Trochlear facet asymmetry (FA)
Trochlear facet asymmetry (FA) refers to the condition of the medial facet being abnormally small if compared to the lateral facet in trochlear dysplasia .
Usage
Facet asymmetry (FA) is also used as a measurement in magnet resonance imaging for trochlear dysplasia to make it more objective , this concept was adapted from axial radiographs .
Measurement
One option to measure facet asymmetry is the ratio between medial versus lateral trochlear facet length calculated as (medial facet) / (lateral facet) . Another option is to calculate it the other way around, that is (lateral facet) / (medial facet) .
Facet asymmetry traditionally measured 3 cm above the joint line .
Interpretation
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 .
History and etymology
The principle has been adapted for magnetic resonance imaging by Pfirrmann .
See also
