Calif. ó Snap, crackle, pop. If youíre a knuckle cracker,
that familiar sound when you consciously pop your joints is
like comfort food. You know it might not be so healthy for
your hands or ankles, but it feels oh-so-good.
G. Cook knows that feeling. The 55-year-old Sacramentan said
heís been popping his knuckles daily for decades. Typically,
itís when he first sits down to work at his computer
keyboard. "Itís like a concert pianist or baseball
player warming up," he said. "Itís a ritual."
many habitual knuckle crackers, heís always been told that
itís bad for his joints, leading to arthritis or enlarged
why Cook jumped at the chance to be one of 40 volunteers in a
recent study by UC Davis radiology professor Dr. Robert Boutin
and orthopedic surgery professor Dr. Robert Szabo, who also
see patients clinically. The pair wanted to resolve two
persistent questions about knuckle cracking: What causes that
popping sound, and is it bad for your joints?
do come in all the time and want to know if knuckle cracking
is bad (for their joints)," Boutin said. "Itís a
real-world question that a lot of patients ask."
popping knuckles is arguably the most common kind of joint
cracking, it can also occur in the ankles, knees, back or
neck. Some say itís a way to release tension or limber up.
For others, itís simply a habit.
said the studyís inspiration came from an unexpected source:
his 11-year-old daughter, who noticed classmates at school ó
mainly boys ó loudly cracking their knuckles. Her curiosity
prompted Boutin to craft the study.
participants included 30 with a history of habitual knuckle
cracking and 10 without. Some said they had never
intentionally cracked their knuckles; others were habitual,
cracking them up to 20 times a day for the past 40 years.
in age from 18 to 63, the volunteers were invited to sit and
methodically crack their knuckles. Techniques varied: Some
pulled their fingers, others flexed or bent them back.
determine what causes the crackle íní pop, a tiny
ultrasound device was hovered over their joints, capturing the
sound effects of knuckles being cracked. More than 400
ultrasound images were taken. The results were startling.
looked like a tiny Fourth of July explosion inside the
hand," said Boutin, who presented his study last month at
the Radiological Society of North America in Chicago.
flash, he explained, is caused by dissolved gas that sits in
joint fluid. When you pull or bend a joint, it creates
negative pressure, which releases the gas, forming tiny
micro-bubbles. When released quickly (i.e. in knuckle
cracking), the escaping gas causes a bright flash that shows
up in imaging. "It has a distinctive appearance on an
ultrasound. Every single case, we heard the crack before we
saw the physical flash."
Davis findings appear to contradict traditional explanations
that the knuckle cracking sound is caused when the gas bubbles
burst, like a balloon being popped.
April, a study by Canadian researchers using MRI imaging came
to a similar conclusion: The knuckle cracking sound is created
by the bubbleís formation itself.
that joint fluid pressure builds up again, it can take up to
20 minutes before someone can re-crack their knuckle.
email, study co-author Szabo, chief of the UC Davis hand,
upper extremity and microvascular surgery department, said,
"What are gratifying about our results are the questions
raised about bubbles in human systems, particularly in joints.
We were able to actually visualize the bubbles and correlate
them with the sounds of cracking, supporting the theory of
cavitation," or the formation of a void or bubble inside
liquid. The formation of bubbles, he said, "is somewhat
mysterious and still not full understood, but we got to see it
in action in the joints of live people. Thatís
second part of the UC Davis study was to assess the potential
harm from knuckle cracking. Each participant was tested Ė
before and after each ultrasound Ė by two hand/wrist
orthopedic surgeons who checked for range of motion, grip
strength and laxity (overextension of ligaments). The surgeons
examined the hands without knowing who was or wasnít a
knuckle cracker, and were not told which joints had
successfully been cracked.
physical examinations by the hand-injury specialists found no
problems in the joints of knuckle crackers. "We did not
find any swelling or adverse results like decreased grip
strength," said Boutin.
conclusion: Thereís no short-term harm in knuckle cracking.
And there might even be a benefit: After a joint was cracked,
it showed a "significantly increased range of
motion" compared to joints that did not crack, Boutin
Davis study appears to contradict a 1990 study that suggested
knuckle cracking can cause joint swelling and weaken the grip
and "should be discouraged."
the prevalence of knuckle crackers Ė itís estimated that
25 percent or more people do so Ė scholars have paid
attention to the topic in recent years. Most studies have
debunked the warnings that knuckle cracking causes arthritis
1975, a study in the then-Western Journal of Medicine
comparing 28 seniors and 28 schoolchildren found there was no
evidence "that knuckle cracking leads to degenerative
changes in the (hand) joints in old age." But, the study
noted wryly, "The chief morbid consequence of knuckle
cracking would appear to be its annoying effect on the
more humorous vein, Dr. Donald Unger, a California allergist
and longtime knuckle cracker in Thousand Oaks, was lauded in
2009 for his own self-styled research. For more than 60 years,
Unger had habitually cracked the knuckles of his left hand,
but not his right. Using his own hands as a test case, he
compared them for arthritis and found no difference. That
"research," originally published in 1998 as a letter
in the Arthritis & Rheumatism journal, earned him the Ig
Nobel prize for medicine, a Harvard University-based parody of
the Nobel Prize given to "improbable" or humorous
step for the UC Davis researchers is to look at long-term
effects of joint cracking on other areas besides hands. Boutin
is in the midst of analyzing results from a global
questionnaire of 1,800 individuals to determine if there are
age, cultural or geographic differences among knuckle
all the research, Boutin knows that some things donít change
about knuckle cracking: "Many people are really quite
fond of knuckle cracking and find it hard to image life
without its existence," he said. "For other people,
itís like fingernails on a blackboard."