*3.3.8 Computed Tomography*

Computed tomography (CT) has been shown to be more accurate in detecting patellar malalignment than conventional axial radiography [70]. Among the advantages of CT over plain radiography are that there is no image overlap or distortion and that there are precise reference points for reliable measurements [50]. The conventional axial radiographs cannot assess the patellofemoral joint with the knee in full extension, whereas the cross-sectional nature of the CT scan allows the patellofemoral joint to be evaluated in such position and enhances detection of early lateral subluxation of the patella (within 0 to 30 degrees of knee flexion). Examination of the knee in extension is crucial because most patellar instability occur in the first 30 degrees of knee flexion, before the patella is constrained by the trochlea [30]. Measurements of congruence angle, lateral patellofemoral angle, patellar tilt, TT-TG distance, and rotational abnormalities of the femur and tibia have been studied extensively using a CT scan.
