"Cracking joints" and "popping knuckles" are an interesting and poorly understood phenomenon. There are many theories as to why joints crack or pop, but the exact cause is simply not known.
Painless cracking of joints is -- as a rule -- not harmful. However, common sense generally would suggest that the intentional and repetitive cracking of one's joints not only is potentially socially bothersome but also could be physically troublesome when it produces pain.
Osteoarthritis (OA) causes joint pain and stiffness for 27 million Americans. Although there are many medications and treatments to ease OA symptoms, many patients with severe OA decide to have joint replacement surgery. In fact, nearly 700,000 Americans undergo hip or knee replacement each year. Another 23,000 have shoulder replacement surgery.
Knuckle "cracking" has not been shown to be harmful or beneficial. More specifically, knuckle cracking does not cause arthritis.
Joint "cracking" can result from a negative pressure pulling nitrogen gas temporarily into the joint, such as when knuckles are "cracked." This is not harmful. "Cracking" sounds can also be heard if tendons snap over tissues because of minor adjustments in their gliding paths. This can occur with aging as muscle mass and action change.
If cracking is accompanied by pain, there could be underlying abnormalities of the structures of the joint, such as loose cartilage or injured ligaments. Some patients with arthritis (inflammation of joints, usually painful), bursitis, or tendinitis notice "cracking" sounds due to the snapping of irregular, swollen tissues.