Alignment and Positioning

Component alignment is important for stability and proper function and mobility of the prosthesis. Optimal component positioning on radiographs is:

AP Radiograph

  • Femoral component
    • 7 +/- 3 degrees in valgus with respect to the long axis of the femur
  • Tibial tray
    • 90 +/- 3 degrees with respect to the long axis of the tibia
  • Both knee joints should be at same level on AP weight-bearing radiographs
  • The mechanical axis, a line drawn from the center of the femoral head to the center of the tibiotalar joint, should go through the mid portion of the knee joint

Lateral Radiograph

  • Femoral component
    • Horizontal portion of component oriented 90 +/- 3 degrees to the long axis of the femur
  • Tibial tray
    • 10 degree posterior slope with respect to the long axis of the tibia

Baseline AP and lateral radiographs demonstrating appropriate positioning of components.

 

AP standing radiograph of both knees demonstrates subtle leg length discrepancy with the right knee joint more distal than the left.

Mechanical axis line drawn from the center of the femoral heads to the mid portion of the tibiotalar joints falls in the mid portion of the knee bilaterally.

Rotational malalignment is difficult to assess on conventional radiographs. This is better evaluated with CT, which may be indicated for patients with chronically painful knee arthroplasties when rotational abnormalities are suspected.

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