sBMJ | You should know you're a medic - Do people who crack their knuckles get arthritis?

You should know you're a medic - Do people who crack their knuckles get arthritis?

Is it anecdotal or is it true? Edward Wild delves into the issue

"My gran used to put the milk in the cup before the tea, and over a period of 40 years all her teeth fell out." This tragic case, based on a real grandmother, highlights the difficulty we encounter when anecdotal evidence becomes established as folklore. These beliefs are often deeply held and immutable, especially when they come from vaguely plausible foundations. One such conviction, that has frequently caused arguments among my colleagues, is the notion that habitual "cracking" of the knuckles causes arthritis in later life.

Some definitions

Let us, in true medical student style, begin with some definitions. By "knuckles," we mean of course the metacarpophalangeal and interphalangeal joints. Some disturbing individuals may boast more extensive repertoires of clickable joints, including the toes, knees, and spine, but the principle remains the same.

The "arthritis" that crackers allegedly get must surely be osteoarthritis, a degenerative disease of cartilage with a well established "wear and tear" causation. This link between chronic abnormal joint stress ("microtrauma") and osteoarthritis provides the basis for the belief that joint cracking causes premature degeneration. The condition can be extremely debilitating. The matter, though apparently trivial, could therefore be an important one, since up to 90% of 65 year olds show some evidence of osteoarthritis.¹ Establishing a link between something as simple as knuckle cracking and osteoarthritis could dramatically reduce the future incidence of the disease.

So what is it?

So what is knuckle cracking? The mechanism by which clicking noises can be produced by extreme pulling, twisting, flexion, or extension of joints is well established. When a joint is deformed in this way, the pressure in the joint space decreased, and a CO₂ filled cavity forms in the synovial fluid. The pressure in the cavity is lower than that in the surrounding fluid, so the fluid quickly rushes into the cavity. This sudden implosion of the cavity is thought to be what causes the distasteful cracking sound. Interestingly, tiny bubbles of CO₂ remain in the synovial fluid, taking about 15 minutes to be reabsorbed. This explains why a knuckle cannot be recracked immediately.²

But is the repeated implosion of tiny CO₂ bubbles enough to cause microtrauma of the kind linked to osteoarthritis? The energy of the cracking sound has been estimated to be 0.07 mJ/mm,³ while the energy required to cause cartilage damage by chondrocyte death is probably around 1.0 mJ/mm.³ This biomechanical analysis suggests that cracking would not be sufficient to cause damage. Some sources, however, argue that the damage may be cumulative, so that even if a single crack does not cause microtrauma, a lifetime of knuckle cracking might. To complicate things further, some renegade rheumatologists claim that the crack comes not from gas bubble implosion, but the snapping of the deformed joint capsule on to the underlying fluid. If this is indeed the case the energy might well be enough to cause microtrauma on each occasion.³

What about epidemiology?

So much for bioengineering. What about epidemiology? There have been several case reports of voracious knuckle crackers who went on to develop osteoarthritis or degenerative radiological changes.⁴ However, individual reports like this are about as useful as the toothless granny story--they do not prove causation.

There have been only two studies involving larger groups of patients. One study of 28 people in a nursing home failed to reveal a link between "habitual knuckle cracking" and osteoarthritis.⁵ In another study, 300 patients attending outpatient appointments were asked whether they were "habitual knuckle crackers." The prevalence of osteoarthritis in the group that answered yes was no higher than in the control group. The cracking group did, however, contain a significantly increased number of people with "decreased hand function."⁶ The authors noted that while some people can crack their knuckles with ease, others cannot. They went on to suggest that individual variations in joint laxity might be the cause of this difference, so that people with floppy joints can crack their knuckles. Such people may subsequently end up with reduced hand function. But this still doe not answer the question of whether people who can crack their knuckles, but choose not to, are safe from possible damage.

Unfortunately, this rather vague position is about as far as science gets us. We know what happens inside knuckles when we crack them--probably. If we believe this, we can calculate that, in theory, the energy from cracking is not enough to kill cartilage in the joint. And studies of large groups of people apparently show that people with osteoarthritis in their knuckles are no more likely to have cracked their knuckles earlier in life. So the evidence seems to suggest that people can crack away without fear of the disease.

The possibilities

However, several worrying possibilities remain to trouble the world's knuckle crackers. What if our mechanism is inaccurate, and the synovial membrane is involved in the crack? In that case, the energy applied directly to the membrane might be enough to cause microtrauma and inflammation. Or what if the ability to crack your knuckles is related to congenital joint laxity? In that case, does cracking make joint degeneration worse? Or are you already predestined to suffer declining hand function even if you do not crack?

As usual--and this is perhaps a valuable message for all doctors to be--we must be satisfied with incomplete answers and a balance of probabilities. From this, we must try to give appropriate advice to the person in front of us demanding a concrete answer. That answer might be along the lines of "You probably won't get osteoarthritis, but if you want to be certain, perhaps you should consider cutting down." Then again, many people find knuckle cracking as unbearable as the sound of fingernails across a blackboard--so perhaps we should forget the evidence and discourage it completely.

I would like to thank Dr Nicholas Sheehan, consultant rheumatologist, Edith Cavell Hospital, Peterborough, for his helpful comments.

Edward Wild, fifth year medical student, University of Cambridge
Email: email

studentBMJ 2001;09:443-486 December ISSN 0966-6494

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