Femoral Trochlear Dysplasia

Femoral trochlear dysplasia can be picked up on lateral knee radiographs and sagittal knee MRs. A too-shallow trochlear groove is suspected of causing patellar instability.

The floor of the trochlear groove forms a dense line that should be posterior to the trochleae along its entire course and is in continuity superiorly with the anterior cortex of the distal femur. This line should also be parallel to the lateral trochlea (Grelsamer and Tedder, 1992).

A positive lateral trochlear sign is seen when the floor of the trochlear groove intersects the anterior femoral condyle (arrow). This is seen in cases of severe femoral trochlear dysplasia (Grelsamer and Tedder, 1992).

Another way for assessing the depth of the trochlear groove is by drawing a line along the anterior cortex of the distal femoral shaft. If the anteriormost point of the floor of the trochlear groove falls more than 3 mm anterior to this line, the trochlear floor is said to be ventrally prominent (Dejour et al, 1994). The midsagittal MRI can also be used for this purpose.

The lateral radiograph shown here is a bit obliqued, but we can see the crossing sign (arrow). We can also see that the anteriormost point of the floor of the trochlear groove falls more than 3 mm anterior to a line drawn along the anterior cortex of the distal femoral shaft (dashed line). The midsagittal MRI also shows this finding as a step-like transition zone from the anterior femoral cortical bone to the trochlea. A small nipple may also be seen at the superior border of the trochlea.

References

• Dejour H, Walch G, Nove-Josserand L, Guier C. Factors of patellar instability: an anatomic radiographic study. Knee Surg Sports Traumatol Arthrosc 1994; 2:19-26.
• Grelsamer RP, Tedder JL. The lateral trochlear sign. Femoral trochlear dysplasia as seen on a lateral view roentgenograph. Clin Orthop Relat Res 1992:159-162.
• Pfirrmann CW, Zanetti M, Romero J, Hodler J. Femoral trochlear dysplasia: MR findings. Radiology. 2000 Sep;216(3):858-64.