The knee is a weight bearing joint and therefore for all intents and purposes, knee x-rays should be taken while weight bearing. The purpose of knee radiographs is to assess the bony structure of the knee and specifically to define the presence of fractures and also to assess for degenerative disease within the joint. Many different types of plain x-rays can be taken, but all investigations should have a purpose and should answer specific questions There may be some condition of the knee that can be seen only from a particular angle. And for that purpose various x-rays are taken from various angles so that the position of knee can be known. Example of these kind of x-rays are AP view i.e. anterior position view, lateral view, weight bearing view, tunnel view etc. The areas covered in lateral view are distal femur, proximal tibia & fibula, patella and knee joint and is done to get a clear image of patella in lateral profile.
For the lateral view of knee there should be specific position that the patient should be in. The position requires for lateral view knee x-ray is as follows- The patient should turn onto the affected side. The patient should bring the knee forward in flexion and extend the other extremity behind it. A flexion of 20 to 30 degrees is preferred. Then the central ray is directed to the knee joint. The radiologist explains the position the patient and the patient should follow strictly follow the instructions of the radiologist.
In good lateral image, the medial and lateral femoral condyles project over each other and the patellofemoral joint is projected free. Oblique images can be obtained by exorotation of the knee from the neutral lateral oblique image or medial oblique image.
In trauma setting, a horizontal beam lateral view may be preferred over the standard lateral image in order to establish lipohemarthrosis.
After the patient is in the preferred position a central ray is directed perpendicular to the film, entering the knee at the anterior margin of the medial epincondyle to lateral. Another kind of lateral view is horizontal beam lateral view in which the x-ray beam enters from lateral end to medial end of the knee thus slight angle downtilt is commonly used.