Measurement in AP and lateral radiographs of the upright patient's pelvis:
(1) Identify the superior most (most cephalad) point on the left and right iliac crests. Draw a horizontal line through these points, perpendicular to the vertical axis.
(2) Identify the tip of the coccyx. Draw a horizontal line through the tip of the coccyx perpendicular to the vertical axis.
(3) Identify the inferior most (most caudal) point at the left and right sacroiliac joints. Draw a horizontal line through these points, perpendicular to the vertical axis.
(4) The lines in 1, 2 and 3 should be parallel to each other.
sacral ratio =
= (vertical distance between the line through the top of the iliac crests and the line through the base of the sacroiliac joint) / (vertical distance between the line through the base of the sacroiliac joint and the line through the tip of the coccyx)
Interpretation:
• The sacral ratio in normal children is about 0.77.
• Children with anorectal malformations may show ratios that vary from almost 0 to 1.0.
• A child with an anorectal malformation who shows a sacral ratio < 0.3 is unlikely to have good bowel control.
