The femoral head in Legg-Calve-Perthes disease may become displaced laterally. The lateral displacement can be expressed as the percent of the femoral head with acetabular coverage.
Measurement:
(1) Perform an anteroposterior X-ray of the pelvis with the patient supine and the legs in the neutral position.
(2) Draw a horizontal line connecting the teardrop figures.
(3) Draw a vertical line perpendicular to the horizontal line from #2 that goes through the lateral lip of the acetabulum.
(4) Determine the maximum horizontal extent of the femoral head that lies perpendicular to the vertical line described in #3. Measure the extent of the femoral head in mm.
(5) Measure the horizontal extent of the femoral head as described in #4 that is medial to the vertical line, in mm.
percentage of acetabular coverage for the femoral head =
= (extent of femoral head medial to vertical line in mm) / (entire extent of horizontal breadth of the femoral head in mm) * 100%
Interpretation:
• Normally 100% of the femoral head is covered by the acetabulum.
• The percentage covered decreases.if the femoral head is displaced laterally.
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