...osteoporosis? Or is that simply a rumor?

my brother is a physical therapist and he says that the sound from cracking knuckles or bones is caused on one of two things. either you have tendons that 'snap' as they pass over the bone or you are forcing air or fluids out of your joint. neither of these leads to arthritis or ostepporosis, although the snapping tendons can become painful (but only in certain people and rarely).

There is no evidence that cracking your knuckles inflames the joints and leads to arthritis. The cracking causes the bones in your knuckles to pull slightly apart, forming a gas bubble and breaking the adhesive seal in the joint. Crack!