From medicaldaily.com
As a boy, Donald Unger was warned by his mother, several aunts, and later his mother-in-law that cracking his knuckles would give him arthritis. So he ran a test that lasted half a century. He cracked the knuckles of his left hand at least twice a day and left his right hand alone as a control.
By his own count, the left-hand knuckles were cracked at least 36,500 times. At the end, he found no arthritis in either hand and no apparent difference between them. The California physician reported the result in a 1998 letter to the journal Arthritis & Rheumatism.
It remains one of the most famous single-person experiments in medicine, and the larger studies that followed have largely backed him up.
A Control Hand and a Lifetime of Pops
Unger, an allergist in Thousand Oaks, California, wrote that the knuckles on his right hand cracked only rarely and on their own. He described his one-man study as a preliminary investigation and wrote that it suggested no link between knuckle cracking and arthritis in the fingers.
The work earned him the 2009 Ig Nobel Prize in Medicine, the satirical award for research that makes people laugh and then think. By then, the prize citation noted, he had kept up the habit for more than 60 years. "I'm looking at my fingers, and there is not the slightest sign of arthritis in either hand," Unger said, according to China Daily's coverage of the ceremony.
Unger stressed that he had used no public funds. He had published scores of research papers as an allergist, yet none brought him the attention his knuckles did. He received the prize on Oct. 1, 2009, at Harvard's Sanders Theatre, where real Nobel laureates hand out the awards.
Larger Studies Reached the Same Answer
A study of one man, who was also the experimenter and the person judging the results, cannot settle a medical question. It was not randomized or blinded, and one person's genes and history may not apply to anyone else.
Bigger studies have since tested the idea. A 1990 study in Annals of the Rheumatic Diseases evaluated 300 consecutive patients age 45 and older, 74 of whom were habitual knuckle crackers. The crackers were no more likely to have hand arthritis. They were, however, more likely to have hand swelling and lower grip strength.
In 2011, Kevin deWeber and colleagues published a case-control study in the Journal of the American Board of Family Medicine. They surveyed 215 people ages 50 to 89 who had right-hand X-rays taken between 2003 and 2008. About 20% cracked their knuckles. Hand osteoarthritis was found in 18.1% of crackers and 21.5% of non-crackers, a difference that was not statistically significant.
Neither how many years people had cracked their knuckles nor how much they had done it in total was linked to arthritis in the joints they cracked. The researchers concluded that habitual knuckle cracking does not appear to be a risk factor for hand osteoarthritis. They listed recognized risk factors as prior joint injury, a family history of hand osteoarthritis, and heavy manual labour involving the hands.
The Pop Itself Took Decades to Explain
Scientists argued for decades about where the sound comes from. A 1947 X-ray study suggested the noise occurred when a gas-filled cavity suddenly formed as the joint surfaces separated. A 1971 study instead blamed a bubble collapsing.
In 2015, a team led by Gregory Kawchuk of the University of Alberta used real-time MRI to watch a volunteer's finger joints as each was pulled until it cracked. Their study in PLOS ONE found that each crack happened faster than a single 310-millisecond imaging frame. Every crack coincided with a cavity forming in the synovial fluid that lubricates the joint, supporting the original 1947 explanation.
The cavity did not collapse while the finger was still being pulled. The researchers said their findings offer a new framework for studying whether joint cracking has any health effects.
Not Arthritis, but Not Entirely Without Effects
The research so far points one way on arthritis, but the evidence has limits. Most studies are observational and rely on people remembering their own habits. No one has randomly assigned people to crack or not crack their knuckles for decades, the kind of trial that would be needed to settle the question.
The 1990 grip-strength finding has also not been consistently reproduced. A later ultrasound study from UC Davis found no immediate difference in grip strength after cracking, though it looked only at short-term effects. Separately, doctors have reported rare acute injuries, including a thumb ligament sprain, from forceful attempts to crack a joint.
For most people, the takeaway is reassuring. The habit may irritate the people nearby, but it has not been shown to cause arthritis. Anyone whose knuckle cracking causes pain, swelling, or reduced hand function should have it checked by a clinician, since those symptoms can signal other joint problems.
Key Questions Answered
What did Donald Unger do?
He cracked the knuckles of his left hand at least twice daily for about 50 years while leaving his right hand alone, then compared them.
What did he find?
Neither hand had arthritis, and he saw no apparent difference between them.
Does knuckle cracking cause arthritis?
Larger studies, including a 215-person case-control study, have not found a link between habitual knuckle cracking and hand osteoarthritis.
What makes the cracking sound?
A 2015 MRI study showed the sound coincides with a cavity suddenly forming in the joint fluid.
Is the habit completely harmless?
Not necessarily. A 1990 study linked it to hand swelling and weaker grip, and rare acute injuries have been reported.
https://www.medicaldaily.com/doctor-cracked-one-hand-knuckles-50-years-arthritis-480656
