Trochlear Dysplasia of Femur

Trochlear dysplasia refers to a pathologic alteration in the shape of the femoral trochlea. Trochlea is also called facies patellaris, intercondylar groove, or intercondylar sulcus.

Normal trochlea is sufficiently concave to guide and retain the patella throughout the normal range of movement.

However trochlear dysplasia may cause the groove to be shallower than normal or even flat or convex.

This predisposes the patient to patellar instability or even dislocation.

The primary contributors to patellar instability are trochlear dysplasia, quadriceps dysplasia/patellar tilt, patella alta and tibial tuberosity–trochlear groove distance.

In trochlear dysplasia, the trochlear joint surface is flattened proximally, and the concavity is less pronounced distally or the trochlear surface may even become convex with increasing hypoplasia of the medial joint surface.

As trochlear dysplasia is frequently bilateral, it is said to be a developmental anomaly.

The criteria for diagnosing trochlear dysplasia were first defined for x-rays but MRI has shown its superiority over conventional x-rays and is the procedure of choice for diagnosis of trochlear dysplasia.

Biomechanics of the Proximal Trochlea

In full knee extension, the patella lies just superolateral to the most superior portion of the trochlea. The patella starts moving at approximately 10° of flexion downward from this position to enter the trochlea.

And this explains from another angle

The most proximal part of the is the shallowest and it is the part that patella encounters in very early flexion.

Being shallow this part offers the least stability to the patella and here the patella is most susceptible to dislocation. [Hence the reason that patellar dislocation often occurs in the presence of patella alta as it articulates with the proximal shallow portion of the trochlea for a longer time]

With further flexion, the trochlear floor deepens and is able to provide greater stability.

provides increased patellar stability. Thus, it is rare for the patella to dislocate from the distal, or more inferior, portions of the trochlear groove. Consequently, any trochlear measurements that are relevant to patellar stability must be made proximally, that is, in the early stages of knee flexion.

Diagnosis of Trochlear Dysplasia

Xrays

Crossing Sign

Crossing sign is seen in about 96% of abnormal cases and is seen in the lateral view of knee x-ray. The crossing sign is positive when the contours of the trochlear floor and of the lateral femoral condyle intersect at any level.

This indicates that, at that level, the lateral side of the trochlea is flat or, equivalently, the trochlear floor is flush with the lateral condyle.

Trochlear Depth

For trocheal depth draw lines as follow

• A vertical line is along the posterior cortex of the femoral diaphysis (Green dotted line)
• Draw a second horizontal line (dashed green line) is drawn at the vertical level of the most proximal point of the posterior contour of the femoral condyles [Shown by blue arrow]. This marks the concavity of the femoral metaphysis giving way to the convexity of the posterior femoral condyles.
• Draw third line anteroinferiorly[beginning at the intersection of above two lines] and angled 15° from the horizontal line (solid green line).
• Measure distance between the osseous trochlear floor and lateral condyle(red line).

A value of 4 mm or less is abnormal. Trochlear depth is shallow in 85% of abnormal cases.

Trochlear Bump

For measuring the trochlear bump draw a line along the most distal 10 cm of the anterior cortex of the femoral diaphysis. [green line]

Measure the distance from this line at the most anterior extent of the trochlear floor(red line).

A value of +3 mm or greater is considered pathological.

[Trochlear beak/spur or spur is an angular projection of the most proximal portion of the trochlea and should not be confused with bump].

Double Contour Sign

The double contour sign indicates that the contour of the medial femoral condyle is significantly smaller than that of the lateral femoral condyle.

Trochlear Angle or Sulcus Angle

The trochlear angle refers to the opening angle of the trochlea as visualized on a 30° flexion axial radiograph. The normal value is >145 degrees.

It is shallow in 65% of abnormal cases.

Classification of Trochlear Dysplasia

Signs of trochlear dysplasia are found in more than 85% of patients with patellar dislocation.

Following classification by Dejour describes four types of trochlear dysplasia

Type A

The normal shape of the trochlea 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 the medial facet being hypoplastic, which results in the flattened joint surface forming an oblique plane

Type D

Findings in Different Types of Trochlear Dysplasia

Type A

•  Crossing sign
• Axial CT shows shallow trochlea [but not flat.]

Type B

• Trochlear spur
• On axial images trochlea is flat.

Type C

• No trochlear spur
• Double contour sign
• On axial images, these trochlea is convex, with a hypoplastic medial condyle.

Type D

• Double contour sign
• Crossing sign
• Trochlear spur
• On axial images, these trochleae have a sharp convex “cliff” separating the medial and lateral facets.

In addition to the features of type C, a vertical link between the medial and lateral facets (cliff pattern on parasagittal images).

MRI in Trochlear Dysplasia

Axial and sagittal MR images allow better and accurate identification of the type of trochlear anomaly.

Trochlear dysplasia can be evaluated at MR imaging by determining lateral trochlear inclination, trochlear facet asymmetry, or trochlear depth.

Lateral Trochlear Inclination

It is the angle formed between the plane of the lateral trochlear facet subchondral bone and a tangential line through posterior femoral condyles. An angle of <11º is considered abnormal.

Trochlear facet asymmetry

This is the ratio of a width of the medial trochlear facet width to lateral trochlear facet width and measured in the axial plane. A ratio of <0.4 is considered abnormal [that means the medial facet is <40% the width of the lateral facet].

Trochlear depth

This measures the inset depth of the trochlear groove or sulcus relative to the mean of the medial and lateral femoral condyle outsets.  It is determined by axial imaging at the same level as the trochlear facet asymmetry. A trochlear depth of <3 mm is considered abnormal.

Treatment of Trochlear Dysplasia

Trochlear dysplasia can be corrected using surgery.

Albee trochleoplasty

It involves a  coronally-oriented opening osteotomy of the lateral femoral condyle which steepens the inclination of the lateral trochlea.

But it does not address the apparent cause i.e. elevated trochlear floor. The procedure tends to lift the patella further away from the femur, narrowing the angle between the vectors of the quadriceps and patellar tendons and causing more cartilage loading.

Deepening Trochleoplasty

This procedure involves removing a near full-width coronal slab of cancellous bone from the anterior femur and then bluntly depressing the central trochlear surface into the surgical defect to create the new trochlear floor (Figure 15). A procedure described by Bereiter et al.29 is similar but involves a thinner flap of the trochlear surface.

Wedge Resection Trochleoplasty

For those trochleae that contain a prominent bump. A wedge of cancellous bone is removed from the anterior femur, and the superior aspect of the trochlea is tilted posteriorly into the surgical defect to make it flush with the anterior diaphyseal cortex.

References

• Fulkerson JP. Disorders of the Patellofemoral Joint. 4th ed. Philadelphia, PA: Lippincott, Williams, & Wilkins; 2004.
• Rémy F, Chantelot C, Fontaine C, Demondion X, Migaud H, Gougeon F. Inter- and intraobserver reproducibility in radiographic diagnosis and classification of femoral trochlear dysplasia. Surg Radiol Anat. 1998;20(4):285–9.
• Dejour D, Saggin P. The sulcus deepening trochleoplasty—the Lyon’s procedure. Int Orthop. 2010 Feb;34(2):311–6.
• Pfirrmann CW, Zanetti M, Romero J, Hodler J. Femoral trochlear dysplasia: MR findings. Radiology. 2000 Sep;216(3):858–64.

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