The Components of Your Joints

Before you can understand the causes of and treatments for arthritis, it is helpful to know more about your joints. A joint is the place where two or more movable bones come together (articulate). Think of your knees, hips, and the many joints in your hands and feet. Healthy joints move smoothly, thanks to a complex system of lubrication and shock absorption. The components of a joint include:

• Cartilage: A tough slippery material that coats the ends of bones and provides a shock-absorbing cushion to prevent them from rubbing together. Joint (articular) cartilage is composed primarily of water and strong protein fibers called collagen. The knee joints, which bear most of the body’s weight, have an extra shock-absorbing layer called meniscal cartilage, which looks like two C-shaped pads.

• Joint capsule: The membrane sac enclosing the entire joint.

• Joint space: The narrow, open area between two bones, where the cartilage meets.

• Synovial membrane (synovium): The inner membrane lining the joint capsule. It secretes a slippery substance called synovial fluid.

• Synovial fluid: The fluid that fills the space around and between bones and helps to keep joints moving with little friction.

• Ligaments, tendons and muscles: Structures located just outside a joint that support the bones and help the joint bend and move. Ligaments are strong, band-like tissues that connect one bone to another at a joint. Tendons are fibrous cords that connect muscles to bones. Muscles are strong, fibrous tissues that work in pairs, flexing and contracting to produce movement in the joints.

• Bursae: Fluid-filled sacs located between ligaments, tendons, muscles and bones. Bursae help these structures move smoothly against each other.

Different forms of arthritis cause inflammation of various structures within and around joints. Osteoarthritis primarily affects the joint cartilage, whereas rheumatoid arthritis generally starts with inflammation of the synovial membrane.