Cracking your knuckles does not cause arthritis

What actually makes the sound

Finger joints are sealed capsules filled with synovial fluid. When you pull or bend a knuckle past its normal range, the capsule expands, pressure inside falls, and dissolved gas comes out of solution as a bubble. In 2015 a team at the University of Alberta filmed the process inside an MRI scanner and showed the pop coincides with the sudden formation of a cavity in the fluid, not its collapse ³. The joint then needs roughly 20 minutes to re-absorb the gas, which is why you cannot crack the same knuckle twice in a row ³⁴.

The 60-year experiment

The most famous piece of evidence is also the simplest. Donald Unger, a physician in California, cracked the knuckles of his left hand at least twice a day for more than 60 years and left his right hand alone. In a 1998 letter to the journal Arthritis & Rheumatism he reported no arthritis in either hand and no difference between them ². It is a sample size of one, but it is a remarkably clean one, and it earned him an Ig Nobel Prize in 2009 ⁴.

What the larger studies found

• A 1990 study of 300 people aged 45 and over found no more arthritis among knuckle crackers than among non-crackers, though it did report more hand swelling and lower grip strength in the cracking group ¹⁴.
• A 2011 study of 215 people, which compared X-rays of the hands with self-reported cracking habits, found no association between cracking and osteoarthritis in any joint, however long or often people had done it ¹⁴.
• Reviews by hand surgeons and rheumatologists since then have reached the same conclusion: the habit is harmless to the joint itself ¹.

The grip-strength and swelling findings from 1990 have not been replicated and are generally treated as a curiosity rather than a warning ¹.

Why the myth persists

The pop sounds like something breaking, and osteoarthritis is common in the hands of older adults, so the two are easy to connect in the mind. Parents also have an obvious motive to discourage a noise that annoys them. But arthritis in the fingers is driven mostly by age, genetics, previous injury and occupation, not by the cavitation of joint fluid ¹⁴.