More than 100 conditions are classified as arthritis, according to WebMD. However, many arthritis cases fall under two main types, osteoarthritis and rheumatoid arthritis. Other types include psoriatic arthritis, gout and lupus.
Osteoarthritis, also known as degenerative arthritis or degenerative joint disease, is the most prevalent kind of arthritis, says WebMD. It happens most often in the knees, spine, hips and feet. It occurs when joints are overused and is most frequently seen as people age. Joint overuse can result from too much weight, which overly stresses joints, and sports injuries that damage the joints. Athletes and people who had childhood injuries may experience osteoarthritis at a younger age than other people. With osteoarthritis, cartilage is worn away and provides less cushioning of joints, leading to pain.
Rheumatoid arthritis is an autoimmune system disorder, meaning that the immune system goes on the offensive against the body, according to WebMD. Joints are the most frequent target, and chronic inflammation over long periods of time leads to serious joint damage and deformities. Rheumatoid nodules are lumps that may form over heels, elbows and knuckles.
With psoriatic arthritis, the skin and joints become inflamed, according to WebMD. Psoriasis causes patchy, red and white areas on skin, and the arthritis aspect sometimes leads to fingers swelling so that they resemble sausages. Knees, the spine and other joints may also be affected.