Vox explains the physical process that occurs when a person cracks their knuckles and what research has been done to determine if knuckle cracking causes any physical harm to the body, including that of Donald Unger. Unger had, over the course of 60 years, only cracked the knuckles on his left hand, but left his right hand alone to determine what difference, if any, abstaining on one side made.
Consider the findings of Donald Unger, arguably the most dedicated researcher of knuckle cracking. In his teens, after his mother warned him the habit would give him arthritis, he started cracking his knuckles in the name of science, according to a 2009 Los Angeles Times story. He continued to pop the joints in his fingers for over 60 years — but only on his left hand. He left the right hand untouched as a comparison. And his decades-long self experiment found no arthritis in either hand.
via The Presurfer