How is the necrotic angle in the femoral head calculated on MRI for Avascular Necrosis (AVN)?

Calculating the Necrotic Angle in Femoral Head AVN on MRI

The necrotic angle in femoral head AVN is calculated using the modified Kerboul method, which measures the arc of necrotic involvement on mid-coronal and mid-sagittal MRI slices, then sums these angles to create a combined necrotic angle that predicts collapse risk. 1

Step-by-Step Measurement Technique

Image Selection and Plane Orientation

• Identify the mid-coronal MRI slice that passes through the center of the femoral head 1
• Identify the mid-sagittal MRI slice that passes through the center of the femoral head 1
• Note that oblique axial slices (parallel to the femoral neck axis) may provide more accurate prognostic information than standard axial slices (parallel to the body axis), as they better capture the posterior extent of necrosis 2

Angle Measurement Process

• On the mid-coronal slice: Draw lines from the center of the femoral head to the anterior and posterior margins of the necrotic lesion, measuring the arc angle in degrees 1
• On the mid-sagittal slice: Draw lines from the center of the femoral head to the superior and inferior margins of the necrotic lesion, measuring the arc angle in degrees 1
• Calculate the combined necrotic angle by adding the coronal angle and sagittal angle together 1

Risk Stratification Based on Combined Angle

The combined necrotic angle directly predicts collapse risk and guides treatment decisions 1:

• Grade 1 (<200°): Low risk - no collapse expected, <5% progression 1
• Grade 2 (200°-249°): Moderate risk - 50% collapse rate 1
• Grade 3 (250°-299°): High risk - nearly universal collapse 1
• Grade 4 (≥300°): Very high risk - universal collapse 1

Treatment Implications by Angle

• <190°: Core decompression with or without bone marrow aspirate concentrate (BMAC) has excellent outcomes 3, 1
• 190°-240°: Moderate risk zone where BMAC addition to core decompression improves outcomes 3
• ≥240°-250°: Core decompression has high failure rates; consider more aggressive intervention 3
• ≥300°: Arthroplasty should be strongly considered as joint-preserving procedures have poor outcomes 3

Critical Technical Considerations

Common Pitfalls to Avoid

• Using standard axial slices instead of oblique axial slices can underestimate posterior necrotic involvement by approximately 15.7% 2
• The posterior intact ratio differs significantly between axial (mean 49.2%) and oblique axial (mean 33.5%) planes, with oblique axial providing better prognostic accuracy (sensitivity 88.4%, specificity 82.3%) 2
• Visual estimation from serial MRI images is as accurate as angular measurements for clinical use, though 3D volumetric measurements remain the gold standard for research 4

Alternative Measurement Methods

• The index of necrosis correlates more closely with 3D volumetric measurements than the Kerboul angle alone 4
• 3D MRI volumetric measurements are most accurate and should be used when available, particularly for research purposes 4
• Simple visual estimates of lesion size are satisfactory for routine clinical decision-making 4

Clinical Context and Prognostic Value

Why This Measurement Matters

• Necrotic volume >30% of the femoral head progresses to collapse in 46-83% of cases, while <30% involvement progresses in <5% of cases 5
• The combined necrotic angle serves as a surrogate for necrotic volume and directly predicts articular collapse risk 1
• Early accurate measurement allows for stage-appropriate intervention to prevent collapse and delay or avoid total hip arthroplasty 5

Additional Risk Factors to Document

Beyond the necrotic angle, document these factors that worsen prognosis 5:

• Joint effusion presence
• Bone marrow edema extent
• Patient age >40 years
• BMI >24 kg/m²