Normal Trochlear Facet Asymmetry Ratio Size on MRI
The trochlear facet asymmetry ratio is an MRI-derived measurement that compares the relative depths of the medial and lateral facets of the femoral trochlea. It is calculated by dividing the difference in facet depths by the depth of the deeper facet, yielding a dimensionless ratio. Accurate assessment of this ratio is clinically important because trochlear dysplasia is a well-recognized structural risk factor for recurrent patellar instability and dislocation.
Normal Reference Values
| Measurement |
|---|
| <0.4 |
Clinical Significance
A trochlear facet asymmetry ratio of less than 0.4 is considered normal, indicating relatively balanced medial and lateral facet development. When the ratio meets or exceeds 0.4, the trochlea is considered asymmetric, reflecting underdevelopment of the medial facet relative to the lateral facet — a hallmark feature of femoral trochlear dysplasia. This morphological abnormality reduces the bony constraint on the patella, predisposing patients to lateral patellar subluxation and dislocation.
Trochlear dysplasia is graded using the Dejour classification, and asymmetry ratio is one of several MRI parameters — alongside the trochlear groove depth and sulcus angle — used to characterize dysplasia severity. Clinicians should interpret asymmetry ratio alongside other instability markers such as the tibial tubercle–trochlear groove (TT-TG) distance, patellar height indices, and medial patellofemoral ligament (MPFL) integrity.
- Recurrent lateral patellar dislocation or subluxation
- Femoral trochlear dysplasia (Dejour types A–D)
- Trochleoplasty or MPFL reconstruction candidacy assessment
- Post-traumatic trochlear remodeling in skeletally immature patients
- Incidental trochlear flattening without symptomatic instability
Reference: Pfirrmann CW, Zanetti M, Romero J et al. Femoral trochlear dysplasia: MR findings. Radiology. 2000;216(3):858–64.
Imaging Notes
Trochlear facet asymmetry ratio is measured on axial MRI sequences through the proximal femoral trochlea, typically on proton density or T1-weighted images where cartilage and bony contours are clearly delineated. The measurement is made at the level where the trochlear groove is most evident, proximal to the distal femoral condyles. The depths of the medial and lateral facets are measured perpendicularly from the facet surface to a reference line connecting the facet peaks, and the ratio is computed as (lateral facet depth − medial facet depth) / lateral facet depth.
Consistent slice selection is essential, as oblique or distally positioned axial planes may underestimate asymmetry. A dedicated knee coil with high spatial resolution (slice thickness ≤3 mm) improves measurement reproducibility. Cartilage surface measurements should be used consistently across examinations to minimize variability between observers.