What does the alpha angle suggest in this hip?

You are shown two MRI images. One is a standard orthogonal coronal with a line through it. This shows the angle of acquisition to evaluate the alpha angle. The second image is the axial oblique with the lines used for alpha angle measurement. Questions:

  • What is a normal alpha angle?
  • What does an abnormal alpha angle suggest?
  • Is an abnormal alpha angle pathognomonic of a symptomatic disease process?

Answers:

  • Normal alpha angle is 55 to 60 degrees or less.
  • Increased alpha angles suggest cam impingement syndrome or impingement syndrome type 1.
  • The yellow line in the second image is placed from the center of the femoral head to the junction of the medial transition of the upper femoral head to the bump. The green line bisects the center of the femoral head, neck, and trochanteric region as demonstrated on the axial oblique image.
  • Athletic, large men often have abnormal alpha angles in the absence of symptoms. Therefore, the alpha angle is not pathognomonic of symptomatic cam impingement, only suggestive.
  • For more orthopaedic case review, head to MRI Online.

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