Trochlear facet asymmetry

Last revised by Jeremy Jones on 27 Jan 2024

Trochlear facet asymmetry (FA) refers to the condition of the medial facet being abnormally small if compared to the lateral facet in trochlear dysplasia 1.

Facet asymmetry is also used as a measurement in magnet resonance imaging for trochlear dysplasia to make it more objective 1-3, this concept was adapted from axial radiographs 3.

One option to measure facet asymmetry is the ratio between medial versus lateral trochlear facet length calculated as (medial facet) / (lateral facet)  1. Another option is to calculate it the other way around, that is (lateral facet) / (medial facet) 2.

Facet asymmetry is traditionally measured 3 cm above the joint line 1,3.  

A threshold of <0.4 or 40% was suggestive of trochlear dysplasia and showed a sensitivity and specificity of 100% and 96% in that study 2.

The principle has been adapted for magnetic resonance imaging by Pfirrmann 1.

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Cases and figures

  • Case 1: small medial facet
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