Knee: AP Oblique Lateral

What routine SID was used for the knee views in the review videos?

40 inches (102 cm).

Why is knee thickness measured with calipers before an exposure?

To help select the exposure technique.

Which measurement determines the CR angle for AP and AP oblique knee projections in Merrill?

The vertical distance from the ASIS to the tabletop.

What CR angle is used for AP or AP oblique knee when the ASIS-to-tabletop distance is less than 19 cm?

3–5° caudad.

What CR angle is used for AP or AP oblique knee when the ASIS-to-tabletop distance is 19–24 cm?

Perpendicular; 0°.

What CR angle is used for AP or AP oblique knee when the ASIS-to-tabletop distance is greater than 24 cm?

3–5° cephalad.

Where is the CR centered for AP and both AP oblique knee projections?

½ inch (1.3 cm) inferior to the patellar apex.

How is the patient positioned for an AP knee projection?

Supine, pelvis without rotation, and knee fully extended if the patient's condition permits.

How should the femoral epicondyles align with the IR for a true AP knee?

Parallel to the IR.

How should the patella appear on an AP knee radiograph?

Completely superimposed over the distal femur.

Which findings indicate an AP knee without rotation?

Symmetric femoral condyles and a centrally positioned tibial intercondylar eminence.

How much fibular head overlap is normally seen on an AP knee?

Slight superimposition of the fibular head by the tibia.

How should the femorotibial joint space appear on a properly positioned AP knee?

Open, with approximately equal medial and lateral widths if the knee is normal.

How is the leg positioned for an AP medial oblique knee?

Rotate the entire extremity 45° medially; the knee and foot turn inward together.

Which femoral and tibial condyles are demonstrated in profile on an AP medial oblique knee?

The lateral femoral and lateral tibial condyles.

Which proximal tibiofibular findings identify a correctly positioned medial oblique knee?

Open proximal tibiofibular joint; fibular head and neck free of tibial superimposition.

Where does the patellar margin project on an AP medial oblique knee?

Slightly beyond the medial side of the femoral condyles.

An image shows lateral condyles in profile and an open proximal tibiofibular joint. Which knee projection was performed?

AP medial oblique knee; 45° internal rotation.

How is the leg positioned for an AP lateral oblique knee?

Rotate the entire extremity 45° laterally; the knee and foot turn outward together.

Which femoral and tibial condyles are demonstrated in profile on an AP lateral oblique knee?

The medial femoral and medial tibial condyles.

How should the fibula appear on an AP lateral oblique knee?

Superimposed over the lateral half of the tibia.

Where does the patellar margin project on an AP lateral oblique knee?

Slightly beyond the edge of the lateral femoral condyle.

Which knee joint space should remain open on both AP oblique projections?

The femorotibial joint space.

An image shows medial condyles in profile and the proximal fibula superimposed over the tibia. Which knee projection was performed?

AP lateral oblique knee; 45° external rotation.

A medial oblique knee patient has an ASIS-to-tabletop distance of 26 cm. What CR angle is indicated by Merrill?

3–5° cephalad.

How is the patient positioned for a routine mediolateral knee projection?

Lying on the affected side, with the pelvis without rotation.

How much knee flexion is used for a routine lateral knee?

20–30°.

For a new or unhealed patellar fracture, what maximum flexion does Merrill specify for the lateral knee?

No more than 10° to help prevent fragment separation.

How should the femoral epicondyles align with the IR for a true lateral knee?

Perpendicular to the IR.

What CR angle and direction are used for a routine mediolateral knee?

5–7° cephalad.

Where does the CR enter for a routine mediolateral knee?

1 inch (2.5 cm) distal to the medial femoral epicondyle.

Why is a cephalad CR angle used for the routine lateral knee?

The medial femoral condyle extends farther distally; angulation helps superimpose the condyles and open the joint space.

How does femoral epicondyle alignment differ between AP and lateral knee positioning?

AP: parallel to the IR. Lateral: perpendicular to the IR.

How should the femoral condyles appear on a correctly positioned lateral knee?

Superimposed.

How should the patella appear on a correctly positioned lateral knee?

In lateral profile.

Which two joint spaces should be open on a correctly positioned lateral knee?

The patellofemoral and femorotibial joint spaces.

How should the fibular head appear on a correctly positioned lateral knee?

Slightly superimposed by the tibia.

With rotation correctly adjusted, the inferior surface of the medial femoral condyle lies below the lateral condyle on a lateral knee. What CR error is indicated?

Insufficient cephalad angulation.

With rotation correctly adjusted, the inferior surface of the lateral femoral condyle lies below the medial condyle on a lateral knee. What CR error is indicated?

Excessive cephalad angulation.

How does CR centering differ between AP/oblique knee and routine lateral knee?

AP/obliques: ½ inch below the patellar apex. Lateral: 1 inch distal to the medial femoral epicondyle.