Which projection of the patella necessitates the knee to be flexed as far as possible?

The tangential projection of the patella, often referred to as the "sunrise" or "merchant" view, requires the knee to be flexed as far as possible to effectively visualize the patella and its relationship with the femoral condyles. This severe flexion allows the patella to move into the groove formed by the femur, providing a clear view of any abnormalities or conditions affecting the patella. By achieving maximum flexion, radiologists can assess the patellofemoral joint more accurately, which is crucial for diagnosing conditions such as patellar fractures, dislocations, or tendinopathy.

Other projections, such as the AP or lateral projections, do not require the knee to be flexed to this extent, as they focus on different anatomical orientations and relationships that do not emphasize the patella's position in relation to the femur in the same way the tangential projection does. The oblique projection also does not necessitate extreme flexion as it is designed to visualize other aspects of the knee joint, thus distinguishing the tangential projection as the one that specifically requires maximum flexion for optimal visualization of the patella.