Knee Projections
Patient Preparation
- Explain the procedure to the patient.
- Remove clothing or metallic objects from the area of interest.
- Ensure the patient is comfortable and the knee is free of rotation.
- Use lead shielding as appropriate.
BASIC PROJECTIONS
1. Anteroposterior (AP) Projection — Knee
Positioning:
- Patient supine on the table.
- Leg fully extended.
- Femoral condyles parallel to the image receptor (IR).
- Ensure no rotation of the limb.
Central Ray (CR):
- Directed 1.25 cm (½ inch) below the patellar apex.
- Angle depends on patient size:
- Average patient: Perpendicular to IR.
- Thin patient: 3–5° caudad.
- Thick patient: 3–5° cephalad.
Collimation:
- Include distal femur and proximal tibia/fibula.
Evaluation Criteria:
- Open femorotibial joint space.
- Equal appearance of femoral condyles.
- Patella centered between femoral condyles.
- Sharp trabecular detail.
2. Lateral Projection (Mediolateral) — Knee
Positioning:
- Patient lies on affected side.
- Opposite leg brought forward.
- Flex affected knee 20–30° for comfort and accurate joint visualization.
- Femoral condyles superimposed and patella perpendicular to IR.
Central Ray (CR):
- Directed 5–7° cephalad.
- Enters 2.5 cm (1 inch) distal to the medial epicondyle.
Collimation:
- Include distal femur, proximal tibia/fibula, and patella.
Evaluation Criteria:
- Open patellofemoral joint space.
- Superimposed femoral condyles.
- Sharp soft tissue and bony detail.
OTHER PROJECTIONS
3. AP Oblique Projection — Medial (Internal) Rotation
Positioning:
- Patient supine with leg extended.
- Rotate entire leg medially 45°.
Central Ray (CR):
- Perpendicular to IR.
- Directed 1.25 cm (½ inch) below patellar apex.
Collimation:
- Include distal femur and proximal tibia/fibula.
Evaluation Criteria:
- Open proximal tibiofibular joint.
- Lateral condyles of femur and tibia in profile.
- Patella projected slightly beyond the medial side of femur.
4. AP Oblique Projection — Lateral (External) Rotation
Positioning:
- Patient supine with leg extended.
- Rotate leg laterally 45°.
Central Ray (CR):
- Perpendicular to IR.
- Directed 1.25 cm (½ inch) below patellar apex.
Collimation:
- Include distal femur and proximal tibia/fibula.
Evaluation Criteria:
- Medial condyles of femur and tibia in profile.
- Fibula superimposed over lateral half of tibia.
- Patella projected slightly beyond the lateral side of femur.
5. AP or PA Weight-Bearing Projection — Knees
Positioning:
- Patient standing erect on the IR (posterior surface of knees against IR for AP, or anterior surface for PA).
- Toes pointed straight ahead.
- Weight distributed evenly on both feet.
- Knees fully extended (or slightly flexed if instructed).
Central Ray (CR):
- Horizontal and perpendicular to the IR.
- Centered 1.25 cm (½ inch) below the patellar apex for each knee.
Collimation:
- Include both knees, distal femora, and proximal tibiae.
Evaluation Criteria:
- Open joint spaces under weight-bearing conditions.
- Comparison of both knees.
- Assessment of joint space narrowing and alignment.
6. PA Axial Weight-Bearing Bilateral Projection — Knees (Rosenberg Method)
Positioning:
- Patient standing with both knees in contact with the IR.
- Knees flexed 45°.
- Weight equally distributed on both legs.
Central Ray (CR):
- Directed 10° caudad (downward).
- Enters midway between the knees at the level of the patellar apices.
Collimation:
- Include both knees and adjacent femur/tibia portions.
Evaluation Criteria:
- Intercondylar fossa visible.
- Open femorotibial joint spaces.
- Visualization of cartilage degeneration and joint space narrowing (for arthritis evaluation).
Image Evaluation Checklist
- Correct exposure and contrast.
- Clear joint spaces without rotation.
- Sharp trabecular and cortical bone detail.
- Proper centering and labeling.
Common Pathologies Demonstrated
- Fractures (tibial plateau, condyles, patella).
- Osteoarthritis and degenerative joint disease.
- Joint space narrowing under weight-bearing.
- Osteochondral defects.
- Alignment and soft tissue abnormalities.