RE: knuckle cracking and arthritis
From what I have read there is no definitive research on this subject. To say there is or is not a correlation is a myth. Some studies have been done but are flawed in that they did not take occupations of participants into account. There was also some question of what constituted habitual knuckle cracking.
If you are really curious you could Google some information on the subject. If you just want to talk about knuckle cracking in an RC forum than you're just really bored and out of RC related topics to discuss, like me.