Until I was diagnosed with arthritis, I didn't analyze the anatomy of human joints. It's one of those things people take for granted -- normal joint movement. That is, until something goes wrong that causes joint pain, joint damage, joint deformity, and limited range of motion. Let's compare a normal joint to an arthritic joint.
In the human body, a joint is formed where the ends of two bones meet, allowing for movement. Where two bones form a highly mobile joint, such as the hip or shoulder, it's known as diarthrosis, which is synonymous with the term "synovial joint." Depending on the specific joint, possible joint movements include: abduction (movement away from the midline of the body); adduction (movement toward the midline of the body); extension (straightening); flexion (bending); and rotation (circular movement). A healthy synovial joint can move freely through its normal range of motion.
A synovial joint has a capsule (referred to as the joint capsule) that is lined with synovium (referred to as the joint lining) and is filled with synovial fluid. The bones of a synovial joint are covered by hyaline (articular) cartilage.
- The joint capsule is a ligamentous sac which completely encloses a joint. The term "ligamentous" refers to fibrous connective tissue that is attached to bone. The joint capsule provides passive stability by limiting movements and, through its nerve endings, lets you know the position of your joint.
- The synovium (also called the synovial membrane) is a connective tissue membrane that lines the surfaces of the joint capsule, as well as tendon sheaths and bursae. Tendons are bands of fibrous tissue attached to the muscles that move your bones. Bursae are flat, fluid-filled sacs that provide cushioning where skin, muscles, tendons, and ligaments rub over bones.
- Synovial fluid, produced by the synovium, provides nourishment to the cartilage and helps to reduce friction during movement.
- Articular cartilage is a tough but flexible tissue composed of 85% water and a highly organized matrix of collagen, protein, and sugar. Cartilage provides a slippery surface that allows bones to glide freely, while reducing friction and absorbing shock.