One area of the knee that is not discussed as often as some of the other components, is a section called the trochlea. This area of the knee is easy to identify. When the knee is bent, the undersurface of the kneecap (the patella) lies in an area known as the trochlear groove. The sides of the patella and the walls of the groove should be almost parallel. The normal shape of the trochlea groove is concave. In fact, the outside, or lateral aspect of the trochlea groove has a higher bump than the inside part. This allows the patella, or kneecap, to glide down the central aspect of the distal femur rather effortlessly due to its bony constraints. This is important because the patella serves as a fulcrum to increase the overall strength and efficiency of the quadriceps muscles of the thigh.
When the trochlea is flat, or even possibly has a convex or dome shape, it is known as trochlear dysplasia which usually is referred to as an unstable kneecap. In these circumstances, the trochlea is not shaped normally, and the patella does not have the normal bony constraints to provide stability. Thus, one needs to rely on the medial patellofemoral ligament and their quadriceps mechanism to hold the patella in place.
A diagnosis of trochlea dysplasia is usually made by a thorough physical exam and radiographic work-up. Patients with trochlear dysplasia often have increased medial and lateral patellar translation near full extension and at 45 degrees of knee flexion. They may also have an apprehension test where they feel their patella is going to dislocate laterally. On plain X-rays, one can see a “crossing sign” on the lateral knee X-ray that would indicate that the trochlea groove is both flat and shallow. The 45-degree patella X-ray would also show a decrease in size of the groove, flattening, and sometimes a dome shape. A frontal view, or AP, radiograph may show some hypoplasia, or a decrease in size, of the medial femoral condyle.
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