Juan Pablo Martinez
Trochlear dysplasia is a common problem in patients with patellofemoral instability. Most descriptions and measurements of trochlear dysplasia on slice imaging, generally MRI, are based on the axial plane, which is critical to understanding the relative position of the patella and trochlea. It is also likely, however, that the shape of the trochlea in the sagittal plane can also tell us some important things. The evaluation of the trochlea in the sagittal plane, or lateral on radiographs, is somewhat limited and the literature is sparse.
Trochlear dysplasia is often described using the Dejour classification to help define the type of trochlear dysplasia (types A to D). Often these are further separated into type A as low-grade dysplasia and types B to D as high-grade dysplasia, which has been shown to have good to excellent intraobserver and interobserver agreement. The Dejour classification was initially described on true lateral radiographs, with good overlap of the condyles, both posteriorly and distally. Type A is a shallow trochlea defined by the “crossing sign” where the deepest portion of the bony trochlea is anterior (ventral) enough that it “crosses” or overlaps with the medial and lateral aspect of the trochlea. Type B has a crossing sign present, but there is also a prominence or bump above the trochlea, the “supratrochlear spur.” This protrudes anterior to the anterior femoral cortex. In type C dysplasia, the medial trochlea is hypoplastic, so there is a “double contour” because the shape of both the medial and lateral trochlea are visible on the radiograph; this is in addition to the presence of a crossing sign. In type D, all of these are present. While widely used, the Dejour classification on radiographs does have some limitations, most notably the need to get a true lateral radiograph, as it has been demonstrated that inadequate imaging is unfortunately common in normal daily practice.
Types B and D dysplasia, both with a supratrochlear spur present, are the most common types of trochlear dysplasia that may be treated with a trochleoplasty. This underscores the importance of the spur as a source for directing the patella away from the groove, resulting in instability. Some studies have referenced the supratrochlear spur alone on slice imaging, but this literature is limited. The most anterior (ventral) point in the floor of the trochlea was found, by Pfirrmann and colleagues, to be more anterior in patients with trochlear dysplasia than in patients without trochlear dysplasia. In children, it has been noted that the supratrochlear spur is often present in trochlear dysplasia, even at young ages. Improving our understanding of the supratrochlear spur may help us tailor trochleoplasty techniques most appropriately, including sulcus deepening trochleoplasty vs. grooveplasty where just the spur is removed.
In addition to the supratrochlear spur, it is likely that the length and radius of curvature of the trochlea, in the sagittal plane, are important to patellofemoral mechanics and engagement between the two structures. This area has limited study, with likely the foremost article on the subject from Biedert and colleagues. They describe the lateral condyle index, which compares the superior most aspect of the anterior cartilage on the lateral trochlea to the superior most aspect of the posterior cartilage on the lateral condyle. The posterior cartilage serves as a reference point, and they found patients with patellar instability had more a distal starting point for the anterior cartilage on the lateral trochlea than did patients without patellar instability. This suggests the length of the trochlea, in the sagittal plane, may be a factor in patients with patellar instability.
Given that the anterior femoral cortex serves as a bony constraint to the patella in full extension, anything that protrudes anterior to the femoral cortex, such as the supratrochlear spur, or defines the patellar engagement of the trochlea like medial trochlear hypoplasia or patellar length, can likely impact patellar tracking. Further study of patellofemoral morphology in the sagittal plane may give us new insights into understanding and evaluating trochlear dysplasia.
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Marc Tompkins, MD, is an associate professor of orthopedic sports medicine at the University of Minnesota/TRIA Orthopedic Center.
Juan Pablo Martinez, MD, MSc, is an associate professor of orthopedic sports medicine at Fundación Valle del Lili in Cali, Colombia.
Disclosures: Tompkins and Martinez report no relevant financial disclosures.
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