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Trochlear dysplasia refers to abnormal development of the femoral trochlear groove, resulting in a shallow or misshapen trochlea that fails to adequately engage the patella during early knee flexion.
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It is a key predisposing factor for patellofemoral instability and Lateral patellar dislocation .
| Aspect | Description |
|---|---|
| Normal trochlea | Concave groove accommodating the patella, with higher lateral facet |
| Dysplastic trochlea | Flattened, convex, or irregular → poor patellar engagement |
| Critical flexion window | ~0°–30° flexion is when the patella is most vulnerable to instability |
| Category | Factors |
|---|---|
| Congenital/anatomic | Primary dysplasia (isolated or syndromic) |
| Acquired | Rare; usually congenital |
| Associated abnormalities | Patella alta, MPFL rupture, TT–TG >20 mm, valgus alignment |
X-ray:
| Feature | Finding |
|---|---|
| ‣ | Line of trochlear floor crosses anterior contour of lateral condyle |
| Supratrochlear spur | Bony bump at superior trochlear margin (Dejour types B–D) |
| ‣ | Two lines from asymmetrical facets (medial hypoplasia) – type D |
MRI
| Feature | Description |
|---|---|
| Trochlear depth | Normal ≥3 mm; <3 mm indicates flattening |
| Lateral trochlear inclination (LTI) | <11° = dysplastic |
| Facet asymmetry | Medial-to-lateral facet ratio <0.4 is abnormal |
| Dejour classification | Used to stratify dysplasia severity |
Dejour classification:
| Type | Radiologic Features | Severity |
|---|---|---|
| A | Shallow trochlea, normal contour, LTI <11°, normal X-ray | Mild |
| B | Flat or convex trochlea + crossing sign + supratrochlear spur | Moderate |
| C | Asymmetrical facets (medial hypoplastic, lateral convex) + double contour | Severe |
| D | All above + cliff pattern; abrupt slope drop between facets | Most severe |
Dejour classification system for trochlear dysplasia: (A) Type A dysplasia: Trochlea is shallow but still symmetrical and concave, with the crossing sign on the lateral radiograph. (B) Type B dysplasia: Trochlea is flat or convex, with the crossing sign and a trochlear spur on the lateral radiograph. (C) Type C dysplasia: Lateral facet is convex and the medial facet is hypoplastic, with the crossing sign and double contour sign (subchondral sclerosis of the medial hypoplastic facet) on the lateral radiograph. (D) Type D dysplasia: Cliff pattern is present on the axial view; the crossing sign, a supratrochlear spur, and the double contour sign can all be seen on the lateral radiograph.
Tarchala, M., Kerslake, S. & Hiemstra, L.A. Sulcus-Deepening Trochleoplasty for High-Grade Trochlear Dysplasia: Demystifying the Procedure—a Review of the Current Literature. Curr Rev Musculoskelet Med 16, 538–549 (2023). https://doi.org/10.1007/s12178-023-09868-6
Classification of trochlear dysplasia on MRI. A. Type A dysplasia. Axial T2 with fat suppression image demonstrating a shallow trochlear groove, but the trochlear structures (dashes) are maintained. B. Type B dysplasia. Axial T2 with fat suppression image demonstrating almost flat (dashes) and horizontally oriented trochlear surfaces. C. Type C dysplasia. The axial proton density image demonstrates a flat and obliquely (dashed) oriented trochlear surface. D. Type D dysplasia. Parasagittal proton density image demonstrating prominent bone protrusion (arrow). Lipohemarthrosis with lipid (black asterisk) fluid level (white asterisks) was identified.
Kim HK, Parikh S. Patellofemoral Instability in Children: Imaging Findings and Therapeutic Approaches. Korean J Radiol. 2022 Jun;23(6):674-687. https://doi.org/10.3348/kjr.2021.0577
An example of high-grade trochlear dysplasia with MRI correlation. A Lateral radiograph shows high-grade trochlear dysplasia evidenced by presence of the crossing sign (white arrow) and supratrochlear spur (black arrow). There is also a large joint effusion (arrowhead). B Axial T2-weighted MRI showing trochlear dysplasia with a torn medial patellofemoral ligament and medial patellar contusion (black arrow) with a corresponding contusion of the lateral femoral condyle (white arrow)
Davis, J.R., Yurgil, J.L., Van Geertruyden, P.H. et al. Radiographic clues to the unstable knee: are findings of trochlear dysplasia on lateral knee radiographs reliable and predictive of patellar dislocation?. Emerg Radiol 28, 1143–1150 (2021). https://doi.org/10.1007/s10140-021-01968-5
Associated radiologic measurements: