Patella Projections

Patient Preparation

• Explain the procedure to the patient.
• Remove clothing or metallic objects from the area of interest.
• Ensure patient comfort and immobilization to reduce motion.
• Use a lead shield to protect gonads if applicable.

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BASIC PROJECTIONS

1. Posteroanterior (PA) Projection — Patella

Positioning:

• Patient prone on the table.
• Affected knee extended and leg rotated slightly internally (5°–10°) to place patella parallel to the image receptor (IR).
• Center patella to the midline of the IR.

Central Ray (CR):

• Perpendicular to the IR.
• Directed to the mid-popliteal area (mid-patella).

Collimation:

• Include distal femur and proximal tibia/fibula.

Evaluation Criteria:

• Patella fully superimposed on femur.
• No rotation (equal condylar appearance).
• Sharp trabecular detail and adequate density.

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2. Lateral Projection — Patella (Mediolateral)

Positioning:

• Patient lies on affected side.
• Knee flexed 5–10° for comfort (to avoid separation of fragments if fracture is suspected).
• Opposite leg brought forward.
• Patella perpendicular to IR.

Central Ray (CR):

• Directed perpendicular to the IR.
• Enters at the mid-patellofemoral joint.

Collimation:

• Include patella, patellofemoral joint, and surrounding soft tissue.

Evaluation Criteria:

• Open patellofemoral joint space.
• Patella in true lateral profile.
• Anterior and posterior borders of femoral condyles superimposed.

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OTHER PROJECTIONS (TANGENTIAL VIEWS)

3. Tangential Axial Projection — Patella (Settegast Method)

Positioning:

• Patient prone with knee flexed 90°.
• IR placed under the knee joint.
• Ensure patella is perpendicular to the IR.

Central Ray (CR):

• Directed 15–20° cephalad through the patellofemoral joint space.

Collimation:

• Include entire patella and distal femur.

Evaluation Criteria:

• Open patellofemoral joint space.
• Patella in profile.
• Clear visualization of femoral condyles and articular surface.

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4. Tangential Axial Projection — Patella (Merchant Method)

Positioning:

• Patient supine.
• Knees flexed 40° over the end of the table.
• IR placed below the knees using a special support device.

Central Ray (CR):

• Directed 30° caudad from the horizontal.
• Centered midway between patellae at the level of the patellofemoral joints.

Collimation:

• Include both patellae and distal femora.

Evaluation Criteria:

• Clear visualization of patellofemoral joint spaces.
• No rotation.
• Both patellae in axial projection.

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5. Superoinferior Sitting Tangential Projection — Patella (Hobbs Modification)

Positioning:

• Patient seated on table edge.
• Knees flexed with feet on the floor.
• IR placed on the floor directly under knees.
• Patient leans forward slightly to bring patellae perpendicular to IR.

Central Ray (CR):

• Directed inferosuperiorly (from superior to inferior).
• Perpendicular to the IR and patellofemoral joint space.

Collimation:

• Include both patellae and patellofemoral joints.

Evaluation Criteria:

• Open patellofemoral joint spaces.
• Patellae in profile with minimal overlap.
• No rotation or motion blur.

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Image Evaluation Checklist

• Correct exposure and sharp detail.
• Patella centered and free from rotation.
• Patellofemoral joint space clearly demonstrated.
• Anatomical markers correctly placed.

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Common Pathologies Demonstrated

• Patellar fractures.
• Chondromalacia patellae.
• Patellar subluxation or dislocation.
• Degenerative changes in the patellofemoral joint.