hey.i was actually trying to find out the answer to your question recently.its how i found this site.i was linked to a site which went into detail on the subject.basically i found out that no tests have conclusivly proven it causes arthritis although they have noticed that it does lessen the grip of the hand.
There is no evidence that cracking one's knuckles can cause arthritis directly. However, repeated injury of a joint or repeatedly causing it to swell can injure the cartilage and potentially lead to degenerative joint disease.
Cracking your knuckles forces the joint fluid from one part of the joint to another, and the popping sound is just the result of the high pressured rushing of synovial fluid. I have arthritis in my hands and most of my joints.Have never cracked my knuckles either !
It's a bit of an urban myth.
I used to crack my knuckles all the time when I was a teenager, and was always being told it would cause arthritis. When i did get arthritis at 23 I was convinced it was all my own fault. But then i realised that I also have it in my wrists, shoulder and knee, which I (obviously) have never cracked so now I think it's rubbish.
The theory is that every time you crack your knuckles you lose synovial fluid which is your lubricant. Where this fluid goes is a mystery to me, I don't think there is any link to be honest