PA Patella is a radiography positioning projection of the Lower Extremity. Prone, leg extended with the patella flat against the IR. SID 40". The centering point is located midpatella, usually at the midpopliteal crease, patient prone and leg extended. The central ray is perpendicular to the IR, directed to the midpatella (usually at the midpopliteal crease). Image-quality criteria include no rotation: femoral condyles symmetric, patellar outline sharp and distinct. A sample technique is 85 kVp with DR 3.2 or CR 6.3 mAs, at an SID of 40 inches (102 cm).
Anatomy demonstrated
- Patella in direct PA projection.
- Both femoral condyles, symmetric.
- Knee 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 patient prone on table with affected knee forward.
- Extend the leg and turn it about 5 degrees internally (heel slightly outward) so the epicondyles are parallel to the IR.
- Support the femur above the knee so there is no direct pressure on the patella.
- Ensure pelvis is level and not rotated.
Position & centering point
Midpatella, usually at the midpopliteal crease, patient prone and leg extended.
Central ray
Perpendicular to the IR, directed to the midpatella (usually at the midpopliteal crease).
Exposure / technique
- kVp
- 85
- mAs
- DR 3.2 / CR 6.3
- SID
- 40" (102 cm)
- Notes
- PA gives better recorded detail of the patella from decreased OID. Bontrager prints digital 75 +/- 5 kV; the sample is Merrill's chart. Sample technique: DR 3.2 / CR 6.3 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
- No rotation: femoral condyles symmetric.
- Patellar outline sharp and distinct.
- Patella centered on the femur, from the slight internal rotation of the knee.
- Patella outline seen through the distal femur.
- Patella superimposed on the femoral condyles, as expected on a PA.
- Collimation: closely to the patella and knee joint (Bontrager); 6 x 6 inches (Merrill's).
Common errors
- 1 Leaving out the slight internal rotation leaves the patella off-centre on the femur.
- 2 Rotation: femoral condyles not symmetric.
- 3 Flexing the knee when a patellar fracture is possible; take extra care not to flex.
- 4 CR not directed to the midpatella.
Clinical indications
- Patellar fracture evaluation and alignment assessment.