When positioning for an AP knee, how should the lower extremity be oriented?

For an anteroposterior (AP) knee projection, the lower extremity should be oriented in a manner that accurately captures the anatomical structures of the knee joint. Rotating the knee internally approximately 5 degrees is crucial because this adjustment aligns the femoral condyles parallel to the film or sensor. When the knee is positioned correctly in this manner, it minimizes the risk of distortion and ensures that the resulting image presents a true representation of the joint anatomy. This technique enhances the visibility of critical details such as joint space and bony landmarks, which are essential for accurate diagnosis.

In contrast, other positioning options may lead to inaccurate imaging. A neutral position does not provide the correct alignment necessary for the AP projection, while external rotation can cause misalignment of the femoral condyles, leading to a less accurate image. Internally rotating more than 5 degrees could also distort the anatomy of the knee and affect the quality of the radiograph, thereby making 5 degrees the optimal choice for proper anatomical positioning.