Arthritis means inflammation (itis) of the joint (arthro). There are over a hundred types of arthritis that can affect any joint, but the three most common types are osteoarthritis, rheumatoid arthritis, and post traumatic arthritis. Osteoarthritis inflames the joint over time, as the articular cartilage deteriorates, bone rubs against bone, and stiffness and pain ensue. In rheumatoid arthritis, the immune system targets and harms the articular cartilage. In post traumatic arthritis, arthritis develops after an injury to the knee, from broken bones to meniscus tears.