Above is the Library of Congress— it is impressive looking, but it's very physical presence dooms it to obsolescence. Information wants to be free of physical presence and physical restraints. Wikipedia and Google "own" the Library of Congress, as my kids would say, when it comes to the accessibility of information.
Background information on the hip joint, straight from Wikipedia, one of those free sources of information:
There are three bones of the os coxae (hip bone) that come together to form the acetabulum. Contributing a little more than two-fifths of the structure is the ischium, which provides lower and side boundaries to the acetabulum. The ilium forms the upper boundary, providing a little less than two-fifths of the structure of the acetabulum. The rest is formed by the pubis, near the midline. The word acetabulum means "little vinegar cup", and was the Latin word for a small vessel for storing vinegar.
In the last few years, femoroacetabular impingement (FAI) has become recognized as a cause of hip pain in adults. Much has been written about this entity, and most physicians are now aware of FAI. The topic is complex, from a diagnostic as well as treatment standpoint.
The two main types of FAI are termed the "cam" and "pincer" forms. In this entry, we will not review FAI; rather, we will focus on how to recognize the condition of acetabular retroversion, which is associated with the pincer form of FAI.
In the normal hip, the acetabular opening is anteverted (opens anteriorly). In the retroverted condition, the superior aspect of the acetabulum is tilted posteriorly. In both normal and retroverted hips, the opening gradually tilts anteriorly as one proceeds inferiorly.
On X-ray, the anterior rim of acetabulum should always project medial to the posterior wall, in a normal anteverted acetabulum:
(A) Edge of anterior acetabular wall (green arrow) is medial to the edge of posterior wall, even in superior aspect of acetabulum. (B) Green and yellow lines denote the anterior and posterior edges of acetabular wall. Note that these lines never cross.
In cranial acetabular retroversion, the anterior rim will project lateral to the posterior wall in the superior aspect of the acetabulum. Views from a CT topogram, in a hip with cranial acetabular retroversion:
(A) Edge of anterior wall (green arrow) is medial to the edge of posterior wall (yellow arrow) in inferior segment of the hip, but in the superior segment, this relationship reverses. This leads to the "crossover sign". (B) Illustration of crossover sign, with lines drawn.
On CT and MRI, cranial acetabular retroversion is recognized by examining the first axial image that includes the femoral head. If the acetabulum is retroverted, the anterior rim of the acetabulum will be lateral to the posterior rim:
(A,C) Axial images that contain the top of the femoral head demonstrate cranial acetabular retroversion (B,C) Axial images from a different patient at a similar slice position demonstrate normal acetabular configuration at this level (anteversion).
Thus, one can recognize acetabular retroversion on both conventional radiography and cross-sectional imaging. Just know what to look for....
Vic David MD