Arthritis is inflammation of one or more of the joints, causing pain, swelling, and stiffness. Arthritis is particularly common in the knee joint. Knee arthritis can make it hard to do many everyday activities, such as walking or climbing stairs. The most common types of arthritis are osteoarthritis and rheumatoid arthritis, but there are more than 100 different forms. Symptoms of knee arthritis include stiffness, difficulty bending and straightening the knee, pain, swelling, joint locks during movement, grinding noise (crepitus), and weakness.