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 distal to the apex of the patella, in true ap.. The central ray is 0-5 degrees cephalad (varies with body habitus), entering 1/2 inch distal to the patellar apex.. Image-quality criteria include knee joint space open and clearly defined., distal femur, proximal tibia and fibula all included.. Standard exposure ranges from 60 to 75 kVp, 5 to 10 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Distal third of femur with condyles.
- Tibial plateau with medial and lateral condyles.
- Proximal fibula articulating with tibia.
- Patella centered on femoral groove.
- Femorotibial joint space.
- Anterior and posterior joint recesses.
Patient preparation
- Verify patient identity using two identifiers.
- Remove shoes and any radiopaque items from the leg.
- Position patient supine with leg extended on the IR.
- Ensure knee is fully extended in true AP alignment.
- Place a sandbag under distal femur for support.
Position & centering point
1/2 inch distal to the apex of the patella, in true AP.
Central ray
0-5 degrees cephalad (varies with body habitus), entering 1/2 inch distal to the patellar apex.
Exposure / technique
- kVp
- 60–75
- mAs
- 5–10
- SID
- 40" (102 cm)
- Notes
- Knee must be fully extended. No grid required.
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 centered on distal femur, not rotated.
- No superimposition of proximal tibia and fibula.
- Collimation 1 inch on sides, 1.5 inches above and below knee.
Common errors / ARRT traps
- 1 Slight knee flexion closes joint space and obscures anatomy.
- 2 Internal or external rotation of leg distorts condylar relationship.
- 3 Off-centered CR clips femoral condyles or tibial plateau.
- 4 Patella rotated or tilted due to improper leg rotation.
Clinical indications
- Traumatic knee injury evaluation, fracture or dislocation.
- Assessment of knee effusion or fat pad sign.
- Degenerative or inflammatory joint disease of the knee.
- Post-operative follow-up of knee surgery or hardware.
Aligned to the 2025 ARRT Content Specifications.