AP Knee is a radiography positioning projection of the Lower Extremity. Supine, leg fully extended on IR. SID 40". The centering point is located 1/2 inch (1.25 cm) distal to the apex of the patella, in true AP. The central ray is perpendicular to the IR for the average patient (ASIS to tabletop 19-24 cm), parallel to the tibial plateau, directed 1/2 inch distal to the apex of the patella; otherwise 3-5 degrees caudad if under 19 cm, 3-5 degrees cephalad if over 24 cm. Image-quality criteria include knee joint space open and clearly defined, distal femur, proximal tibia and fibula all included. A sample technique is 85 kVp with DR 2.5 or CR 5 mAs, at an SID of 40 inches (102 cm).
Watch the setup
All steps (3)
- Position the patient supine, no rotation of the pelvis, a pillow under the head, the leg fully extended.
- Align and center the leg and knee to the CR and the midline of the table.
- Rotate the leg internally 3-5 degrees so the interepicondylar line is parallel to the IR.
3D anatomy: BodyParts3D, © The Database Center for Life Science. Posed, rendered and labelled by RadTechPrepper. These animations are licensed CC BY-SA 4.0. Credits
Anatomy demonstrated
- Distal third of femur with condyles.
- Tibial plateau with medial and lateral condyles.
- Proximal fibula articulating with tibia.
- Patella outline through the distal femur, slightly off center medially (Merrill's).
- Femorotibial joint space.
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 shoes and any radiopaque items from the leg.
- Position the patient supine, no rotation of the pelvis, a pillow under the head, the leg fully extended.
- Align and center the leg and knee to the CR and the midline of the table.
- Rotate the leg internally 3-5 degrees so the interepicondylar line is parallel to the IR.
- Place sandbags by the foot and ankle to stabilize if needed.
Position & centering point
1/2 inch (1.25 cm) distal to the apex of the patella, in true AP.
Central ray
Perpendicular to the IR for the average patient (ASIS to tabletop 19-24 cm), parallel to the tibial plateau, directed 1/2 inch distal to the apex of the patella; otherwise 3-5 degrees caudad if under 19 cm, 3-5 degrees cephalad if over 24 cm.
Exposure / technique
- kVp
- 85
- mAs
- DR 2.5 / CR 5
- SID
- 40" (102 cm)
- Notes
- Knee fully extended. Grid or Bucky if the knee is over 10 cm thick; tabletop if under 10 cm. Bontrager prints digital 75 +/- 5 kV; the sample is Merrill's grid row. Sample technique: DR 2.5 / CR 5 mAs at 85 kVp, 40 in SID, for a 12 cm part. Adjust for your equipment.
kVp and mAs are sample values. Use caution when applying them in your department.
Image-quality criteria
- Knee joint space open and clearly defined.
- Distal femur, proximal tibia and fibula all included.
- Femoral condyles and tibial plateau symmetric.
- Patella outline visible through the distal femur.
- About the medial half of the fibular head superimposed by the tibia.
- Collimation: sides to skin margins, ends to IR borders (Bontrager); 1 inch beyond the sides (Merrill's).
Common errors
- 1 Slight knee flexion closes joint space and obscures anatomy.
- 2 Too much or too little internal rotation makes the femoral and tibial condyles asymmetric.
- 3 Off-centered CR clips femoral condyles or tibial plateau.
- 4 Patella rotated or tilted due to improper leg rotation.
- 5 Knee not fully extended: Merrill's criteria call for full extension if the condition permits.
- 6 CR not directed 1/2 inch distal to the patellar apex.
Clinical indications
- Traumatic knee injury evaluation, fracture or dislocation.
- Degenerative or inflammatory joint disease of the knee.