Update on Trochlear Dysplasia Classifications

Juan Pablo Martinez-Cano, MD, MSc, PhD, COLOMBIA Tomas Pineda, MD, CHILE

ISAKOS eNewsletters   Current Perspective 2025   Not yet rated

Introduction

Trochlear dysplasia classifications have evolved since the first classification described by Henri Dejour et al. in 1990.¹ While the first classifications were based on morphological features or signs, the most recent proposal involves objective data that should be measured. This quantification has improved the interobserver reliability. This article reviews the available classifications along with their features, advantages and disadvantages.

Henri Dejour Classification

Together with Gilles Walch, Philippe Neyret, and Patrice Adeleine, Henri Dejour described the first trochlear dysplasia classification system with use of true lateral radiographs. They described the crossing sign and identified three different types of dysplasia: Type I (with a proximal crossing sign as the mild variant), Type II (with condylar asymmetry, or medial condyle hypoplasia, which David Dejour would later call the double contour sign), and Type III (with a distal crossing sign [a longer groove with trochlear dysplasia] as the more severe variant).¹ This system was criticized for showing low interobserver reliability but was the basis for the future classifications.

David Dejour’s First Classification

David Dejour’s first trochlear dysplasia classification system, which combines lateral radiographs and an axial computerized tomography (CT) scan or magnetic resonance imaging (MRI) slice, is the most popular system. This system includes 4 types: Type A (characterized by the crossing sign only), Type B (characterized by the crossing sign as well as the presence of a supratrochlear spur), Type C (characterized by the crossing sign as well as a double contour), and Type D (characterized by the crossing sign, a double contour, and a supratrochlear spur) (Figure 1).² Today, the term bump or anterior prominence is preferred over the term supratrochlear spur as this sign does not always have a spur shape. Similar to Henri Dejour’s classification, this classification showed poor interobserver reliability.³

Figure 1. The 4 types of trochlear dysplasia according to first David Dejour classification system.²

The Oswestry-Bristol Classification

The Oswestry-Bristol Classification (OBC) is a MRI-based system that categorizes trochlear dysplasia into four intuitive grades on the basis of cartilage surface morphology in the axial plane: normal, shallow, flat, and convex.⁴ This simplicity has led to high intraobserver agreement, making it attractive in clinical settings. While the classification effectively reflects general trochlear shape and correlates with instability patterns, its qualitative, single-plane perspective may oversimplify complex anatomy. One of the limitations of this system is that it does not allow for the identification or quantification of the anterior prominence or bump, which is critical for determining the need for trochleoplasty, thereby limiting its utility in surgical planning.

David Dejour’s MRI-Based Classification

The recently published MRI-based Dejour classification (v3.0) represents a significant shift from the original radiograph and CT-based Dejour classification by relying exclusively on quantitative MRI parameters.⁵ It is based on a biplanar MRI analysis and includes 3 objective measurements: the sulcus angle (Figure 2), the lateral trochlear inclination (LTI) (Figure 3), and the central bump height (Figure 4). According to the value of these measurements, the classification has 4 grades: 0 (normal), 1 (low grade), 2 (moderate grade), and 3 (high grade) (Table 1).

The classification is structured to represent a progressive spectrum of trochlear dysplasia severity and is intended to assist with treatment decision-making. Moreover, the classification’s complexity, based on multiple precise measurements, may challenge its adoption in everyday practice. It remains to be seen whether this level of anatomical detail will prove practical and impactful in guiding real world surgical decision-making.

Figure 2. The sulcus angle is measured on the first axial MRI slice in which the trochlear cartilage is visible, showing the sulcus formation and the medial facet. If the trochlea is convex, it is described as unmeasurable.

Figure 3. The lateral trochlear inclination (LTI) angle is measured on the first axial MRI slice in which cartilage is seen for the lateral and medial trochlear facets. If the trochlea is convex, it is described as unmeasurable.

Figure 4. The bump height is measured in the central-groove sagittal MRI slice with use of a line that follows the anterior cortex of the distal femur and the highest point of the anterior prominence.

Table 1. Dejour MRI-Based Classification

Table 2. Advantages, Disadvantages, and Highlights of Different Classification Systems

Conclusion

The present article summarizes several different trochlear dysplasia classification systems. The first 3 systems are morphology-based and help to provide a quick general idea of the shape of the trochlear groove, whereas Dejour’s MRI-based classification includes objective data, has greater interobserver reliability, and can be useful for treatment decision-making. Understanding these morphological subtypes of the trochlea is important for planning the surgical approach to be used for correcting lateral patellar instability, although further investigations are needed to refine surgical planning and decision-making in this regard.

References

1. Dejour H, Walch G, Neyret P, Adeleine P. La dysplasie de la trochlée fémorale. Revue de chirurgie orthopédique et réparatrice de l'appareil moteur. 1990;76(1):45-54.
2. Dejour D, Le Coultre B. Osteotomies in patello-femoral instabilities. Sports Med Arthrosc Rev. 2018;26(1):8-15.
3. Martinez‐Cano JP, Tuca M, Gallego A, Rodas‐Cortes Y, Post WR, Hinckel B. The Dejour classification for trochlear dysplasia shows slight interobserver and substantial intraobserver reliability. Knee Surgery, Sports Traumatology, Arthroscopy. 2024;32(6):1363-9.
4. Sharma N, Brown A, Bouras T, Kuiper JH, Eldridge J, Barnett A. The Oswestry-Bristol classification: a new classification system for trochlear dysplasia. The bone & joint journal. 2020;102(1):102-7.
5. Dejour DH, de Sanctis EG, Müller JH, Deroche E, Pineda T, Guarino A, et al. Adapting the Dejour classification of trochlear dysplasia from qualitative radiograph‐and CT‐based assessments to quantitative MRI‐based measurements. Knee Surgery, Sports Traumatology, Arthroscopy. 2024.

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