Beta angle (developmental dysplasia of the hip)

Last revised by Andrew Murphy on 23 Mar 2023

The beta angle is a measurement used in the ultrasonographic assessment of developmental dysplasia of the hip (DDH).

The angle is formed by the cartilaginous roof to the vertical cortex of the ilium and thus reflects the femoral head cartilaginous coverage

Technique for drawing the beta angle:

  1. First draw the baseline, which is the vertical line parallel to either the outer cortex or inner cortex of the ilium as in the alpha angle measurement.
  2. Next draw the cartilage roofline, which is the line from the turning point to the center of the bright echogenic labrum, the angle between the two lines is the beta angle.

N.b. The turning point is also referred to as the bony rim in German literature. It is defined as the first point where the concavity of the acetabulum turns to a convexity when you start at the lower limb of the os ilium and move upwards, i.e. in a cephalad direction.

When both alpha and beta angles are drawn, the three lines in the majority of cases do not cross at a single point. There is greater variability in the beta angle as compared to the alpha angle as the center point of the echogenic labrum is a subjective point to mark. 

The beta angle determines the subtype of hip dysplasia, for example:

  •  type Ia vs. Ib,
  •  type IIc vs. type D

See article: sonographic classification of developmental dysplasia of the hip.

Reinhard Graf originally described both the beta angle and the alpha angle in 1980 1

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Cases and figures

  • Figure 1: labeled ultrasound
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  • Case 1: Graf type I
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  • Case 2: normal hip ultrasound
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