "fracture" as cavitation would not occur where there are no fluids)
- Do a demonstration by hitting a glass bottle full of water with a rubber mallet (see the embedded video below)
- I've done this with an unopened glass bottle of ketchup by upturning it and hitting the base with my palm
Want to know more?
- Real-Time Visualization of Joint Cavitation
- GN Kawchuk, et al. PLoS|ONE. April 15, 2015 DOI: 10.1371/journal.pone.0119470
- The original journal article. Includes downloadable images and PowerPoint slides, plus a link to a video showing the MRI results.
- Researchers pull fingers to solve why knuckles crack | Finding bursts bubble of popular theory
- Tina Hesman Saey Science News April 15, 2015
- A plain-English summary of the new work on knuckle cracking with an embedded video (see below)
Check out this video from the research article
Here's a video of the glass bottle demo of cavitation
Photo credit: Kawchuk et al.