Well, aside from being slapped in the face for annoying people with the noise, no, there are no studies that show long-term side-effects from knuckle cracking.
There have been several long-term, well-designed cohort studies performed on this, and none of them show a statistically significant increase or decrease in any negative physiological processes or diseases. Joints crack naturally, and cracking them on your own does not make a difference in their structure.
This myth probably arose due to the high incidence of osteoarthritis (“normal” arthritis - that which is not from rheumatism or infection) among the elderly, many of whom either had/have a habit of knuckle cracking at some point in their life. It’s a common habit that many people have at some point or another, even if it’s not a lifelong thing.
A humorous paper that took this question to the extreme was the winner of a 2009 Ig Nobel prize: Donald Unger was chided by his mother, mother-in-law, and teachers for cracking his knuckles, so for 50 years he only cracked the knuckles on one hand and not the other, and found that he developed no arthritis on either side, and his flexibility was roughly the same on both sides.
Of course, a single-patient experiment is necessarily unblinded and biased, but it does follow the same line as the results of prior experiments.
I will say you don’t want to crack Knuckles, though. That mofo is crazy as is.
What you will need:
The first documentation of gout was in Egypt, in a description of an arthritic big toe with unique symptoms, around 2,600 BC.
Hippocrates knew of gout, and noted its absence in eunuchs and pre-menopausal women.
The realization that the urate crystals found in the urine and joints of gouty patients (first noted by Leeuwenhoek in 1679) was directly related to the symptoms caused wasn’t reached until Alfred Baring Garrod proposed the mechanism of pain in 1848.
Humans and other great apes lost the ability to produce uricase, which is part of the reason that we develop gout, and most other animals don’t…though if the ability to produce that enzyme is interfered with, gout CAN occur. Sue the Tyrannosaurus rex is thought to have been gouty, after all. It’s not known if some larger reptiles did not have the ability to produce uricase in general, or if Sue simply had a genetic defect.