Saturday, February 28, 2015



Pathology Demonstrated:

  • Subluxation of the patella and other abnormalities of the patella and femoropatellar joint are demonstrated.

Technical Factors:

  • IR size - 24 x 30 cm (10 x 12 inches), crosswise
  • Detail screen, small focal spot (grid is not needed because of air gap caused by increased OID)
  • 65 +- 5 kV range: Some type of leg support and cassette holder should be used.
  • mAs: 11


  • Place lead shield over entire pelvic are.

Patient Position:

  • Take radiograph with patient in the supine position with knees flexed 40 degree over the end of the table, resting on a leg support. Patient must be comfortable and relaxed for quadriceps muscles to be totally relaxed.

Part Position:

  • Place support under knees to raise distal femora as needed so they are parallel to tabletop.
  • Place knees and feet together and secure legs together below the knees to prevent rotation and to allow patient to br totally relaxed.
  • Place cassette on edge against legs about 12 inches ( 30 cm) below the knees, perpendicular to x-ray beam.

Central Ray:

  • Angle CR caudad, 30 degree from horizontal (CR 30 degree to femora).
  • Adjust CR angle if needed for true tangential projection of femoropatellar joint spaces.
  • Direct CR to a point midway between patellae.
  • SID is 48 inches (120 cm) to 72 inches (180 cm) (increased SID reduces magnification).


  • Collimate tighly on all sides to patellae.

Note: Patient comfort and total ralaxation are essential. The quadriceps femoris muscles must be ralaxed to prevent subluxation of the patellae, wherein they are pulled into the intercondylar sulcus ar groove, which may result in false readings.

Radiographic Criteria:

Structure Shown:

  • The intercondylar sulcus (trochlear groove) and the patella of each distal femur should be visualized in profile with the femoropatellar joint space open.

patellar x-ray


  • No rotation of the knee is present, as evidenced by the symmetric appearance of the patella, anterior femoral condyles, and intercondylar sulcus.

Collimation and CR:

  • Correct CR angle and centering are evidenced by open femoropatellar joint spaces.
  • Four-sided rectangular collimation field should be limited to area of patellae and femoral condyles.

Exposure Criteria:

  • Optimal exposure should clearly visualize soft tissue and joint space margins and trabecular markings of patellae.
  • Femoral condyles will appear underexposed with only the anterior margins clearly defined.

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