The hip socket (acetabulum) is the bony portion of the pelvis that makes up the concave part of the hip joint.  The socket is covered by smooth cartilage and joins with the cartilage-covered ball (femoral head) composing the hip joint.  A normal socket covers the ball to an appropriate amount to allow for normal, functional ranges of motion.  Any deviations in this coverage (too much or too little coverage) can lead to injuries of the hip joint.  Additionally, the hip socket is normally forward facing (about 10-15 degrees).  Some people are born with or develop a hip socket that is neutral or backward facing (less than 10 degrees or sometimes less than 0 degrees).  A socket that is backward facing is called acetabular retroversion.  This backward rotation causes significant limitations in range of motion, especially with inward rotation of the thigh and knee (internal rotation), and can lead to impingement.  Impingement can cause tears of the hip labrum (o-ring around the rim of the socket) and cartilage, causing pain and wear and tear to the joint over time (arthritis).  This diagnosis requires a thorough examination and detailed assessment of radiographs, but it is important for proper treatment.  Treatment of acetabular retroversion and the resulting impingement should begin with activity modification, rest, therapy, and injections in to the hip.  If non-surgical treatments fail, then arthroscopic or corrective surgery may be indicated.  Mild cases may adequately be treated with hip arthroscopy, but most moderate to severe cases frequently require a corrective osteotomy (PAO) to properly and completely treat the rotational problem.