PA patella, Merchant, and Settegast

What patient position is used for a PA patella projection?

Prone.

How can pressure on the patella be reduced during PA positioning?

Place supports under the thigh and lower leg.

How should the patella align with the IR for a PA projection?

Parallel to the IR.

According to your Merrill page, what rotation usually places the patella parallel to the IR for PA positioning?

Approximately 5–10° lateral rotation; adjust until the patella is parallel to the IR.

What CR angle is used for a PA patella projection?

Perpendicular to the IR.

Where is the CR centered for a PA patella projection?

The midpatellar area.

How should the patella appear on a PA patella radiograph?

Completely superimposed over the femur, without rotation.

Why does PA patella positioning provide better spatial resolution than AP positioning?

The patella is closer to the IR, reducing OID.

A prone patient has the patella parallel to the IR and a perpendicular CR centered to the midpatellar area. Which projection is being performed?

PA patella.

What patient position is used for the Merchant method?

Supine, with both knees at the end of the table.

How are the femora positioned for the Merchant method?

Parallel to the tabletop, with the knees elevated approximately 2 inches.

What knee flexion is recommended for the standard Merchant setup in your Merrill page?

40°.

What knee-flexion range may be used for Merchant according to Merrill?

30–90°, although 40° is the recommended standard setup.

Why should the knee-flexion angle be recorded for Merchant examinations?

Patellofemoral appearance changes with flexion; recording it permits consistent follow-up images.

Why must the quadriceps be relaxed during Merchant positioning?

Contraction may reduce patellar subluxation and create a falsely normal appearance.

How is rotation controlled during Merchant positioning?

Strap the legs together at calf level and support them comfortably.

With 40° knee flexion, what CR angle and direction are used for Merchant?

30° caudad from the horizontal.

Where does the CR enter for a bilateral Merchant projection?

Midway between the patellae at the level of the patellofemoral joints.

How should the Merchant IR align with the CR?

Perpendicular to the CR.

Where is the IR placed for Merchant?

Against the anterior shins, approximately 1 foot distal to the patellae.

What SID does Merrill recommend for Merchant, and why?

6 feet (72 inches) to reduce magnification.

Which joint spaces should be open on a Merchant image?

The patellofemoral joint spaces.

Which structures should be demonstrated on a Merchant image?

Patellae in profile, femoral condyles, and intercondylar sulci.

What patellar relationship can be evaluated with bilateral Merchant images?

Patellar alignment and congruence with the femoral grooves.

A supine patient has both knees flexed 40°, a CR directed 30° caudad from horizontal, and an IR perpendicular to the CR. Which method is being performed?

Merchant.

Which patient position is preferred for Settegast, and what alternatives does Merrill describe?

Prone is preferred; supine and seated variations are also described.

Why is prone positioning preferred for Settegast?

It permits greater knee flexion and easier immobilization.

How is knee flexion determined for Settegast?

Flex slowly, as the patient's condition permits, until the patella is perpendicular to the IR.

What CR relationship is required for Settegast?

Perpendicular to the joint space between the patella and femoral condyles.

What CR angulation does Merrill describe for Settegast when angulation is needed?

Typically 15–20°, depending on knee flexion and joint-space alignment.

Where is the CR centered for Settegast?

The patellofemoral joint space.

How is the IR placed for Settegast?

Transversely under the knee, centered to the patellofemoral joint space.

What must be ruled out before attempting the acute knee flexion required for Settegast?

A transverse patellar fracture, using a lateral image.

Why is acute knee flexion avoided when a transverse patellar fracture is present?

It may separate or displace the fracture fragments.

What does Merrill state about attempting Settegast when the patient is in pain?

The acute-flexion procedure should not be attempted.

Which fracture orientation can be demonstrated by a tangential Settegast image?

A vertical patellar fracture.

What are the key image-evaluation findings for Settegast?

Patella in profile, open patellofemoral joint space, and demonstrated femoral condyles/intercondylar sulcus.

What SID was used for Settegast in your review video?

40 inches (102 cm).

How does knee-flexion selection differ between Merchant and Settegast?

Merchant uses a standard 40° setup; Settegast flexion is adjusted to place the patella perpendicular to the IR.

How does patellar appearance differ between PA patella and tangential patella projections?

PA: patella superimposed over femur. Tangential: patella in profile with an open patellofemoral joint.

How do tangential patella views differ in purpose from intercondylar-fossa views?

Tangential patella views demonstrate the patellofemoral articulation; fossa views open the notch between the femoral condyles.