To understand the pathological processes in the joint, we need to take a look at the normal healthy joint. Joints are held together by a joint capsule and designed to allow smooth movement between adjacent bones. In the type of joint commonly affected by arthritic diseases, the highly movable joints, we find the bone ends covered by articular cartilage and the joint space enclosed by a synovial membrane. This thin membrane secretes synovial fluid that lubricates the space between the cartilage-covered joint-forming bones. The cartilage contains no blood vessels or nerves and receives its nutrients by diffusion from the synovial fluid and from the bone.
Joint function depends on the health of the cartilage in the joint. Cartilage is a gel-like substance that acts as a shock absorber, essential for smooth and easy movement in the joint. Cartilage gets its elasticity from collagen fibers and its sponge-like quality from water, held by a structure of big molecules called proteoglycans. Collagen and proteoglycans are produced by special cells, called chondrocytes, in the cartilage. Joints can withstand enormous pressure by slowly releasing water from the cartilage.
As we age the ability to restore and maintain a normal cartilage structure begins to decline. The activity of important repair zymes is reduced, the water content diminished, and the joints become more prone to damage. But the full pathological mechanism for development of arthritis is not yet known.