Mom always told you not to crack your knuckles. For starters, it's rude. Worse, she warned, you'd wind up with giant malformed fingers one day, and maybe arthritis, too.

The problem is, there are so many good reasons to crack your knuckles. It can gross out your big sister. It can make your algebra teacher (and, in later years, your irritating co-worker) wince. If you're really good at it, it may give you a certain status among your peers.

Besides, it feels good. After a 45-minute white-knuckled commute home, lacing those digits together and bending 'em backward for a bone-deep crick-crick-crick-crick can mark a satisfying start to your downtime.

So, did Mom just not appreciate these subtle pleasures? Or was she truly just trying to protect her young from something they'd regret later in life? Let's look at the evidence.

When you "pop open" a can of soda, the loud cracking noise you hear is gas expanding when the air pressure inside the can suddenly drops. The mechanism that makes your knuckles and other joints produce a loud cracking sound is very similar, explains chiropractor Raymond Brodeur, an adjunct faculty member of Michigan State University's Department of Biomechanics.

A joint, of course, is the meeting of two bones. It's tightly encased by tendons (which hold muscles to the bones) and ligaments (which hold the bones to each other). Inside the capsule formed by tendons and ligaments is a substance called synovial fluid, which lubricates the joint.

When you crack a knuckle, you are pulling the two bones of the joint slightly apart, thereby stretching that capsule of connective tissue. Because there's suddenly more room inside the capsule, the pressure of the synovial fluid drops. As the fluid pressure drops, the gases within the fluid (primarily carbon dioxide -- the same gas used to carbonate soft drinks) expand. When the gas expansion reaches a certain point, a bubble pops up, and in doing so makes that very satisfying (or disgusting) craaack. (Experts believe that it's the sudden formation of the bubble that causes the cracking sound, says Brodeur, not the exploding of the bubble. In fact, the bubble actually shows up in post-knuckle-cracking X-rays.)

The released gases keep the joint slightly expanded until they dissolve back into the fluid over the next 15 to 30 minutes. That explains not only why you might feel increased mobility or relief from stiffness for a while after cracking your knuckles, but also why you can't crack them again right away. You need to allow time for shrinkage.

So why can't everyone crack their knuckles? Some people's joints are too tight, says Brodeur, and some people's are too loose. Only those with joints of medium tightness possess cracking ability.

Feeling the Hurt

But back to the question: Can knuckle-cracking really hurt you?

When a knuckle cracks, the resulting vibration submits the joint to impact stress, say researchers. That's the same sort of stress that erodes ship propellers and the blades of hydraulic machinery. (Moms: Feel free to cite this fact in future lectures.)

Jorge Castellanos and David Axelrod of Mount Carmel Mercy Hospital in Detroit reasoned that if those vibrations could damage propellers, they might damage knuckles as well. That hypothesis led to what became medicine's most respected study of knuckle-cracking to date, published in the Annals of the Rheumatic Diseases in November 1990.

Castellanos and Axelrod examined the hands of 300 patients age 45 and older, of whom 74 identified themselves as habitual knuckle-crackers. Interestingly, the proportion of knuckle-crackers was the same (25 percent) among men and women.

The good news for crackers is that there was no significant difference between the crackers' and non-crackers' prevalence of gout, osteoarthritis, carpal tunnel syndrome, or trauma or surgery to the hand.

But . . . you know those swollen hands Mom predicted you'd get? She could be right. Nearly 85 percent of the knuckle-cracking group had swollen hands, versus 6 percent of the non-crackers. The habitual knuckle-crackers also showed an average grip strength of about 25 percent less than that of the non-crackers.

Since enlarged, gnarly knuckles and weak grip in a grandma or grandpa would probably make many of us jump to the conclusion of "arthritis," that might explain the source of the knuckle-cracking/arthritis myth.

It has often been pointed out, though, that correlation doesn't necessarily equal cause and effect. Maybe people prone to swollen or weak hands are more likely to crack their knuckles, rather than vice versa. Alas, medical research has yet to delve into this mystery. (And probably won't anytime soon. There's a serious lack of grant money for knuckle-cracking research.)

If It Hurts, Etc.

Hand surgeon Peter Chan of Morristown, N.J., says he has published numerous serious research papers in medical journals. But the one publication that journalists call him about regularly is the one in which he reports two cases in which common-sense-impaired individuals sprained their fingers knuckle-cracking (American Journal of Orthopedics, Feb. 1999).

While a resident at University of Pennsylvania Medical Center, Chan treated two patients with knuckle-cracking injuries within a matter of weeks. The first, a 25-year-old administrative assistant, had tried to force her thumb to crack, which it apparently didn't want to do. So she yanked, hard. Bad idea. She showed up at the hospital's orthopedic clinic with a painful swollen thumb. She'd torn a ligament, and the thumb had to be splinted for four weeks.

The second patient, a 26-year-old graduate student, told doctors he'd been cracking his right pinky frequently for two to three months. Then one time he wanted to crack it and it wouldn't cooperate. He forced the issue -- and ruptured a tendon. Result: Six weeks in a hand splint.

Surely, two mothers somewhere were shaking their heads.

On the other hand . . . the only published long-term study of the consequences of knuckle-cracking seems to weaken Mom's argument.

Donald Unger of Thousand Oaks, Calif., past president of the American College of Allergy, Asthma and Immunology, proclaims himself one of "the world's foremost experts" on knuckle-cracking. He's certainly one of the best-known research subjects.

In the May 1998 issue of Arthritis & Rheumatism (the official journal of the American College of Rheumatology), Unger wrote that "during the author's childhood, various renowned authorities (his mother, several aunts, and later, his mother-in-law) informed him that cracking his knuckles would lead to arthritis of the fingers. To test the accuracy of this hypothesis, the following study was undertaken.

"For 50 years, the author cracked the knuckles of his left hand at least twice a day, leaving those on the right as a control. Thus, the knuckles on the left were cracked at least 36,500 times, while those on the right cracked rarely and spontaneously. At the end of the 50 years, the hands were compared for the presence of arthritis.

"There was no arthritis in either hand, and no apparent differences between the two hands . . . While a larger group would be necessary to confirm this result, this preliminary investigation suggests a lack of correlation between knuckle cracking and the development of arthritis of the fingers." Five years later, both hands are still perfect, says Unger. No swelling, no weakness, nada.

Experts say no other scientific studies have shown a correlation between knuckle-cracking and arthritis, either.

The bottom line: Cracking your knuckles won't give you arthritis. It might make your hands swollen and weaker or, if you're overly enthusiastic about it, give you a nasty sprain. Or, like Unger's, your fingers might stay just fine. It's a calculated risk -- only you can decide the path that's right for you.

But you already know what Mom would say.

*

Lisa Barrett Mann, a Washington area freelance writer, last wrote for Health about Asperger's syndrome.

How a knuckle cracks: Force is applied (red arrow), stretching the joint's fibrous capsule and creating more room inside the capsule. This results in lower pressure in the joint's synovial fluid, which in turn causes the gasses inside the fluid (mostly carbon dioxide) to expand, forming a bubble. It's the formation of the bubble, not its dissipation, that makes the cracking sound. In fact, intact bubbles show up on x-rays of already cracked knuckles. Synovial FluidBubble in FluidFibrous Capsule