The human body has 143 different joints, categorized into three areas (see more under Anatomy and Physiology)
- fixed (synarthroses)
- slightly movable (amphiarthroses)
- highly movable (diarthroses)
All of these joints have other structures that help them function. Since arthritis affects the synovial, or highly movable joints, most often, it is these joints that will be dealt with here. Their basic structure includes:
- articular cartilage caps the ends of bones, absorbing shocks while providing a slick surface so that the bone ends can easily glide across each other during movement. It is a smooth, glistening, bluish-white, gel-like substance, composed of 65-80% water, collagen, and proteoglycans which form the cartilage matrix. It is a fact that there is no man-made substance that can compare to, or be manufactured like, the low-friction, shock-absorbing properties of healthy cartilage.
- a capsule made of a tough membrane encloses the joint and connects one bone to another holding them firmly in place.
- synovium is the inner lining of the capsule which secretes synovial fluid to lubricate and nourish the cartilage.
- ligaments attach bone to bone and help to provide stability.
- tendons attach muscle to bone, allowing for movement and acting as secondary joint stabilizers.
- muscles contract to provide the force for movement and are critical for much of the shock absorption around a joint.
- bursae are small, fluid-filled sacs positioned at strategic points to cushion ligaments and tendons, protecting them against friction and wear and tear.