Perkin line is a line drawn perpendicular to Hilgenreiner line, intersecting the lateral most aspect of the acetabular roof.
The upper femoral epiphysis should be seen in the inferomedial quadrant: it should lie below Hilgenreiner line, and medial to Perkin line. If the nucleus of the femoral head is not visible because it is not ossified yet, the femoral metaphysis should be used.
Lateral displacement of the femoral head occurs in developmental dysplasia of the hip (DDH).
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