Knee (AP weight-bearing view)

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The knee AP weight-bearing view is a specialized projection to assess the knee joint, distal femur, proximal tibia and fibula and the patella.

Indications

Knee AP weight-bearing views will often be used in the context of orthopedic appointments to assess the alignment and degree of arthropathy when weight-bearing.  This view is often used to assess osteoarthritis as non-weight bearing views can underestimate the degree of joint space loss.  It is common for the AP view to include both knees so to use the contralateral side as a comparison.

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
  • centering point
    • center 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 visualize 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.

Radiographic views