What happens when joints crack… and a mysterious white flash

Osteoarthritis

16 Apr 2015

Australian and Canadian scientists have used MRI scans to show what really happens when a joint cracks.

Their findings prove that the mechanism of joint cracking is related to cavity formation rather than bubble collapse, they say.

The authors visualized ten finger joints from one participant by inserting them one at a time into a tube connected to a cable that was slowly pulled until the knuckle joint cracked. MRI video captured each crack in real time—occurring in less than 310 milliseconds.

As traction forces increased, real-time cine magnetic resonance imaging demonstrated rapid cavity inception at the time of joint separation and sound production after which the resulting cavity remained visible.

“Our results offer direct experimental evidence that joint cracking is associated with cavity inception rather than collapse of a pre-existing bubble,” the research team wrote in PLOS One. 

Their observations are consistent with tribonucleation, a known process where opposing surfaces resist separation until a critical point where they separate rapidly resulting in vapor cavities that do not collapse instantaneously.

However they also observed a white flash that appeared just before cracking, which they describe as a transient  bright signal in the intra-articular space.

They speculate that this ‘phenomenon’ may be related to changes in fluid organization between cartilaginous joint surfaces and specifically may result from evacuation of fluid out of the joint cartilage with increasing tension.

If so, this sign may be indicative of cartilage health and therefore provide a non-invasive means of characterizing joint status, they say.

The research was funded by the Canadian Chiropractic Research Foundation. 

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