- A mathematical model proves the older theory that knuckle-cracking
is due to the popping of bubbles or gas pockets present in the synovial fluid
- A complete collapse of the bubbles is not needed to produce the
cracking sound; hence bubbles can persist even after the generation of the
Researchers Abdul Barakat and Vineeth Chandran Suja
have revealed a mathematical model of a cracking
knuckle that suggests that bubble popping in the synovial fluids of knuckles
could accurately explain the knuckle-cracking sound.
The study has been published in Scientific Reports.
‘A mathematical model now explains that it is the collapsing of bubbles or gas pockets present in the synovial fluid in joints that causes the sound when knuckles are cracked.’
Cracking the knuckles is very common - some proven facts about them are
that not all joints can be cracked and among those that can be cracked there
has to be a gap of 20 minutes to crack them again.
But how can we explain
the source of the sound?
So far, the joints most studied for
explaining the source of the sound has been
the metacarpophalangeal joints. Metacarpophalangeal joints are the joints
between the metacarpal
bones (those in between the finger bones and the wrist bones) and the phalanges of the fingers.
the metacarpophalangeal joints, there is a little lake of synovial
from grinding on each other. Gas, mostly carbon dioxide
dissolves and stays in
the synovial fluid. When we
crack our knuckles, the bones are pulled away from each other. This causes a sudden drop
in the pressure in the
middle of the joint. The dip in
pressure allows the gases to come together, forming bubbles.
Cavitation is the formation of
bubbles in the synovial fluid
as a consequence of forces acting on the liquid.
1971, Unsworth and co-workers
through extensive experiments concluded that cavitation and the
subsequent collapse or popping
of cavitation bubbles in the synovial fluid was the source of the cracking sound
. This theory was widely accepted for
over 40 years.
In 2015, Greg Kawchuk of the
University of Alberta
and co-workers challenged the cavitation theory
They used an MRI scanner
to record what
was happening inside the fingers of a frequent knuckle-cracking volunteer.
images showed a sudden appearance of a bulge in the knuckle as it
is cracked. This prompted Kawchuk and his colleagues to hypothesize that
the formation of the bubble, when the joint is pulled apart, might be
responsible for the cracking noise.
- The images also showed that the bubbles persisted
long after the cracking sounds were observed and provided evidence that gas
bubbles existed in the synovial fluid long after the cracking sounds were
Hence, they hypothesized that the cracking
sound does not occur due to the collapse of the bubbles as Unsworth and his
coworkers had concluded but rather due to the creation of the bubbles.
Abdul Barakat, a professor of biomechanics and a master's
student in his lab, Vineeth Suja at the Ecole Polytechnique in France grew
interested in knuckle-cracking on reading the 2015 paper.
They aimed to support the available experimental data and
to test the older theory, whether bubbles collapsing could produce a sound of
They developed a mathematical model of the hand's
metacarpophalangeal joint with a bubble in it
and experimented with a lot
of factors that could lead to the generation of the sound, like the thickness
of the surrounding fluid, or the speed at which the joints.
The idea was very simple - they compared the theoretical
sounds of the bubble collapsing in the model with recordings of Suja and others
cracking their knuckles
"We wanted to look at it mathematically because all the previous
work was based on observation or imaging, so we tried to build a mathematical
model that described the physical phenomena that governed this," Barakat
of the study
- The sounds predicted by the model matched the
volume and frequency of the recordings fairly well, even if the bubble
disappeared suddenly, rather than slowly.
- The model also showed that a
partial collapse (30 to 40 percent) of the bubble is enough to replicate
the recorded sound, thus agreeing with Kawchuk's images of a bulge in the knuckle long after the
sound had passed.
- Another finding was that an important factor that
seemed to make a difference to the type of sound generated was how much
force is applied to the knuckle.
"The sound that is generated when one cracks his or her knuckles
is due to the partial collapse of a cavitation bubble that's in the fluid in
the joint," explained Abdul Barakat, a professor at the Ecole
Polytechnique. "It could be multiple bubbles, but we showed that the
collapse of a single bubble is sufficient to give you the signature sound you get".
of the study
The researchers did not study what happens as the bubble
forms — the model merely assumes its existence.
The plausible next step would be to see whether the sound is
created during the formation of the bubble in a similar model as well. This
model would study how both bubble formation and bubble collapse might
contribute to the sound.
Knuckle-cracking is neither beneficial or harmful and does not cause
arthritis, contrary to popular belief. Understanding knuckle-cracking can provide some insight to physiologists about how healthy joints move.
- V. Chandran Suja et al. "A Mathematical Model for the Sounds Produced by Knuckle Cracking",
Scientific Reports (2018). DOI: 10.1038/s41598-018-22664-4.