What is the role of the Karabo angle in assessing avascular necrosis (AVN) on Magnetic Resonance Imaging (MRI)?

Necrotic Angle Measurement in Humeral Head AVN on MRI

The "necrotic angle" (not "Karabo angle") is measured on mid-coronal MRI planes to quantify the volume of avascular necrosis in the humeral head, with angles <90 degrees predicting no collapse over 24 months of follow-up. 1

Measurement Technique and Clinical Application

Anatomic Location and Measurement

• The necrotic volume in the humeral head is most commonly located in the superior medial aspect 1
• The necrotic angle is measured on the mid-coronal plane of MRI sequences 1
• This measurement quantifies the extent of necrotic bone involvement within the humeral head 1

Prognostic Value

• Humeral heads with a necrotic angle <90 degrees did not collapse during 24-month follow-up 1
• This measurement serves as a prognostic indicator for risk of articular collapse 1
• The angle measurement helps predict which lesions may be managed conservatively versus those requiring surgical intervention 1

MRI Protocol for AVN Assessment

Optimal Imaging Approach

• MRI without IV contrast is the most sensitive and specific modality for diagnosing osteonecrosis, with sensitivity and specificity approaching 100% 1
• MRI allows comprehensive characterization including location, volume, presence of bone marrow edema, and joint effusion 1
• Coronal sequences are essential for measuring the necrotic angle in humeral head AVN 1

Volume Quantification Beyond Angle Measurement

• For femoral head AVN, lesions involving <30% of the femoral head have <5% progression to collapse 2, 3
• MRI can predict necrotic volume accurately, which is the main determining factor in outcome 4
• The necrotic angle measurement in the humerus serves a similar prognostic function to volume measurements in the femoral head 1

Clinical Context and Pitfalls

Important Clarification

• The term "Karabo angle" does not appear in the medical literature; the correct terminology is "necrotic angle" 1
• This measurement has been specifically validated for humeral head osteonecrosis, not femoral head AVN 1

Detection of Multifocal Disease

• Whole-body MRI protocols have led to increased detection of multifocal osteonecrosis in conditions like multiple myeloma, polymyositis, and lymphoma 1
• When AVN is detected in one location, imaging of other weight-bearing joints should be considered, particularly the contralateral side 1

Complementary Imaging

• While MRI excels at early detection and volume quantification, CT is superior for showing location and extent of articular collapse once it occurs 1
• CT plays a critical role in surgical planning when collapse has developed 1