Bones have several important purposes in our bodies. Besides producing blood in the marrow, bones provide a rigid support system for the rest of the body and allow you to move. While bones themselves are sturdy, durable and immobile, the places where bone connects to bone -- the joints -- enable us to perform a wide variety of movements. There are six different types of joints, such as the ball-and-socket joint in your hip and the hinge joint that is your elbow.
Bones are held together with various types of connective tissue such as cartilage (protein-based material that cushions the bone ends), tendons (which connect muscle to bone) and ligaments (which connect bone to bone). This tissue holds the joint together and provides stability while enabling a range of motion. Cartilage covering the bone ends prevents them from rubbing together. Surrounding the joint is a joint cavity, a sac lined with a membrane called the synovium. The synovium produces synovial fluid that nourishes the joint and cartilage.
Sturdy and well designed as these joints are, they aren't immune to damage and decay. The knee joint, for example, has an intricate design that allows for complicated maneuvering. Bone ends must be shaped just right, cartilage and tendons must be properly oriented and the trochlear groove must be perfectly aligned with the kneecap that slides back and forth in it. If you take out random amounts of bone, remove a little cartilage and swell everything else up, everything in the knee can get off track. If it stays even slightly off track for too long, the impaired motion will start grinding bones together, pinching connective tissue or wearing away tendons as they slide against bones they weren't designed to slide against.
Different types of arthritis may affect some joints more than others. Psoriatic arthritis tends primarily to affect the hands and feet, the shoulder is more susceptible to injury-related arthritis, and osteoarthritis most often affects the hands, hips, knees and spine.
In the early stages of arthritis, you might not even feel any pain. As the damage increases, symptoms such as discomfort, tightness or a feeling of heat in the joint will appear and then worsen. If any of your joints begin feeling stiff, tender or like they're grating together, contact a doctor. The earlier arthritis is diagnosed, the better your options and chances for treating it.