From: Ian A. York
Subject: Re: Cracking Knuckles
Date: Feb 20 1996
Newsgroups: sci.med

In article <4g87eo$lbu@cnct.com>,  <ethan@cnct.com> 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] 

Ian

-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
  Knuckle cracking: key to a better life, or the road to a living hell? 

                                             An investigative report by
                                                               Ian York

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
clear. 

Two papers ([4] and [5]) were essentially case reports, or were too small to
be reliable.  One [2] was a larger study, one [1] was a review with some 
original data, and the other one [3] was a letter commenting on [2].  
The first two can be pretty much dismissed; [4] 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.

[5] 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
knuckles?"  instead.)

Now we get to the interesting papers.  [2] 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
problems. 

However,  the followup letter [3] 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 
remains open.  

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 [1].  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-------

[1] 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).  [ ... ]


[2]  Castellanos J.  Axelrod D.
 Effect of habitual knuckle cracking on hand function.
 Annals of the Rheumatic Diseases.  49(5):308-9, 1990

Discussion
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[1].  Indeed, metacarpophalangeal
osteophytes were found in patients who had not been habitual knuckle
crackers. 
  Yet a bioengineering study of cracking joints suggested the potential
for significant joint damage.[2] 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.[3]
  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. 


[3] 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.  


[4]  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
osteoarthritis. 
  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. 


[5]  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 ]

Conclusion
  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.  


[6]  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   (iayork@panix.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