Several measurements obtained from the axial, or sunrise, view have traditionally been used to evaluate for malalignment. There are three main radiographic patterns of malalignment (described with CT). These include subluxation of the patella with and without tilt, and tilt without subluxation . Conditions that lead to malalignment and influence patellofemoral stability, such as depth of the trochlea, can be measured with imaging. The most common indices described in the literature are the lateral patellofemoral angle (tilt), the congruence angle, and lateral patellar displacement [38-40]. Other measurements that have been shown to potentially have merit are the Q angle and the trochlear-tubercle distance [41,42].
The lateral PF angle is calculated on an axial radiograph obtained at 20 degrees of flexion by measuring the angle of the lateral patellar facet compared with a line drawn across the femoral condyles. A study by Laurin and colleagues  showed that 97% of normal patellae open laterally. If the angle opens medially, or is parallel, then the patella is tilted (external rotation) (Fig. 11) . Lateral patellar displacement is measured by comparing the medial margin of the patella on an axial view to the medial femoral condyle. If is more than 1 mm lateral in relation to the medial condyle apex, it is considered subluxed (Fig. 12) [39,43].
The congruence angle, as described by Merchant , is measured from a 45 degree of flexion axial film. The measurement is made by bisecting the sulcus angle to create a zero reference line. Then a line is drawn from the lowest point on the patella to the sulcus angle point. The angle created is then measured. The ''normal'' value is -6 ± 11 degrees. Values outside this range are an
indicator of patellar subluxation (Fig. 13) . These values may also be obtained with CT and MR.
The Q angle is a measure of the angle formed between a line drawn from the tibial tubercle and a line drawn from the middle of the patella and anterior superior iliac spine. The normal value is 15 degrees . An increased Qangle implies that the tibial tubercle is more lateral than normal. Thus, the patella experiences a lateral force with contraction of the quadriceps, predisposing it to lateral subluxation or dislocation. The Q angle can be measured by scout CT image or physical exam.
The tibial tubercle distance (T-T distance) can be measured with axial imaging, and can substitute for the Qangle. This is a measure of the tibial tubercle in relation to the trochlear nadir. Two sagittal lines are drawn, one through the tibial tubercle, the other through the bottom of the trochlear groove. The lines' difference in position in the axial plane is the T-T distance. One study showed high specificity for maltracking if the tibial tubercle distance was 2 cm or greater .
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