Dr. Donald Unger was awarded the 2009 Ig Nobel Prize in medicine for his more than 60-year-long personal investigation of knuckle-cracking. Here’s an earlier, though much more time-compressed, study by other scientists:
“The Consequences of Habitual Knuckle Cracking,” Robert L. Swezey and Stuart E. Swezey, Western Journal of Medicine, vol. 122, no. 5, May 1975, pp. 377–9. [AIR 15:6] The authors, at the University of Southern California School of Medicine, Los Angeles, report:
“Habitual knuckle cracking in children has been considered a cause of arthritis. A survey of a geriatric patient population with a history of knuckle cracking failed to show a correlation between knuckle cracking and degenerative changes of the metacarpal phalangeal joints.”