The pisiform bone is located in the wrist on the side of the little finger. It has one joint that moves against the slightly larger triquetrum bone. Degenerative arthritis of this joint is really not uncommon because it gets a lot of use.
When the wrist bends, the pisiform flexes. When the wrist extends, the pisiform also extends and moves slightly toward the thumb side. With normal motion, multiple forces act on the pisiform-triquetrum joint. Forceful or repetitive use of this joint is the main reason for arthritic changes.
Acute injury from trauma with tendon rupture is the second most common reason for arthritis in this joint. Tight ligaments and tight fibrous bands of tissue across the pisiform can also create compression and eventual problems.