"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.
Symptoms of osteoarthritis may include joint pain and progressive stiffness that develops gradually.
Symptoms of rheumatoid arthritis may include painful swelling, inflammation, and stiffness in the fingers, arms, legs, and wrists occurring in the same joints on both sides of the body, especially upon awakening.
Symptoms of infectious arthritis may include fever, chills, joint inflammation, tenderness, and sharp pain that is associated with an injury or infection elsewhere in your body...
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.