Crowe’s classification is based on three easily identifiable anatomic landmarks: (1) the height of the pelvis; (2) the medial head-neck junction in the affected hip; and (3) the inferior margin of the acetabulum (the teardrop).

The measurements are made on AP radiographs of the pelvis. The reference line is drawn joining the inferior margins of each teardrop. The medial head-neck junction is identified and its distance from the reference line is noted. In the absence of subluxation this distance is close to zero. This distance was described as a measure of the degree of dysplasia by Crowe et al. [4]. According to the original investigation, the height of the pelvis is measured as the vertical distance from the highest point on the iliac crest to the inferior margin of the ischial tuberosity. The normal ratio of vertical diameter of the femoral head to the height of the pelvis was approximately 1:5 [4]. A hip was considered subluxated 50% or greater only if the medial head-neck junction was situated above the reference line by at least 10% of the measured height of the pelvis.