The alpha angle is a measurement used in the ultrasonographic assessment of developmental dysplasia of the hip (DDH).
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Terminology
The term alpha angle is also used in a separate and unrelated context in the evaluation of femoroacetabular impingement.
Measurement
The angle is formed by the acetabular roof to the vertical cortex of the ilium and thus reflects the depth of the bony acetabular roof. This corresponds to 90 minus the acetabular angle.
first a baseline is drawn, which is the vertical line parallel to either the outer cortex or inner cortex of the ilium
next, the lowest and lateral-most point of the bright spot of the lower limb of the os ilium is chosen
using that point, a line is drawn that just tangentially touches the bone of the ilium; this line must not cut the white echo of the ilium but just touch it
Alpha angles are only drawn on centered hips in the Graf system; decentred hips do not need any angle measurements.
Interpretation
≥60°: normal
-
<60°
>12 weeks old: acetabular dysplasia
0-12 weeks old: could represent a physiologically immature hip that could go on to mature without treatment
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