When performing a recumbent lateral projection of the knee, what angle should the patient's knee be flexed?

In a recumbent lateral projection of the knee, the optimal flexion of the knee is typically set at 20-30 degrees. This degree of flexion is crucial for adequately visualizing the joint space and the surrounding structures. When the knee is flexed within this range, it helps to ensure proper alignment of the femur and tibia, facilitates better radiographic contrast, and minimizes superimposition of the anatomical structures.

Flexing the knee too much, such as in the range of 30-40 degrees or even 40-50 degrees, can lead to distortion of the joint space on the radiograph, complicating the interpretation of the image. Less flexion, such as 10-20 degrees, may not provide sufficient separation between the femoral and tibial condyles, which is essential for a clear view of potential joint pathology. Therefore, utilizing a 20-30 degree angle achieves the balance needed for an appropriate lateral projection of the knee.