Most types of arthritis are caused by the breakdown of cartilage and synovium (a pocket of fluid between cartilage) within joints. Rheumatoid arthritis and juvenile rheumatoid arthritis are both caused when the body's immune system attacks the tissues that line the joints, which can destroy the joint's cartilage and bone over time. Osteoarthritis is caused by wear and tear of joints, or excessive stress or injury to joints. Post-traumatic arthritis is caused by sudden trauma to a joint, usually as a result of an accident or injury.
Risk factors for arthritis can vary depending on the type, but in general, certain things may increase your risk. Genetics may play a role in the development of certain types of arthritis, so if your family members have arthritis, you're more likely to develop it. Women are more likely than men to develop many types of arthritis. Also, any kind of excessive stress or injury to the joints may increase the risk of developing arthritis, so people who are overweight or who play sports are often at a higher risk. Age also plays a role in the development of some types of arthritis, so being over the age of 40 may increase your risk.