*3.3.5 Assessment of trochlear morphology*

The morphology of the trochlea should be carefully assessed on a true lateral view of the knee as trochlear dysplasia is a known risk factor for recurrent patellar instability. On the true lateral radiograph, three anterior lines are visualized: the most anterior line is a projection of the medial femoral condyle, the middle line is a projection of the lateral femoral condyle, and the remaining line is a projection of the floor of the trochlea. Dejour et al. [61] have evaluated trochlear morphology and reported two separate measures in a radiographic study of the factors of patellar instability; First measure is the trochlear bump and the second is trochlear depth. The trochlear bump is defined as the distance between the projection of the anterior femoral cortex and the projection of the trochlea, which can be anterior positive or posterior negative. The trochlear bump was greater than +3 mm in 85% of patients with objective patellar instability [61]. The trochlear depth is defined as the depth of the trochlea along a line 15 degrees from the perpendicular to the tangent of the posterior femoral cortex. A depth of less than 4 mm was found in 85% of patients with objective patellar instability and in only 3% of controls [61].

One should also look for supratrochlear spur, crossing sign and double contour sign on the lateral radiograph of the knee. The supratrochlear spur is a global prominence of the trochlea. The crossing sign represents an abnormally elevated floor of the trochlear groove rising above the top of the wall of one of the femoral condyles. On the lateral radiograph of the knee, trochlear dysplasia is defined by the crossing sign [62] which refers to the crossing over of the trochlear floor condensation with the condensation of the most prominent aspect of the lateral trochlea and is found in 96% of the population with a history of true dislocation but in only 3% of healthy controls [61, 63]. The double contour sign is a radiographic line that represents the hypoplastic medial facet on the lateral view [64, 65].

Radiographically, trochlear dysplasia is defined by a sulcus angle of greater than 145 degrees as seen on axial radiographic views of the patellofemoral joint [66, 67]. Dejour and colleagues [67, 68] have classified trochlear dysplasia into 4 types as shown in **Figure 4**.

• In type A dysplasia, there is a crossing sign on the lateral radiographs and the trochlear groove is symmetric but shallower than normal, with a sulcus angle greater than 145° on axial images.

#### **Figure 4.**

*Dejour classifications of trochlear dysplasia. Type A: Crossing sign, trochlear morphology preserved (fairly shallow trochlea,* >*145°). Type B: Crossing sign, supratrochlear spur, flat or convex trochlea. Type C: Crossing sign, double contour (projection on the lateral view of the hypoplastic medial facet). Type D: Crossing sign, supratrochlear spur, double contour, asymmetry of trochlear facets, vertical link between the medial and the lateral facet (cliff pattern). Reprinted with permission from: Onor et al. [69].*


The Dejour classification is widely referred to in the literature and currently considered the gold standard for the description of trochlear dysplasia.

#### *3.3.6 The Axial Radiograph*

The axial views as described by Merchant and colleagues [36] and Laurin and colleagues [37, 38] are commonly used for the evaluation of the patellofemoral joint. The axial view is helpful for diagnosing lateral patellar tilt and also provides valuable information about any persistent subluxation or dislocation of the patella. The sulcus angle can be measured on the axial view. Tangential osteochondral fracture of the medial facet of the patella or osteochondral fracture of the lateral femoral condyle may be visualized on an axial radiograph.

#### *3.3.7 Stress Radiographs*

Stress radiography is widely practiced in Europe and less commonly utilized in USA. Stress radiographs are helpful in identifying patients with patellofemoral instability. Measurements on stress radiographs are more reliable predictors of lateral, medial, and multidirectional patellar instability than measurements made

#### *Patellofemoral Instability DOI: http://dx.doi.org/10.5772/intechopen.99562*

on static radiographs [50]. Moreover, they can provide useful objective information when evaluating the results of different treatment regimens. Patients who are unable to relax the extensor mechanism due to pain or who have bilateral symptoms are not candidates for stress radiography [50].
