Hips Don't lie Inside the Joint Each hip is a ball-and-socket joint. The ball is the top of your thighbone (femoral head). The socket (acetabulum) is in your pelvic bone. Smooth, slippery tissue called cartilage lets the ball and socket glide against each other when you move. A thin lining (synovium) tops the cartilage and makes a bit of synovial fluid, which further eases rubbing. Tendons, ligaments, and muscles complete the joint. Osteoarthritis It’s the “wear and tear” type of arthritis that many people get in middle age. Cartilage on the ball end of the thighbone and in the hip socket slowly breaks down and causes grinding between bones. You’ll have stiffness, and you might feel pain in your crotch and at the front of your thigh that radiates to your knee and behind. It’s often worse after a hard workout or when you don’t move for a while. Rheumatoid Arthritis With RA, your immune system attacks parts of your body, which can include the syn...