So after discussing arthritis in class I became interested in finding the truth about what Dr. Hens said about knuckle cracking leading to arthritis due to cartilage damage in the joints. I occasionally crack my knuckles, particularly after writing a lot of notes in a class or after playing a lot of tennis (both of which happen more frequently than would be expected), so I wanted to find out if I was doing my body harm. Based on my research the answer is... yes and no. It has been found that knuckle cracking does not lead to arthritis, however, it has been shown to be related to developed hand weakness. So what happens when we crack our knuckles? Joints are filled with what is called synovial fluid that acts as a lubricant in the joints, within this fluid are dissolved gases (oxygen, nitrogen, etc.), so when we crack our knuckles we create a bubble of these gases that then "pops". Once popped, it takes time for the gases to be re-dissolved in the synovial fluid, which is why you cannot crack your knuckles too frequently. But all is not lost! You can strengthen joints by exercise because building mucle will build the cartilage and tendons that are associated with them, so if you are a knuckle cracker it would be wise to also be a frequent exerciser. The link below is a short primary research article discussing knuckle cracking and arthritis... enjoy.
http://osteoarthritis.about.com/gi/o.htm?zi=1/XJ&zTi=1&sdn=osteoarthritis&cdn=health&tm=30&f=20&tt=12&bt=1&bts=1&zu=http%3A//www.pubmedcentral.nih.gov/articlerender.fcgi%3Fartid%3D1004074