AP Weight-Bearing Knees is a radiography positioning projection of the Lower Extremity. Upright standing, both feet flat, knees extended. SID 40". The centering point is located center point between both knees at 1/2 inch below patella apexes. The central ray is horizontal, perpendicular to the IR (5-10 degrees caudad on a thin patient), midway between the knees 1/2 inch below the patellar apices. Image-quality criteria include both knee joint spaces visible and symmetric, femoral condyles and tibial plateaus symmetric bilaterally. A sample technique is 85 kVp with DR 2.5 or CR 5 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Bilateral femoral condyles under weight-bearing stress.
- Bilateral tibial plateaus; weight bearing often reveals narrowing (Merrill's).
- Faint outlines of the patellae through the femora.
- Femorotibial joint spaces bilaterally.
- Varus or valgus alignment at both knees.
- Enough femur and tibia to determine long axes for alignment.
Patient preparation
- Verify patient identity using two identifiers.
- Remove shoes and any radiopaque items from both legs.
- Have patient stand upright on both feet on a low platform.
- Position feet forward and parallel with weight equally distributed.
- Keep knees fully extended in neutral position.
Position & centering point
Center point between both knees at 1/2 inch below patella apexes.
Central ray
Horizontal, perpendicular to the IR (5-10 degrees caudad on a thin patient), midway between the knees 1/2 inch below the patellar apices.
Exposure / technique
- kVp
- 85
- mAs
- DR 2.5 / CR 5
- SID
- 40" (102 cm)
- Notes
- Weight-bearing view shows joint space narrowing and alignment under load. Bontrager prints digital 75 +/- 5 kV; the sample borrows Merrill's nonweight-bearing 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
- Both knee joint spaces visible and symmetric.
- Femoral condyles and tibial plateaus symmetric bilaterally.
- Faint outlines of the patellae seen through the femora.
- No rotation; both knees in true AP alignment.
- Distal femurs, knees, and proximal tibias included.
- Collimation: enough femur and tibia for long axes (Bontrager); 1 inch beyond the sides (Merrill's).
Common errors
- 1 Feet not straight ahead.
- 2 Knees not fully extended, or weight not equal on both feet (Merrill's setup).
- 3 CR not midway between the knee joints, 1/2 inch below the patellar apices.
Clinical indications
- Degenerative joint disease severity assessment under weight-bearing stress.
- Varus or valgus alignment evaluation with weight on both legs.
- Joint space narrowing or cartilage loss assessment.