Knee (AP weight-bearing view)

The Knee AP weight bearing view is a specialised projection to assess the knee joint, distal femur, proximal tibia and fibula and the patella. Often used in the context of orthopaedic appointments to obtain images of the knees in their natural anatomical position. 

This view is often used to asses osteoarthritis, it is not uncommon for the AP view to be bilateral knees. 

Patient position

• the patient is erect against the upright detector with the knee and ankle joint in contact with the detector
• leg is extended
• ensure the knee is not rotated

Technical factors

• anteroposterior projection
• centring point
  • centre of the knee 1.5 cm distal to the apex of the patella
• collimation
  • 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
• orientation
  • portrait
• detector size
  • 24 cm x 30 cm
• exposure
  • 60-70 kVp
  • 7-10 mAs
• SID
  • 100 cm
• grid
  • no

Image technical evaluation

The femoral and tibial condyles should be symmetrical, the head of the fibula is slightly superimposed bit the lateral tibial condyle. The patella is resting on the superior portion of the image superimposing the distal femur.

Practical points

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 wit the image detector.

Very slim patients may require a slight caudal angle to better visualise the joint space in an AP fashion. The opposite applies for larger patients (larger thighs mean the leg may be naturally flexed at rest), requiring a slight cephalic angle. Both angles roughly 5-8 degrees.