AP Distal Femur is a radiography positioning projection of the Lower Extremity. Supine, leg extended in true AP. Knee joint included. The centering point is located midpoint of the IR (Bontrager); midfemur and center of the IR (Merrill's). Knee joint must be on the IR. The central ray is perpendicular to mid femur, centered to include the knee joint. Image-quality criteria include distal two-thirds of femur including knee joint visible, femoral condyles symmetric with no rotation. A sample technique is 87.5 kVp with DR 3.6 or CR 7.1 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Distal two-thirds of the femoral shaft.
- Medial and lateral femoral condyles.
- Patella over the distal femur.
- Knee joint, its space not fully open.
- Trabecular pattern of the distal femur.
Patient preparation
- WARNING: with possible spinal injury or severe trauma, do not attempt to move the patient; do not rotate a severely injured part or remove splints (Bontrager). Merrill's: do not attempt true AP or lateral positioning of a severely injured limb; take two projections 90 degrees apart, moving it as little as possible, and move the IR and tube if the patient cannot reach the position.
- Verify patient identity using two identifiers.
- Remove radiopaque items from the leg.
- Position patient supine with leg fully extended.
- Turn the leg in about 5 degrees, the same rotation used for an AP knee, so the femur is in true AP.
- Place the lower IR margin about 2 inches below the knee joint.
- Confirm pregnancy status when applicable.
- Collimate closely. Bontrager says to shield outside the region of interest; Merrill's shields the gonads when the patient or caregiver requests it. Follow your department's policy.
Position & centering point
Midpoint of the IR (Bontrager); midfemur and center of the IR (Merrill's). Knee joint must be on the IR.
Central ray
Perpendicular to mid femur, centered to include the knee joint.
Exposure / technique
- kVp
- 87.5
- mAs
- DR 3.6 / CR 7.1
- SID
- 40" (102 cm)
- Notes
- Grid required for adult femur. 14"x17" lengthwise IR. Bontrager prints digital 80 +/- 5 kV; the sample is Merrill's chart. Sample technique: DR 3.6 / CR 7.1 mAs at 87.5 kVp, 40 in SID, for a 15 cm part. Adjust for your equipment.
kVp and mAs are sample values. Use caution when applying them in your department.
Image-quality criteria
- Distal two-thirds of femur including knee joint visible.
- Femoral condyles symmetric with no rotation.
- Knee joint included; its space is not fully open because of beam divergence.
- Patella outline slightly toward the medial side of the femur.
- Trabecular detail of distal femur visible.
Common errors
- 1 Rotation of the leg distorts the femoral condyles.
- 2 Failure to include the knee joint on the receptor.
- 3 CR not directed to the midpoint of the IR.
Clinical indications
- Suspected distal femoral fracture or stress injury.
- Assessment of bone lesions in the distal femur.