AP OBLIQUE PROJECTION-MEDIAL (INTERNAL) ROTATION: KNEE

Thursday, September 27, 2012

Pathology Demonstrated:
  • Pathology involving the proximal tibiofibular and femoral (knee) joint articulations is demonstrated, as are fractures, lesions, and bony changes ralated to degenerative joint disease, especially on the anterior and medial or posterior and lateral portions of the knee.


Note: A common departmental routine is to include both medial and lateral rotation oblique projections for the knee. If only one oblique is routine, it is most commonly the medial rotation oblique.

Technical Factors:

  • IR size - 18 x 24 cm (8 x 10 inches), lenghtwise
  • Grid or Bucky, >10 cm (70+- 5 kV)
  • Screen, tabletop, <10 cm (65 +- 5 kV)
  • mAs 5


Shielding:

  • Place lead shield over gonadal area.


Patient Position:

  • Take radiograph with patient in the semisupine position with entire body and leg rotated partially away from side of interest; place support under elevated hip; give pillow for head.


Part Position:

AP oblique
  • Align and center leg and knee to CR and to midline of table or IR.
  • Rotate entire leg internally 45 degree. (Interpicondylar line should be 45 degree to plane of IR.)
  • If needed, stabilize foot and ankle in this position with sandbags.


Central Ray:

  • Angle CR zero degree on average patient (see AP knee)
  • Direct CR to midpoint of the knee at a level 1/2 inch (1.25 cm) distal to apex of patella.
  • Minimum SID is 40 inches (100 cm)


Collimation:

  • Collimate on both sides to skin margins, with full collimation at ends to IR borders to include maximum femur and tibia-fibula.


Note: The terms medial (internal) oblique and lateral  (external) oblique position refer to the direction of rotation of the anterior or patellar surface of the knee. This is true for descriptions of AP or PA oblique projections.

Radiographic Criteria:

Structure Shown:

  • Distal femur and proximal tibia and fibula with the patella superimposing the medial femoral condyle and shown.
  • The lateral condyles of the femur and tibia are well demonstrated and the medial and lateral knee joint spaces appear unequal.


Position:

  • The proper amount of part obliquity will demonstrate the proximal tibiofibular articulation open with the lateral condyles of the femur and tibia seen in profile.
  • The head and neck of the fibula will be visualized without superimposition, and approximately half of the patella should be seen free of superimposition by the femur.


Collimation and CR:

  • CR and center of the collimated field is to the femorotibial (knee) jont space.


Exposure Criteria:

  • Optimal exposure with no motion should visualize soft tissue in knee joint area, and trabecular markings of all bones should appear clear and sharp. Head and neck area of fibula should not appear overexposed.

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