Alpha angle (cam morphology)

Last revised by Dr Henry Knipe on 17 Aug 2021

The alpha angle is a radiological measurement proposed for the detection of cam morphology, which is sometimes associated with femoroacetabular impingement (FAI)

The term alpha angle is also used in a separate and unrelated context in the evaluation of developmental dysplasia of the hip (DDH) in children.

Measured on obliquely oriented radiographs. The Dunn projection of the hip is used.

Measured on oblique axial images as the angle between 4:

  • a line from the center of the femoral head to the center of the femoral neck at its narrowest point
  • a line from the center of the femoral head to a point where the distance from the bone to the center of the head is greater than the radius of the cartilage-covered femoral head

There is no one value that can define cam morphology 5. Initially, in 2002, an alpha angle of ≥50° was proposed as pathological 4, subsequently, a value of ≥57° was proposed in symptomatic patients 2, and more recently (c. 2021) a value of ≥60° has been proposed as definitional of cam morphology 5

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Cases and figures

  • Case 1: with measurement on plain film
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