Required radiograph: flat plate of the pelvis showing both hip joints
Measurement at each hip:
(1) Draw a line connecting the greater and lesser trochanters (intratrochanteric line),
(2) Draw a line across the midportion of the femoral neck.
(3) Identify the midpoints of the intratrochanteric and neck lines.
(4) Draw a third line through these 2 midpoints.
(5) Draw a perpendicular line "b" from the line in step 4 to the most prominent point on the lateral femoral head margin.
(6) Draw a perpendicular line "a" from the line in step 4 to the most prominent point on the medial femoral head margin.
femoral head ratio =
= (length of line a in mm) / (length of line b in mm)
Interpretation:
• In the normal femoral head the ratio is close to 1.
• A ratio >= 1.35 in one or both hips indicates a tilt deformity of the femoral head.
