From: Ian A. York
Subject: Re: Cracking Knuckles
Date: Feb 20 1996
In article <email@example.com>, <firstname.lastname@example.org> wrote:
>What are the side effects of cracking your knuckles?
>I am 21 and have been doing it since I was a kid. I sometimes feel the
>need to do it maybe as a habit or because to relieve tension in my fingers.
>I have heard about arthritis and those things but are those myths?
I looked into this a bit recently, and this is what I posted a couple of
months ago on a different newsgroup ... [posted and mailed]
Knuckle cracking: key to a better life, or the road to a living hell?
An investigative report by
The question is usually, "Will cracking my knuckles cause arthritis?" and
there are probably two myths given as the answer. One is "yes", and the
other is "no."
Oddly enough, medical science has not devoted great attention to this
vital question. A while ago I posted some abstracts about it; more
recently I read some 83% of the world's literature on this subject (the
sixth paper was torn out of its binding) and the answers are far from
Two papers ( and ) were essentially case reports, or were too small to
be reliable. One  was a larger study, one  was a review with some
original data, and the other one  was a letter commenting on .
The first two can be pretty much dismissed;  simply found a guy with
arthritic hands who cracked his knuckles (and had no obvious predisposing
signs for arthritis) and concluded the knuckle-cracking was causative;
this is a meaningless correlation.
 was slightly better; they surveyed elderly people for arthritis of the
hands and then asked which of them were 'habitual knuckle-crackers'.
However, the numbers were too small to give much information; I also have
serious questions about their method of identifying knuckle-crackers.
(They found that 11 of the 23 women they asked self-identified themselves
as knuckle-crackers. Perhaps I've been leading a sheltered life, but this
sounds awfully high to me. Does anyone out there find that half of the
little old ladies they meet habitually crack their knuckles? Perhaps
their subjects took the question to mean 'Have you ever cracked your
Now we get to the interesting papers.  looked at a larger number of
people (300 people, of whom 74 admitted to being habitual
knuckle-crackers - a number that sounds closer to my expectations than
the previous one) and studied a variety of hand functions. Most were
normal; in particular, there was no increased incidence of arthritis.
However, the KC group was more prone to swelling of the hand, and also
had reduced grip strength.
When I posted the abstract to this paper, someone on the group pointed out
that the KC group were also found to be more likely to be manual
labourers, and noted that they therefore might have been more prone to
hand trauma, which (rather than the KC itself) could have led to the hand
problems. This was a fair point that wasn't addressed by the abstract,
although I assumed the study would have considered this rather obvious
potential confounding factor; this is why I checked the full paper. The
authors do indeed consider it, and conclude that it seems unlikely, since
the KC group was no more prone to trauma or hand surgery, or to other hand
However, the followup letter  makes another point - which is that the
correlation doesn't demonstrate cause. The writer suggests that, for
example, people with unusually loose joints may be more prone both to
crack their knuckles and to joint damage because of the loose joints.
This is fair too - although pretty speculative - and so the question
One thing we can safely conclude, though, is that KC does not cause
arthritis. Even in the relatively large study, the KC group was not
found to be more prone to arthritis. So technically, the "no" answer is
correct. However, since I doubt that most people draw a sharp
distinction between developing osteoarthritis of the metacaarpophalangeal
joints and chronic inflammatory damage to the ligamentous capsule of the
metacarpophalangeal joint (both hurt) I don't think a "no" answer here
really addresses the question.
This brings us to the last study . Here Brodeur considers some
potential mechanisms by which KC could induce damage. They point out
that the mechanism responsible for the noise - cavitation - is the same
principle that causes damage to plane and ship propellers. However, a
caculation of the energy released by a single crack puts it at 1/10 the
level necessary to cause damage. If we accept this - and it seems
reasonable - then the question becomes whether chronic low-level assault
can cause damage in the long term. There's no evidence either way for
this, and it's as easy to come up with rationales on both sides.
There's another mechanism that Brodeur mentions more in passing, that I
think is at least as plausible as cavitation. During the KC process, the
ligaments around the joint are rapidly stretched. Could this lead to
chronic damage? It seems to me that it certainly could. A similar effect
strikes, for example, pitchers; the trauma involved in pitching a single
game may not cause any particular damage, but as we all (where "we all"
equals North American baseball fans) know pitchers' joints and ligaments
are more prone to damage over the long term than are the average
persons'. The stress involved in cracking a knuckle may be less than
that involved in throwing a baseball 97 miles per hour, but on the other
hand (Ha!) the ligaments involved are less robust, and while a pitcher
may deliver some 100-odd insults to the ligaments every five days, I
imagine a confirmed knuckle cracker is well above that.
So what can we conclude? There's weak evidence for some damage to the
joints; there's a reasonable, but speculative, model for the mechanism.
There is hard evidence that the effect is not inevitable; not all members
of the KC groups showed damage. I would conclude, personally, that it's
quite likely that cracking your knuckles could lead to some unpleasantness
down the road; and since there's little reason to continue an annoying and
asocial habit, I'd suggest quitting. But since your loved ones have
probably already suggested this many a time, it's unlikely you'll take my
word for it.
If I were writing the FAQ, I'd say something like this:
F: Cracking your knuckles causes arthritis.
Tb/U: It may damage your hand in other ways.
----here are selected bits of the references in question-------
 Brodeur R.
The audible release associated with joint manipulation.
Journal of Manipulative & Physiological Therapeutics. 18(3):155-64, 1995
Damage Due to Habitual Joint Cracking
There has been very little work done to determine the long-term effects
of habitual joint cracking. The scant literature that is available on
this topic indicates that habitual knuckle cracking may have a direct
effect on the soft tissue of the hands and there is a case study that
indicates it may also cause damage that leads to radiologic change (17).
However, there is insufficient evidence to make any conclusive statement
regarding the long-term effects of habitual knuckle cracking.
[review of the Swezey and Swezey report: no significant difference.]
[review of the Catellanos and Axelrod report.] The knuckle crackers had
approximately 75% less grip strength and a higher incidence of hand
swelling. Because the average duration of the knuckle cracking habit was
35 +/- 18 years, the habit seems to have little effect on joint cartilage.
The increase in joint swelling and the decrease in hand function indicates
that habitual knuckle cracking has a greater effect on the soft tissue
than on the bone or cartilage. However, damage to the cartilage cannot be
ruled out. Watson et al [review of Watson et al.]
[ ... ]
Joint Damage from Habitual Joint Cracking
[ ... ]
Watson et al estimated the energy released by the cracking sound to be
0.07 mJ/mm^3 (15). Cartilage requires impact energies on the order of
1.0 mJ/mm^3 to cause damage to the articular structure and chondrocyte
death (26). The effect of a single joint crack is less than 10% of this
value and the energy released would be released in terms of damage to
cartilage. Watson et al. argue that the effects of habitual joint
cracking may be additive: the energy released during caviatation may
be, over a period of time, sufficient to damage the articular cartilage
(5,17,23). Although Watson et al. proposed this thery as a cause of
direct damage to the joint cartilage, there is little clinical evidence
to support this mechanism. [ ... other possible damage mechanisms ...]
The evidence of tissue swelling with habitual joint cracking indicates
that the only area of injury would be at the proposed snap-back interfce
between the capsular ligament and the synovial fluid. Microtrauma may
occur at the portion of the ligament involved in the snap-back and
excessive joint cracking may ultimatly lead to swelling of the ligament.
This mechanism is completely speculative, but at least it has the support
of clinical evidence (19). [ ... ]
 Castellanos J. Axelrod D.
Effect of habitual knuckle cracking on hand function.
Annals of the Rheumatic Diseases. 49(5):308-9, 1990
The development of arthritis of the hand as a result of habitual
knuckle-cracking has been considered an old wives' tale. Swezey reviewed
28 nursing home patients who could recall whether or not they had cracked
their knuckles. Among these patients, a relationship between knuckle
cracking and arthritis could not be found. Indeed, metacarpophalangeal
osteophytes were found in patients who had not been habitual knuckle
Yet a bioengineering study of cracking joints suggested the potential
for significant joint damage. When tension is applied to the joint,
ccavitation occurs within the synovial fluid. This creates an unstable
condition as the pressure within the bubble is ower than that of the
surrounding fluid. Bcause the joint separation occurs at a high rate the
net flow of synovial fluid is toward the low pressure regions, with a
collapse of the vapour phase of the cavity. There is a release of
vibratory energy, which may be responsible for the cracking sound.
It is this phenomenon which is responsible for the erosion of ship
propellers and the blades of hydraulic turbines.
Given the potential damage caused by this caviation phenomenon, one
might expect habitual knuckle cracking to cause some decrement in hand
function, if not accelerate the onset of osteoarthritis of the hand.
Of the 300 patients studied, 74 admitted to habitual knuckle cracking
for 35 (18) years. Their sex distribution was similar to that of those
denying knuckle cracking. Those patients who were habitual knuckle
crackers were more likely to have swelling of the hand and lower grip
strength (table 2). Other factors which might influence hand function,
such as carpal tunnel syndrome, contractures, surgery or trauma to the
hand, and the presence of Heberden's or Beouchard's nodes, were equally
present in both patient groups. Habitual knuckle crackers however, were
more ikely to be manual labourers with higher incomes (tables 1 and 2).
Although the cause of habitual knuckle cracking was not considered in
this study, patients admitting to it were more likely to bite their nails,
smoke, and drink alcohol, as were members of their families (table 2).
This study suggests that although habitual knuckle cracking does not
relate to osteoarthritis of the hand, it may relate to decreased hand
function. Therefore, habitual knuckle cracking should be discouraged.
 Simkin PA.
Habitual knuckle cracking and hand function.
Annals of the Rheumatic Diseases. 49(11):957, 1990
SIR: In a recent survey Castellanos and Axelrod evaluated 300
consecutive outpatients at Mount Carmel Mercy Hospital to determine
whether habitual knuckle cracking is a risk factor for hand dysfunction.
They found no relation with osteoarthritis, but noted that 'knuckle
crackers were more likely to have hjand swelling and lower grip strength'
and concluded that 'habitual knuckle cracking results in functional hand
impairment. I believe they have not established cause and effect in
these interesting correlations.
Not everyone can crack their kncukles. Some do so with ease, whereas
others are quite incapable of performing this feat. No one has
determined how the joints of these groups differ. It is quite possible,
for instance, that metacarpophalangeal joint laxity may both facilitate
knuckle cracking and impair hand function. As this hypothesis implies
that hand swelling and diminished grip occur secondary to articular
structure rather than abuse, it may be that nervous citizens of Detroit
can continue to crack their knuckles without ferar of injury.
'Will cracking my knuckles hurt my hands?' remains a common gambit when
a rheumatologist is identified as such among new acquaintances striving
to make conversation. I still believe that the answer to this question
is no, but perhaps it is time that we really found out.
 Watson P. Hamilton A. Mollan R.
Habitual joint cracking and radiological damage.
BMJ. 299(6715):1566, 1989
A 25 year old Malaysian man who habitually elicited cracking sounds from
many of his joints was investigated during a study of joint cracking. He
had no symptoms or obvious anrmalities of his joints, but a radiograph of
his right hand showed ligamentous ossification on the ulnar side of his
third metacarpophalangeal joint and chondrocalcinosis in the first and
fourth metacarpophalangeal joints (figure). There was no evidence of
Distraction of the articular surfaces during finger pulling lowers the
pressure of the articular fluid. When the vapour pressure is reached the
fluid evaporates, giving a cracking sound and forming an intra-articular
bubble. Previous studies have reached conflicting conclusions about the
radiological changes found in habitual finger cracking,[1,2], but we
suggest that excessive joint cracking caused the changes seen in this
subject, who had no signs of any other underlying disease.
 Swezey RL. Swezey SE.
The consequences of habitual knuckle cracking.
Western Journal of Medicine. 122(5):377-9, 1975
[ ... ] The patient population consisted of 28 persons, with an average
age of 78.5 years, of whom 23 were women and 5 were men. [...] [ from
Table 2: 11 women and 4 men were habitual knuckle-crackers ]
The data fail to support evidence that knuckle cracking leads to
degenerative changes in the matacarpal phalangeal joints in old age. The
chief morbid consequences of knuckle cracking would appear to be its
annoying effect on the observer.
 Unsworth A. Dowson D. Wright V.
'Cracking joints'. A bioengineering study of cavitation in
the metacarpophalangeal joint.
Annals of the Rheumatic Diseases. 30(4):348-58, 1971
Ian York (email@example.com) <http://www.panix.com/~iayork/>
"-but as he was a York, I am rather inclined to suppose him a
very respectable Man." -Jane Austen, The History of England