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 (FA) 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 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.
History and etymology
The principle has been adapted for magnetic resonance imaging by Pfirrmann 1.
- 1. Pfirrmann CW, Zanetti M, Romero J, Hodler J. Femoral trochlear dysplasia: MR findings. (2000) Radiology. 216 (3): 858-64. doi:10.1148/radiology.216.3.r00se38858 - Pubmed
- 2. Charles MD, Haloman S, Chen L, Ward SR, Fithian D, Afra R. Magnetic resonance imaging-based topographical differences between control and recurrent patellofemoral instability patients. (2013) The American journal of sports medicine. 41 (2): 374-84. doi:10.1177/0363546512472441 - Pubmed
- 3. Steven S. Ngai, M.D., Edward Smitaman, M.D., Donald Resnick, M.D Trochlear Dysplasia – Radsource MRI Web Clinic June 2018
- 4. Paiva M, Blønd L, Hölmich P, Steensen RN, Diederichs G, Feller JA, Barfod KW. Quality assessment of radiological measurements of trochlear dysplasia; a literature review. (2018) Knee surgery, sports traumatology, arthroscopy : official journal of the ESSKA. 26 (3): 746-755. doi:10.1007/s00167-017-4520-z - Pubmed