Four routine x-ray views are necessary for a good evaluation, AP standing of both knees with the quadriceps contracted, true lateral, flexed AP (notch view), and Merchant view.
If the AP standing view is taken with the quadriceps relaxed, the patella will not be seen in its functional position. The position may appear normal but not be normal. A well aligned patella with no patella Alta will be centered over the femoral trochlea. If there is patella Alta and increased Q Angle, an AP x-ray taken with the quadriceps contracted will show the patella above and lateral to the trochlea, high and outside (like in baseball!). This patellar position is frequently associated with patellar instability.
Patella Baja can be assessed on the lateral view. The AP view will show the patella low in the trochlea. This position is associated with patellar pressure.
The Merchant view, described by Dr. Allan Merchant, is done with the knee in 45 degrees of flexion. It shows the femoral trochlea of both knees. Shape and contour of the trochlea and patella, patellar position relative to the trochlea, and thickness of the articular cartilage can be assessed. Degenerative arthritis of the patellofemoral joint can be assessed as well. However, it is common to have significant patellar or femoral articular surface breakdown not be obvious by x-ray.