Trochlear dysplasia

Last revised by Ayla Al Kabbani on 30 Jul 2022

Trochlear dysplasia is a morphological deformity of the femoral trochlea and a known association with patellofemoral instability.

The reported prevalence of trochlear dysplasia in recurrent patellar dislocations is ~80% (range 74-85%) 1,15. The latter is most common in the adolescent age group 4,5.

The diagnosis of trochlear dysplasia is usually established by typical imaging features. Since cross-sectional images and radiographs show different aspects of trochlear dysplasia it might be worthwhile acquiring both if there are doubts.

Patients usually present with recurrent lateral patellar dislocation and patellofemoral instability.

Trochlear dysplasia can involve a shallow, flattened or convex trochlear groove +/- a hypoplastic (small) or convex lateral femoral condyle 15. This dysplastic deformity of the most superior aspect of the femoral trochlea, in particular, is a risk factor for patellar dislocation during the transition from full knee extension to early knee flexion. The degree of dysplasia for causing patellar instability is not known 15.

The cause of trochlear dysplasia is not known but trochlear dysplasia may be secondary to patellofemoral maltracking during development 15. Breech position and genetics may be risk factors 15.

Different radiographic methods exist to assess for trochlear dysplasia in true lateral radiographic images of the knee and cross-sectional imaging CT and MRI respectively.

The classification of trochlear dysplasia as described by Dejour 6,13 requires the correlation of three radiographic signs from lateral knee radiographs with cross-sectional imaging 16.

  • type A: crossing sign, normal facet geometry but shallow trochlea
  • type B: crossing sign, trochlear spur on lateral radiograph and flat trochlear groove on cross-sectional imaging
  • type C: crossing sign and double contour on lateral radiograph with medial facet hypoplasia and lateral facet convexity
  • type D: crossing sign, trochlear spur and double contour on the lateral radiograph and a cliff-like pattern between the medial and lateral facets

Initial evaluation based on a true lateral knee radiograph 1:

  • crossing sign: the intersection of the trochlear floor and lateral femoral condyle contours
  • supratrochlear spur: bony spur at the most proximal portion of the trochlea
  • double contour sign: significantly smaller medial femoral condyle
  • trochlear bump: the increased distance between the anterior trochlear groove and extension of the anterior cortex of the distal femur
  • sulcus angle

CT can demonstrate the three-dimensional shape of the trochlea. Depending on the degree it might show a shallow or flat contour, a convexity of the lateral facet, hypoplastic medial facet or a cliff-like pattern on the axial images 6,16.

Like CT, MRI will demonstrate abnormalities not only in the bony shape but also in the cartilaginous contour of the femoral trochlea, which does not always follow the osseous anatomy. In addition, different measurements for the assessment have been described 7-11:

  • lateral trochlear inclination (LTI)
    • inclination angle between the lateral trochlear facet and a posterior condylar tangential line
    • evaluation on the most cranial/ proximal axial slice containing trochlear cartilage
    • an angle <11° indicates trochlear dysplasia (reported sensitivity and specificity: 93%/87%) 7
    • generally considered to be the most reliable measurement 15
  • trochlear depth (TD)
    • different methods with different normal values exist also depending on whether bony or cartilaginous contours are used 8-10
    • a simple method is to measure the distance between the floor of the trochlear and a tangential line connecting the most anterior points of the medial and lateral facet 10
    • a trochlear depth <3 mm indicates trochlear dysplasia
  • ventral trochlear prominence
    • distance between the anterior cortex of the distal femur and the most anterior cartilaginous point of the trochlear groove on a sagittal plane through the trochlear groove 8
    • a distance >8 mm indicates trochlear dysplasia (sensitivity and specificity: 75%/83%)
  • medial condyle trochlear offset (MCTO)
    • measured as the distance between the medial condyle and a tangential line through the trochlear groove parallel to the posterior surfaces of the femoral condyles 9
  • facet asymmetry (FA)
    • the ratio between medial versus lateral trochlear facet length calculated as (medial facet) / (lateral facet) is one way to calculate it 8 
    • a ratio of <40%, in this case, indicates trochlear dysplasia

There is a poor correlation between the classical and still commonly used Dejour classification system and the measurements derived from axial MR images. This should be taken into account when indicating trochlear dysplasia. The radiological report should, therefore, contain a qualitative description as well as the metric used for the diagnosis of trochlear dysplasia.

Trochlear dysplasia is a predisposing factor of patellofemoral instability and should be only treated in that context.

The treatment of high-grade trochlear dysplasia is trochleoplasty, which aims at correcting the trochlear depth abnormality by recreating a centralized groove, which facilitates the entry of the patella during early knee flexion.

In symptomatic patients with recurrent patellar dislocations and failure of previous patellar alignment surgery or non-operative management or trochleoplasty can be proposed as an indication 4.

Different surgical techniques of trochleoplasty exist and include 12-14:

  • sulcus-deepening trochleoplasty
  • recession-wedge trochleoplasty
  • elevation of the lateral trochlear facet

The clinical outcome seems to depend on the type of dysplasia and seem to show better results after surgical correction of Dejour type B and type D dysplasia.

The need for a simplified approach in respect to grading of trochlear dysplasia assessed on MR images has been formulated and a simplified grading into low-grade and high-grade trochlear dysplasia has been proposed 9. This is also due to a poor correlation with the original Dejour classification system 9, which is considered to be difficult to understand 15.

A review regarding the quality assessment of radiological measurements identified the following metrics as most useful for the assessment of trochlear dysplasia 11: the lateral trochlear inclination (LTI), the crossing sign, the trochlear bump, the TT-TG for planning treatment, the trochlear depth and the ventral trochlear prominence.

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Cases and figures

  • Figure 1: reporting aid
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  • Figure 2: reporting aid
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  • Figure 3: reporting aid
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  • Case 1: in transient lateral patellar dislocation
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  • Case 2: mild with patellar chondral avulsion fracture
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  • Case 3
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  • Case 4
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  • Case 5: type C
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