From an old issue of Arthritis and Rheumatism:

Does knuckle cracking lead to arthritis of the fingers?
To the Editor:
During the author’s childhood, various renowned authorities (his mother, several aunts, and, later, his mother-in-law [personal communication]) 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.
Knuckle cracking did not lead to arthritis after a 50-year controlled study by the one participant. 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. A search of the literature revealed only one previous paper on this subject, and the authors came to the same conclusion (Swezey RL. Swezey SE. The consequences of habitual knuckle cracking. West J Med 1973;122:377-9.). This result calls into question whether other parental beliefs, e.g., the importance of eating spinach, are also flawed. Further investigation is likely warranted. In conclusion, there is no apparent relationship between knuckle cracking and the subsequent development of arthritis of the fingers.

This study was done entirely ut the author’s expense, with
no grants from any governmental or pharmaceutical source.

Donald L. Unger, MD
Thousand Oaks, CA

[The response is from the fellow whose study Dr. Unger cited in his letter. -Ed]

Reply
To the Editor:
I appreciate the opportunity to review Dr. Unger’s report. His “self-controlled” study adds considerable credence to our 1973 study findings. Dr. Unger exercised amazing self control by performing 50 years of knuckle cracking (KC) on his left hand at least twice daily, “while those on the right cracked only rarely and spontaneously.’’ No evidence of arthritis in either hand was found at the end of 50 years. I have taken the liberty of consulting Dr. John Adams, PhD, at the Rand Corporation. who has generously provided
me with the following statirtical [sic] analysis.

The basic study designed by Dr. Unger is a two-arm trial without randomization. Although it is not clear, it appears that the study was not blinded. Blinding would only be possible if the investigator didn’t know left from right. This is not likely since studies indicate that only 31% of primary care physicians don’t know left from right. (The figure is reportedly somewhat higher for most specialists.) The lack of randomization suggests the need for a multivariate analysis to reduce bias. Controlling for knuckle-to-knuckle variation in race, sex, socioeconomic status, initial severity, comorbidities, and Ecuadorian barometric pressure at the time of measurement would be advisable. The sample size appears too small to support accurate inference. Typically, sample sizes of roughly twice the available research budget are required for valid inference. Restrictive eligibility criteria and convenience sampling limit generalization of the results to knuckle-cracking physicians with a lot of time on their hands.

1 should note that SES, the co-author of our 1973 investigation, was 12 years old at the time of the study and that the study was stimulated because of his grandmother’s concern about the arthritic consequences of his KC. It is now 22 years later and he continues to enjoy frequent KC without manifestations or evidence of arthritis. Closer scrutiny of the data in both studies raises the question of a possible osteoarthritis preventative therapeutic benefit from the exercise effect on joint lubrication resulting from habitual KC. Clearly, further study should be undertaken, with the caveats as given by Dr. Adams.

The possible utilization of KC by managed care providers as an economic, noninvasive, home preventative treatment for arthritis of the hands should be given further consideration. A clear distinction between hand wringing related to managed care procedures and therapeutic KC will have to be made.
Robert L. Swezey, MD
Santa Monica, CA

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