Also known as femoroacetabular or FA impingement, hip impingement is an abnormality in the way the ball of the femur (thighbone) and the acetabulum (hip socket) fit together. It is a fairly common condition that affects more men than women. An improper shape of both the ball and the socket creates excess friction in the joint and may cause the hip to "jam" in front when bending forward. Over the years, hip impingement can tear or wear down hip cartilage (osteoarthritis), causing pain, swelling, stiffness and loss of mobility. The condition may be present at birth but cause no symptoms until later in life. Athletic activity-particularly soccer, football, hockey and running-may exacerbate the problem. If left untreated, the person may require total hip replacement.