An arthritic hip is painful because the joint has broken down. The articular cartilage on the surfaces of the joint has worn away leaving raw bone exposed. Movement between the two joint surfaces is no longer smooth and friction free. As a result, the tissues within the joint become inflamed and the joint begins to hurt. An excess amount of joint fluid may be produced and the resultant swelling of the joint adds to the symptoms.
The exposed raw bony surfaces have a greater nerve supply than the articular cartilage and pressure on these surfaces causes pain. As the joint becomes stiffer, greater strain is placed on the hip muscles as they move the joint. This too can be painful. The friction between the exposed bony surfaces can cause grinding and a feeling of discomfort. When the joint is inflamed, there may be pain both with activity and when the joint is at rest. Anatomically, the hip joint fits deep in the groin. For this reason, groin pain may be the first sign of an arthritic hip. The pain may radiate to the outside or lateral side of the hip or to the back or posterior aspect of the hip.
It will often extend to the mid portion of the thigh and down to the knee. Since the ball of the hip, the femoral head, is part of the femur the pain travels down the length of the entire bone and will be felt in the thigh and the knee. When the pain goes below the knee, it is often a sign of a different problem, such as sciatica or poor circulation.