The acetabular teardrop and its relevance to acetabular migration.

Five pelvises were photographed, roentgenographed, and sequentially sectioned or reamed to determine the location and appearance of the acetabular teardrop figure. The teardrop is located inferomedially in the acetabulum, just superior to the obturator foramen. The lateral lip is the exterior, and the medial lip is the interior of the acetabular wall. The ilioischial line projects over the medial acetabulum only fortuitously on the straight anteroposterior (AP) roentgenogram. Because of parallax, the relationship between the ilioischial line and the teardrop changes for views varying as little as 10 degrees in horizontal obliquity from the true AP roentgenogram. Because the teardrop comprises a well-defined, constant portion of the medial acetabular wall whereas the ilioischial line does not, the authors recommend using the acetabular teardrop rather than the ilioischial line for the detection and measurement of medial and superior acetabular migration.