Requirement: One femoral head must be normal.
Parameters:
(1) radius of the femoral head on the unaffected side
(2) radius of the femoral head on the affected side
(3) height of the unaffected femoral head
(4) height of the affected femoral head
The height of the femoral head is measured as the distance of a line drawn (a) perpendicular to the baseline of the femoral head to (b) the top of the ball surface.
The critical factor is an accurate determination for the center of the femoral head and its radius. Normally this is found easily along the epiphyseal line. If the femoral head is distorted, then the center can be located using a template of concentric circles (Mose, 1964; Figure 2, Mose, 1980) or an ischiometer (Muller, 1955)
joint surface index for the unaffected side =
= (height of femoral head in cm) / (2 * (radius of the femoral head in cm)
joint surface index for the affected side =
= (height of femoral head in cm) / (2 * (radius of the femoral head in cm)
joint surface ratio =
= (joint surface index for affected side) / (joint surface ratio for unaffected side) * 100
radius quotient =
= (radius of affected femoral head) / (radius of unaffected femoral head) * 100
Interpretation:
• A normal spherical femoral head has a JSQ > 85 and RQ < 115.
• A JSQ <= 85 or RQ => 115 would be abnormal.
