AP PROJECTION: KNEE

Pathology Demonstrated:

• Ant fractures, lesions,  or bony changes related to degenerative joint disease involving the distal femur, proximal tibia and fibula, patella, and knee joint may be visualized in the AP projection.

Technical Factors:

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

Shielding:

• Place lead shield over gonadal area.

Patient Position:

• Take radiograph with patient in the supine position with no rotation of pelvis; give pillow for head; leg should be fully extended.

Part Position:

AP knee 3-5 degree cephalad CR

• Align and center leg and knee to CR and to midline of table or IR.
• Rotate leg internally 3 to 5 degree for a true AP knee (or until interpicondylar line is parallel to plane of IR.)
• Place sandbags by foot and ankle to stabilize if needed.

Central Ray:

• Align CR parallel to articular facets (tibia plateau); for average size patient, CR is perpendicular to IR (see Note).
• Direct CR to a point 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 at ends to IR borders.

Note: A suggested guideline for determining that CR is parallel to articular facets (tibia plateau) for open joint space is to measure distance from anterior superior illiac spines (ASIS) to tabletop to determine CR angles as follows.
<19 cm: 3 to 5 degrees caudal (thin thighs and buttocks)
19 to 24 cm: 0 degree angle (average thighs and buttocks)
>24 cm: 3 to 5 degrees cephalad (thick thighs and buttocks)

Radiographic Criteria:

Structure Shown:

AP projetion

• The distal femur and the proximal tibia and fibula are shown.
• The femorotibial joint space should be open, with the articular facets of the tibia seen on end with only minimal surface area visualized.

Position:

• NO rotation will be evidenced by the symmetric appearance of the femoral and tibial condyles and the joint space.
• The approximate medial half of the fibular head should be superimposed by the tibia.
• The intercondylar eminence will be seen in the center of the intercondylar fossa.

Collimation and CR:

• The collimation field should align with the long axis of the IR.
• The center of the collimation field (CR) should be to the mid-knee joint space.

Exposure Criteria:

• Optimal exposure will visualize the outline of the patella through the distal femur, and the fibular head and neck will not appear overexposed.
• No motion should occur; trabecular markings of all bones should be visible and appear sharp. Soft tissue detail should be visible.