Knee (AP view)
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The knee anteroposterior view is a standard projection to assess the knee joint, distal femur, proximal tibia and fibula and the patella.
This view demonstrates the distal femur and proximal tibia/fibula in their natural anatomical position allowing for assessment of suspected dislocations, fractures, localizing foreign bodies and osteoarthritis.
patient is supine on the table with the knee and ankle joint in contact with the table
leg is extended
ensure the knee is not rotated
center of the knee 1.5 cm distal to the apex of the patella
superior to include the distal femur
inferior to include the proximal tibia/fibula
lateral to include the skin margin
medial to include medial skin margin
24 cm x 30 cm
Image technical evaluation
The femoral and tibial condyles should be symmetrical, with the head of the fibula slightly superimposed by the lateral tibial condyle. The patella is resting on the superior portion of the image superimposing the distal femur.
The fibula head is a great indication of rotation, if the fibula head is entirely superimposed, the image is not AP; to correct this you must internally rotate until the knee is in even contact with the image detector.
Very slim patients may require a slight caudal angle to better visualize the joint space in an AP fashion. The opposite applies for larger patients (thicker thighs mean the leg may be naturally flexed at rest) and would require a slight cephalic angle. Whether cephalic or caudal, an angle of approximately 5-8° is adequate.