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. Turn your nervous energy into a less harmful habit that occupies your hands, such as doodling. Most importantly, get to the bottom of what's causing your nervousness in the first place, you may crack your knuckles more often at work than at home, for example, and address those sources directly.