Radiograph: supine pelvic AP X-ray with legs in neutral position
Required lines on radiographs:
(1) midline (center line) thorough sacrum and symphysis
(2) Hilgenreiner's line = horizontal line through the centers of the triradiate cartilages
(3) Perkin's line (Ombredanne-Perkin's line) = vertical line, perpendicular to Hilgenreiner's line, passing through the outermost border of the acetabular roof
(4) horizontal line through the most superior part of the femoral neck
(5) vertical line through the most medial portion of the femoral neck
where:
• In radiographs in infants and young children there is a gap between the ossification center of the femoral head and ossification of the femoral neck and shaft.
distance c =
= distance between the center line (line 1) and the vertical line through the most medial portion of the femoral neck (line 5)
distance b =
= distance between the center line (line 1) and Perkin's line (line 3)
distance h =
= distance between Hilgenreiner's line (line 2) and the horizontal line through the most superior part of the femoral neck (line 4)
where:
• Distance h is given as a negative number if the horizontal line through the most superior part of the femoral neck (line 4) is cephalad to Hilgenreiner's line.
lateral displacement ratio =
= c / b
superior displacement ratio
= h / b
Interpretation:
• The normal c/b ratio is 0.6 to 0.85. A dislocated hip will have a ratio > 0.85 (see Figure 3, page 1083, Smith et al).
• The normal h/b ratio in infants is 0.1 to 0.2. A dislocated hip will usually have a value <= 0 (-0.7 to 0). A ratio from 0 to 0.1 is indeterminate.
