Have you ever measured the alpha angle in a knee?

This 50-year-old female presents with knee pain. There is no known injury, but the patient did have anterior cruciate ligament (ACL) repair 15 years earlier.

Q1 – How would you measure the alpha angle in this case? You may think I have lost my mind, and the wrong case is displayed, but I assure you it is correct.

Q2 – How would you measure the intercondylar notch distance?

Q3 – What is the significance of the alpha angle in the knee?

Q4 – What is the significance of the intercondylar notch distance?

Coronal T1

Sagittal T2 FS (60 TE)

Axial PD FS


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).

A4 – The more narrow the distance, the more at risk the anterior cruciate ligament. A typical distance is 20mm measured at the right level. Patients with distances of approximately 15mm or less are at high risk for ACL deficiency or rupture. Most patients that have intercondylar notch distances of 12mm or less have ACLs that are absent or torn.


  • The alpha angle is similar in normal women and men.
  • The width of the intercondylar distance is more narrow in normal women than men.

Coronal T1

Sagittal T2 FS (60 TE)

Axial PD FS


Individuals with large alpha angles approaching 55-60 degrees, and narrow intercondylar distances measured where the ACL attaches to the lateral condylar wall of less than 15mm, are at high risk for ACL stressors and subsequent rupture.


Check out MRI Online for more case review.

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