This 3D animation from The Human Atlas explains Chronic Rheumatism of the knee.

The skeletal system provides the framework for the body and protects the internal organs. Joints exist where two bones meet to allow movement and flexibility of the skeleton. They are complex structures that contain many types of tissue. Ligaments surround the joints in order to provide support to the joints and prevent dislocation and injury.

Cartilage is a smooth and strong material that lines the connecting surfaces of the bones. This tissue cushions the joints and prevents friction when the bones rub against each other. Synovial membranes line the joints and secrete fluid. This fluid provides lubrication, reduces bone erosion, and aids in joint mobility.

Rheumatoid arthritis, or RA, is an autoimmune disorder in which the body's own immune cells attack this fluid-secreting membrane that lines the joint. This can cause joint swelling, stiffness, and pain. Inflammation can also occur in tissues around the joints, such as the ligaments, tendons, and muscles.

Chronic rheumatoid arthritis can exist over a long period of time and it is one example the many chronic rheumatism conditions. Treatment for RA can include medications such as nonsteroidal anti-inflammatory drugs (NSAIDs), corticosteroids, and disease-modifying anti-rheumatic drugs (DMARDs).