Trochlear dysplasia with transient lateral patellar dislocation
Recurrent dislocation of patella and history of fall.
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Marrow edema in medial patellar facet and in the lateral femoral condyle reflecting transient lateral patellar dislocation; the patella is in joint. There is a tear of the MPFL with moderate joint effusion. Patella cartilage is intact. The trochlear sulcus angle is 156° with flattened trochlear groove. Type A trochlear dysplasia (shallow trochlear groove). Patellar tendon and quadriceps attachment are intact. Insall salvati ratio is 0.79.
Marrow edema is seen in the lateral femoral condyle. Focal cortical defect is seen in the tibial plateau. Type II signal intensity changes seen in the medial meniscus.
Trochlear dysplasia is one of the main factor contributing to chronic patellofemoral instability. Dejour et al proposed a classification distinguishing four morphologic types of trochlear dysplasia:
- Type A: Normal shape of trochlear preserved but a shallow trochlear groove.
- Type B: Markedly flattened or even convex trochlea.
- Type C: Asymmetric trochlear facets with the lateral facet being too high and medial facet being hypoplastic, which results in the flattened joint surface forming an oblique plane.
- Type D: In addition to features of type C, a vertical link between medial and lateral facets ("cliff pattern" on parasagittal images).
Sulcus angle of >150° has been defined as indicating pathologic flattening of the joint surface on axial projections.
- Diederichs, G., Issever, A. and Scheffler, S. (2010). MR Imaging of Patellar Instability: Injury Patterns and Assessment of Risk Factors. RadioGraphics, 30(4), pp.961-981.