Kerboul et al measured the size of an area of osteonecrosis involving the femoral head based on changes seen in radiographs taken in two dimensions. The authors are from the Hopital Cochin in Paris.
Radiographs performed:
(1) anteroposterior view
(2) lateral view
Measurements in each view:
(1) Measure the arc of the articular surface involved by the osteonecrosis.
(2) From the center of the femoral head measure the angle required to subtend the arc.
combined necrotic angle =
= (degrees subtended in the anteroposterior view) + (angle subtended in the lateral view)
Interpretation:
• minimum combined necrotic angle: 2 (1 degree in each view)
• maximum combined necrotic angle: 720 (entire necrosis of the femoral head)
• The greater the combined necrotic angle the greater the degree of avascular necrosis.
Combined Necrotic Angle
Size of the Area Involved
< 160°
small
160 - 200°
medium
> 200°
large
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