How is facet asymmetry measured in a computed tomography (CT) knee scan for a patient with recurrent patellar dislocation?

Measuring Facet Asymmetry in CT Knee for Recurrent Patellar Dislocation

Facet asymmetry is measured on axial CT images as the ratio of lateral trochlear facet length to medial trochlear facet length, with values >2.0 indicating significant asymmetry and increased risk for recurrent patellar dislocation. 1, 2

Measurement Technique

Image Selection

• Obtain axial CT images at the level of the greatest epicondylar width of the femur 1
• This standardized reference point ensures reproducible measurements across patients and follow-up studies 1
• Rotational profile CT scanning is the preferred imaging protocol for comprehensive assessment of all patellofemoral parameters 2

Facet Asymmetry Calculation

The lateral-to-medial facet ratio is calculated by dividing the lateral trochlear facet length by the medial trochlear facet length on the same axial image: 1, 2

• Normal values: Mean ratio of 1.46 (SD 0.19) in stable knees 1
• Pathological values: Mean ratio of 2.14 (SD 0.42) in knees with recurrent dislocation 1
• Korean population data: Control group showed 63.5% facet asymmetry versus 45.16% in patellar dislocation group 2

Measurement Landmarks

• Lateral facet length: Measure from the deepest point of the trochlear groove to the lateral edge of the lateral trochlear facet 1
• Medial facet length: Measure from the deepest point of the trochlear groove to the medial edge of the medial trochlear facet 1
• Both measurements should be perpendicular to the posterior femoral condylar line 1

Clinical Significance

Prognostic Value

• Low trochlear facet asymmetry (mean 23.5 ± 18.8) is significantly correlated with higher rates of redislocation compared to patients without redislocation (43.1 ± 16.5, p = 0.03) 3
• This makes facet asymmetry one of the most important predictors of surgical failure after soft tissue repair alone 3

Additional Patellar Measurements

While assessing facet asymmetry, also measure: 4, 2

• Medial patellar width: Reduced by mean 3.6 mm in recurrent dislocation (83.3% diagnostic accuracy as single discriminator) 4
• Patellar facet asymmetry (Wiberg index): Ratio of medial to lateral patellar facet length, with increased values indicating lateral facet dominance 4, 2
• Patellar volume: Decreased by mean 0.3 cm³ in recurrent dislocation 4

Diagrammatic Representation

AXIAL CT IMAGE AT GREATEST EPICONDYLAR WIDTH:

                    Anterior
                       |
              Medial Facet
                   /   \
                  /     \
    Medial -----●-------●----- Lateral
                  \     /
                   \   /
              Lateral Facet
                       |
                   Posterior

Measurements:
• Point ● = Deepest point of trochlear groove
• Medial Facet Length (MFL) = Distance from ● to medial edge
• Lateral Facet Length (LFL) = Distance from ● to lateral edge
• Facet Asymmetry Ratio = LFL / MFL

Normal: Ratio ≈ 1.46
Pathological: Ratio ≈ 2.14 (lateral facet disproportionately longer)

Comprehensive CT Assessment Protocol

Beyond facet asymmetry, CT evaluation should include: 2

• Sulcus angle: Normal mean 132.5° vs. pathological 143.3° 2
• Trochlear depth: Normal mean 6.04 mm vs. pathological 3.6 mm 2
• Bisect offset: Normal 56.4% vs. pathological 99.9% 2
• Lateral patellar tilting: Normal 9.8° vs. pathological 19.2° 2
• TT-TG distance: Normal 10.91 mm vs. pathological 27.16 mm 2

Common Pitfalls

• Inconsistent axial slice selection leads to non-reproducible measurements; always use the greatest epicondylar width level 1
• Measuring at wrong rotation can falsely alter facet length ratios; ensure proper patient positioning with knees in neutral rotation 2
• Ignoring medial patellar morphology misses the single strongest discriminator for recurrent dislocation risk 4
• Relying on plain radiographs alone is inadequate, as CT without IV contrast is the gold standard for quantifying rotational and morphological parameters 5