Trochlear dysplasia is known to be a major risk factor for patellofemoral instability.¹,²,³,⁴ Dejour et al⁵ found that 96% of patients with a history of a true patellar dislocation had evidence of trochlear dysplasia.

In trochlear dysplasia, the trochlea is shallow, flat, or dome shaped mainly at its proximal part with inadequate bony resistance to lateral patellar dislocations, where the patella engages into the trochlea.⁵,⁶,⁷ It may lead to patellofemoral maltracking, increased contact pressures, patellar instability, and isolated patellofemoral arthritis.¹,³,⁴,⁸

It is recognized that the central/lateral flattening results from an increase in central bone or an elevation of the central trochlear region. The patella engages from knee extension to flexion into the trochlear groove; therefore, the dysplasia of the proximal extent is mainly responsible for the disengagement and possible dislocation of the patella in patients with trochlear dysplasia.¹ The highest risk of lateral patellar dislocation is between 0° and 20° of knee flexion because of disengagement with the trochlea and a slack medial patellofemoral ligament (MPFL) restraint.

Dejour⁵ has emphasized that the aim of treatment in patellar instability with trochlear dysplasia is to restore more normal anatomy by removing this excess central bone, thus deepening the central trochlea.

Deepening trochleoplasty has two main described methods: the Dejour technique and the Bereiter technique.³,⁹,¹⁰,¹¹,¹²,¹³,¹⁴,¹⁵ In the former, the new groove is formed by thick flap trochlear osteotomy and depressing the two sides into a deepened central groove. In the latter, a thin flexible flap of articular cartilage is raised and molded in the deepened groove. The thin flap technique was first introduced by Bereiter in 1994.⁹

Trochlear dysplasia was first described on strictly lateral conventional radiographs by Maldague and Malghem.¹⁶

In 1994, Dejour et al⁵ described three signs on the lateral radiograph that may indicate trochlear dysplasia, including a double contour, a supratrochlear bump, and a crossing sign.

Lippacher et al¹⁷ found that these signs performed adequately among children with trochlear dysplasia, but that magnetic resonance imaging (MRI) should remain the benchmark for assessment.

Dejour et al further developed the analysis of trochlear dysplasia into four types.³,¹⁸

The most commonly used classification system is the four-grade Dejour classification. The four Dejour classes are defined on true lateral radiographs and axial computed tomography or magnetic resonance scans: Type A is characterized by the crossing sign and a fairly shallow trochlea; type B by the crossing sign, a trochlear spur, and a flat or convex trochlea; type C by the crossing sign, a double contour representing a hypoplastic medial condyle, asymmetry of trochlear facets, and a convex lateral facet; and type D by the crossing sign, a supratrochlear spur, a double contour sign, asymmetry of the trochlear facets, and a cliff pattern.

However, the Dejour classification of trochlear dysplasia shows low agreement between lateral conventional radiographs and axial MRI.⁶,¹⁹ The two-type classification of low-grade versus high-grade trochlear dysplasia or dysplastic trochleae with and without a supratrochlear spur might be more precise, clinically more relevant, and, therefore, more adequate.⁶,¹⁹

For clinical practice, the indication for trochleoplasty includes a domed or laterally facing chondral surface with closed or closing physes. A flat trochlea or a shallow groove is not considered an indication for trochleoplasty.⁷,¹²

For clinical purposes, the discrimination between low-grade and high-grade dysplasia is an important distinction because prognosis and treatment mainly depend on the severity of trochlear dysplasia.

Table 19.1 lists indications and contraindications for thin flap trochleoplasty.

The first dislocation frequently occurs while playing sport (72%), less frequently during activities of daily living (21%), or rarely as a result of direct trauma (7%).¹⁸,²⁰

Often, dislocation of the patella is preceded by several years of anterior knee symptoms, including pain, giving way, and a sense of unease.

Patients frequently express poor confidence in their activity levels and sports performance.¹⁸

Knee evaluation should include clinical examination, assessment of symptoms, crepitus, range of motion (ROM), patellofemoral pain, and patellar apprehension.

The presence of a J-sign is a strong indicator for the presence of trochlear dysplasia.

Imaging always includes plain anteroposterior and lateral radiographs and axial MRI of the distal femur, including the supratrochlear region (Figure 19.1).

A true lateral radiographic view is necessary to assess the radiologic signs of trochlear dysplasia, patella alta, and malalignment and to exclude further skeletal abnormalities.

On craniocaudal axial MRI scans, trochlear dysplasia must be analyzed along the entire distal femur and also proximal to the cartilaginous trochlea to investigate the different characteristics of trochlear dysplasia, especially hypoplasia of the medial condyle and the supratrochlear spur⁶ (Figure 19.2).

Because, in most cases, different risk factors can be found in patients with patellofemoral instability, a thorough preoperative planning is essential to evaluate these factors.

The aim should be to identify all relevant anatomic risk factors that may contribute to patellar instability. Based on the findings, surgical intervention should be individualized for any single patient with correction of the underlying relevant pathologies according to the “menu à la carte” introduced by the Lyon Group of Dejour.