Trochlear depth

Last revised by Dai Roberts on 20 Nov 2021

Trochlear depth (TD) measures the depth of the trochlear groove in relation to the femoral condyles.

Trochlear depth is used for the assessment of trochlear dysplasia, a dysplastic deformity of the femoral head, which is a known risk factor for patellofemoral instability 1-6.

A review of the quality assessment of measurements for trochlear dysplasia identified trochlear depth as one of the more useful measurements 3.

On a true lateral radiograph of the knee, a line at an angle of 15° from a cutting point of a tangent to the posterior femoral diaphyseal cortex and a perpendicular running tangent of the most proximal part of the femoral condyle is used to measure trochlear depth as the distance between the trochlear groove and the lateral femoral facet 1,3.

Different methods exist. The method introduced by Pfirrmann measures the distance of the trochlear groove and the medial and lateral trochlear facets to a tangential line of both femoral condyles. To calculate trochlear depth the distance from the trochlear groove from the before mentioned tangent is subtracted from the average of the two distances between the medial and lateral facet and that tangent 2. The measurement was conducted at different levels above the knee joint and found most useful at 3 cm above the joint line 2.

A simpler technique described later just measures the distance between the trochlear floor and a tangential line connecting the most anterior points of the medial and lateral facet 4,5.

A threshold value of ≤4 mm was considered pathological in the view of Henri Dejour.

A trochlear depth of ≤3 mm with Pfirrmann’s method resulted in a sensitivity and specificity of 100% and 96% in that study 2.

The measurement of trochlear depth on the lateral radiograph was described by Henri Dejour 1.

The first trochlear depth measurement on MRI was introduced by CW Pfirrmann 2.

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Cases and figures

  • Figure 1 - MRI
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