Arthritis (from Greek arthro-, joint + -itis, inflammation; plural: arthritides) is a group of conditions involving damage to the joints of the body. Arthritis is the leading cause of disability in people older than fifty-five years.

There are different forms of arthritis and each has a different cause. The most common form of arthritis, osteoarthritis (degenerative joint disease) is a result of trauma to the joint, infection of the joint, or age. Emerging evidence suggests that abnormal anatomy might contribute to the early development of osteoarthritis. Other arthritis forms are rheumatoid arthritis and psoriatic arthritis, autoimmune diseases in which the body attacks itself. Septic arthritis is caused by joint infection. Gouty arthritis is caused by deposition of uric acid crystals in the joint, causing inflammation. There is also an uncommon form of gout caused by the formation of rhomboid crystals of calcium pyrophosphate. This gout is known as pseudogout.

There is also a form of osteoarthritis caused by various forms of hemophilia, and has been diagnosed in patients with Von Willebrand Disease. The theoretical basis of this type of arthritis is explained as being caused by physical exertion accompanied by shock to joints, thus causing small amounts of bleeding into the joints. This is contrary to many medical opinions[citation needed], but has been observed by patients and physical therapists in a few cases where individuals with Von Willebrand Disease also suffered osteoarthritis which worsened after using aspirin which destroys the ability of platelets to function correctly, or other NSAIDS, such as ibuprofen and naproxen sodium. These NSAIDS suppress platelet function rather than destroy it as aspirin does. The result of the use of any of these NSAIDS is a propensity to bleed, as it is more commonly termed medically, a heightened bleeding diathesis.