This is a degenerative joint disease stemming from wear and tear on the knee usually affecting those in middle age and older. Osteoarthritis causes the cartilage to erode away. Symptoms include pain, stiffness, and swelling that at first happens once in a while but can progress to chronic pain.
Unlike osteoarthrits, this type of arthritis does not erode the cartilage slowly, rather it inflames the joint (usually many joints at one time) until the joints are ruined. Rheumatoid arthritis is most common in middle aged women, but can affect people of all ages.
Inflammation of the joints can occur when a small granual of sodium urate (related to gout) or calcium phosphorate (related to chondrocalsinosis) gets lodged in the joint. The effect is similar to getting a piece of sand trapped in your eye. It irritates the surrounding inflammation. Middle-aged men are most prone to this sort of arthritis.