You are shown two MRI images. One is a standard orthogonal c | Figure 1

Pomeranz

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:

  1. What is a normal alpha angle?
  2. What does an abnormal alpha angle suggest?
  3. Is an abnormal alpha angle pathognomonic of a symptomatic disease process?

Similar cases

does this angle have a name? knowing what normal looks like makes it easy to spot the abnormal, although the fracture line may not be immediately visible, note the 'sinking' of femoral head, the loss of neck curvature, and the intertrochanteric hypolucency](https://app.figure1.com/case-detail/30f0522a-967f-46f7-877f-6b3bf9c2b01c)

Radiograph of the pelvis (image 1) shows findings of right-sided developmental dysplasia of the hip. The right femoral head is smaller than the left, the right acetabular angle is steeper than the left, and the right hip is subluxed superolaterally. A fluoroscopic image (image 2) was obtained during arthrogram and hip reduction for hip dysplasia. The arthrogram shows that the majority of the femoral head is cartilaginous and not visible on x-ray. Developmental hip dysplasia is usually treated by placing the child in a Pavlik harness. #MSKMonday](https://app.figure1.com/case-detail/4cd5ad19-af83-4de9-a1f0-4cc5d607b1b5)

A1 - A coronal T1 (image 4, pink arrow) shows a spur encroaching on the base of the femoral notch. A sagittal T2 (image 5), at the level of the posterior cruciate ligament, shows how you would measure the alpha angle. The green line is parallel to the long axis of the femur, and the yellow line is parallel to Blumensaat’s line of the femoral roof. In this patient, the angle is 29 degrees. Increasing alpha angles with a more horizontal femoral tunnel roof elevates the risk of notch impingement. A2 - Have a look at the orange line in axial image 6. At this level, the trochlea is deep, and the insertion of the anterior cruciate ligament along the medial side wall of the lateral femoral condyle is the one to be evaluated (red arrow). The widest transverse dimension of the notch is measured as 1.56cm. A3 - Alpha angles approaching 55-60 degrees or greater lay out the ACL in a more horizontal position in the native ungrafted knee, and place the ACL at greater risk for stress forces in flexion and extension. On the other hand, several authors have commented that a very vertical roof, and a very narrow alpha angle in ACL grafted knees, may result in an “unforgiving knee”. This would place grafted knees at greater risk for graft failure (American Journal of Sports Medicine, 1995).

1a (no arrows): Coronal T1-weighted MR image of a 2-year-old with right hip pain. What are the abnormalities? Comment below with your thoughts, answer to follow later today! #FridayQuizDay](https://app.figure1.com/case-detail/5d9d13ba-b933-4e20-b589-8547319d4c82)