Hip X-Ray Positioning: AP, Frog-Leg Lateral, Cross-Table, and Dunn Methods
Why Hip X-Ray Positioning Matters
Hip X-rays are among the most commonly performed radiographic examinations in clinical practice. Whether the patient arrives from the emergency department with a suspected hip fracture after a fall, the orthopedic clinic with chronic hip pain, or the pediatric unit with developmental dysplasia of the hip (DDH), the radiologic technologist's ability to produce diagnostic-quality images directly affects patient management.
The hip joint is a ball-and-socket synovial joint that bears significant weight and undergoes tremendous stress throughout life. Osteoarthritis, avascular necrosis (AVN), femoral neck fractures, and developmental dysplasia represent just a fraction of the pathology that hip radiographs help diagnose. The ARRT registry exam consistently includes hip positioning questions — from the standard AP projection to specialized trauma views like the cross-table lateral and Dunn method.
This guide covers every major hip projection you need to know for clinical practice and the ARRT exam, drawing on positioning standards from Clark's Pocket Handbook and current clinical protocols.
Key Bony Anatomy of the Hip Joint
Before positioning the patient, you must understand the anatomy you are imaging. The hip joint consists of:
- Acetabulum — the cup-shaped socket formed by the fusion of the ilium, ischium, and pubis (the triradiate cartilage in children)
- Femoral head — the spherical ball that articulates with the acetabulum; approximately two-thirds of the head is covered by articular cartilage
- Femoral neck — the narrow segment connecting the femoral head to the shaft; this is the most common site of hip fracture
- Greater trochanter — the palpable bony prominence on the lateral proximal femur; attachment site for the gluteal muscles
- Lesser trochanter — the smaller medial prominence; the iliopsoas tendon inserts here
ARRT Exam Tip — Trochanters and Rotation
The orientation of the lesser trochanter tells you whether the femur is properly rotated. On a correctly positioned AP hip with 15–20° of internal rotation, the lesser trochanter is not visible (it faces posteriorly). If you can see the lesser trochanter, the femur is externally rotated and the femoral neck is foreshortened — a classic ARRT scenario question.
The neck-shaft angle of the femur is approximately 125° in adults (coxa valga >135°, coxa vara <120°). This angle is important for positioning because the femoral neck must be brought parallel to the IR through internal rotation of the leg.
AP Hip (Unilateral Anteroposterior Projection)
The AP hip is the most frequently requested hip projection. It is performed as a unilateral study when the clinical concern is limited to one hip (e.g., unilateral hip pain, suspected fracture after a fall on one side).
Positioning Steps
- Patient position: Supine on the radiographic table with the midsagittal plane aligned to the midline
- Leg rotation: Internally rotate the affected limb 15–20° to bring the femoral neck parallel to the IR. The foot should be positioned so the patella faces directly anterior (straight up). Use sandbags or tape at the ankle for support
- IR size and orientation: 24 × 30 cm (10 × 12 inch) IR, placed lengthwise (portrait orientation) centered at the hip joint. For larger patients, 35 × 43 cm (14 × 17 inch) may be necessary
- Centering point: Central ray perpendicular to the IR, directed to the hip joint — located at the midpoint between the anterior superior iliac spine (ASIS) and the superior border of the pubic symphysis on the affected side. Alternatively, this is approximately 2.5 cm (1 inch) distal to the midpoint of the inguinal ligament
- SID: 100–110 cm (40–42 inches)
- Collimation: Include the proximal femur, hip joint, and lower iliac bone. Collimate closely to the skin margins
- Respiration: Suspend respiration on expiration
Evaluation Criteria
- The femoral neck should be visualized without foreshortening (evidence of adequate internal rotation)
- The lesser trochanter should be not visible or barely visible (facing posteriorly)
- The acetabulum and femoral head should be well demonstrated
- The greater trochanter should be seen in profile laterally
- No motion blur; adequate penetration to visualize trabecular detail in the femoral head and neck
- Symphysis pubis should be aligned with the midline (no rotation)
Clinical Pearl — The 15° Rule
How do you know when 15–20° of internal rotation is correct? The patella should face directly anterior (straight up toward the ceiling). If the patella faces medially, the femur is externally rotated. If it faces laterally, you have over-rotated. This is a simple check before exposing — train yourself to look at the patella, not just the foot.
AP Bilateral Hips
When the clinical question involves both hips (arthritis evaluation, AVN, bilateral hip pain), the request is often for "AP bilateral hips." This projection is identical in positioning to the AP pelvis — same centering, same IR size (35 × 43 cm crosswise), same collimation.
| Feature | AP Pelvis | AP Bilateral Hips |
|---|---|---|
| Clinical focus | Pelvic ring, SI joints, general survey | Femoral heads, necks, acetabula specifically |
| IR size | 35 × 43 cm, crosswise | 35 × 43 cm, crosswise |
| Centering | 2" below ASIS-symphysis midpoint | Same as AP pelvis |
| CR angle | Perpendicular | Perpendicular (occasionally slight 5–10° cephalad depending on department protocol) |
The distinction on the ARRT exam: the AP pelvis focuses on the pelvic ring and its stability, while the AP bilateral hips focus on the femoral heads and acetabula for comparison.
Frog-Leg Lateral (Modified Cleaves / Lauenstein Method)
The frog-leg lateral is performed to evaluate the hip joint in a lateral projection when no acute fracture is suspected. It is used for osteoarthritis, avascular necrosis assessment, hip dysplasia in children, and follow-up of known hip pathology.
Positioning Steps
- Patient position: Supine on the radiographic table
- Affected leg positioning: Flex the hip 60–90° and abduct the thigh 40–45° from vertical. The lateral aspect of the foot of the affected side should rest against the medial aspect of the opposite knee (or as close as comfortably possible)
- IR size and orientation: 24 × 30 cm (10 × 12 inch), lengthwise, centered to the hip joint. For bilateral frog-leg lateral, use 35 × 43 cm crosswise
- Centering point: Central ray perpendicular to the IR, centered at the hip joint (midpoint between ASIS and pubic symphysis on the affected side)
- SID: 100 cm (40 inches)
- Respiration: Suspend on expiration
Evaluation Criteria
- The femoral head and neck should be seen in a true lateral profile
- The greater and lesser trochanters should be visible, demonstrating the proximal femur in lateral orientation
- The acetabular joint space should be visible without overlap
- The femoral head should be superimposed on the acetabulum (or partially superimposed) — not displaced
When to AVOID the Frog-Leg Lateral
Do not perform the frog-leg lateral on any patient with a suspected hip fracture or dislocation. The abduction motion can:
- Displace a nondisplaced femoral neck fracture
- Convert a stable fracture to an unstable one
- Worsen an existing dislocation
- Cause severe pain and patient distress
The rule: when in doubt about trauma, use the cross-table lateral instead.
ARRT Exam Tip — Frog-Leg Contraindication
The ARRT frequently tests this as a safety question: "A patient with suspected hip fracture is brought to the emergency department. Which lateral projection of the hip should be performed?" The correct answer is the cross-table lateral (Danelius-Miller method). The frog-leg lateral is contraindicated because hip flexion and abduction can displace a fracture. This is one of the most commonly tested hip positioning questions on the registry.
Cross-Table Lateral (Danelius-Miller Method)
The cross-table lateral (also called the horizontal beam lateral or Danelius-Miller method) is the projection of choice for evaluating the hip in trauma patients. It requires no movement of the affected hip — the patient remains supine throughout.
Positioning Steps
- Patient position: Supine on the table or stretcher, with the affected hip as close to the center as possible
- Unaffected leg: Flex the unaffected hip and knee, and elevate the leg on a support (firm padding or a radiolucent support stand) to move it out of the beam path. The affected leg remains in a neutral position — no rotation, no lifting
- IR placement: Place a grid cassette (or DR detector) vertically against the lateral aspect of the affected hip, perpendicular to the table. The top of the IR should be at the level of the iliac crest; the bottom extends distally to include the proximal femur
- Central ray: Horizontal beam (cross-table), directed perpendicular to the IR, entering the medial aspect of the affected thigh. The centering point is the hip joint — same as the AP projection (midpoint between ASIS and pubic symphysis)
- SID: 100 cm (40 inches) if possible; may need to be adjusted based on patient size and bed clearance
- Respiration: Suspend on expiration
Evaluation Criteria
- Femoral head and neck in a true lateral projection
- No superimposition of the unaffected leg over the anatomy of interest
- The acetabulum should be seen in profile
- Fractures of the femoral neck should be clearly visualized
- Adequate penetration without motion
Clinical Pearl — Danelius-Miller Setup
The most challenging part of the cross-table lateral is getting the unaffected leg out of the way. Use a firm foam positioning block or a folded sheet — it must be radiolucent and stable. The elevated knee must be supported so the patient can relax; any muscle spasm in the elevated leg can cause the patient to shake and introduce motion artifact. Take the time to make the patient comfortable before centering the tube.
Dunn Method (Dunn/Risser Projection)
The Dunn method is a specialized projection used to evaluate the femoral head — specifically for assessing femoral head fractures, posterior hip dislocations, and Perthes disease (Legg-Calvé-Perthes disease) in children. It is named after orthopedic surgeon A.W. Dunn and is part of the trauma hip series in some institutions.
Positioning Steps
- Patient position: Supine on the radiographic table
- Hip positioning: Flex the affected hip to 90° (may need to be reduced if patient tolerance is limited). The degree of abduction varies depending on the specific modification:
- Standard Dunn: 45° abduction
- Modified Rippstein: Neutral abduction (femur vertical)
- IR: 24 × 30 cm (10 × 12 inch), lengthwise
- Centering: CR perpendicular to the femoral neck, entering the hip joint
- SID: 100 cm (40 inches)
Clinical Indications
| Indication | Dunn Method Role |
|---|---|
| Posterior hip dislocation | Evaluates femoral head for associated fracture |
| Femoral head fracture | Profiles the head to detect fracture lines |
| Legg-Calvé-Perthes disease | Assesses femoral head shape and containment |
| Slipped capital femoral epiphysis (SCFE) | Lateral evaluation of the proximal femoral epiphysis |
Lowenstein Lateral (For Pediatric Hip Evaluation)
The Lowenstein lateral is used primarily in pediatric imaging for developmental dysplasia of the hip (DDH) surveillance and for evaluating Legg-Calvé-Perthes disease.
- Position: Supine with the affected hip flexed approximately 30–40° and moderately abducted
- IR: 24 × 30 cm (10 × 12 inch), lengthwise
- Centering: CR perpendicular to the hip joint
- Key difference from frog-leg: Less flexion overall; the emphasis is on positioning the femoral neck parallel to the IR while keeping the patient comfortable
In children under 6 months, ultrasound (the Graf method) is the preferred screening modality for DDH. The Lowenstein lateral becomes relevant for older infants and children who are walking.
Comparison Table of Hip Projections
| Projection | CR Angle | IR Size | Centering Point | Primary Indication |
|---|---|---|---|---|
| AP Hip (unilateral) | None (perpendicular) | 24 × 30 cm, lengthwise | Midpoint between ASIS & pubic symphysis | Standard screening, unilateral hip pain |
| AP Bilateral Hips | None (perpendicular) | 35 × 43 cm, crosswise | 2" below ASIS-symphysis midpoint | Bilateral comparison (arthritis, AVN) |
| Frog-Leg Lateral | None (hip flexed/abducted) | 24 × 30 cm, lengthwise | Hip joint (ASIS-symphysis midpoint) | Non-traumatic hip evaluation, DDH |
| Cross-Table Lateral | Horizontal beam | 24 × 30 cm, vertical | Hip joint, medial thigh entry | Trauma, suspected fracture |
| Dunn Method | None (hip flexed 90°) | 24 × 30 cm, lengthwise | Femoral neck | Femoral head fracture, SCFE, Perthes |
| Lowenstein Lateral | None (hip 30–40° flexed) | 24 × 30 cm, lengthwise | Hip joint | Pediatric hip surveillance (DDH) |
Trauma Considerations for Hip Radiography
Hip fractures are among the most serious injuries in the elderly population, with a one-year mortality rate of approximately 20–25% after hip fracture. The radiologic technologist plays a critical role in obtaining diagnostic images safely.
Key Safety Rules for Trauma Hip Exams
- Minimal patient manipulation — Do not rotate the affected extremity if a fracture is suspected. The cross-table lateral allows you to image the hip without moving the injured leg
- Two-person assist — Always have assistance when moving a patient with a suspected hip fracture. Use a sheet slide or transfer board
- Document rotation — If you cannot achieve the ideal 15–20° of internal rotation due to pain or fracture, document the degree of rotation on the image
- Pain management — Administer imaging before or after analgesic administration based on department protocol. Some pain relief can improve patient cooperation and reduce motion
Portable Hip Technique
For patients who cannot be moved to the radiographic table:
- Place the DR detector under the patient's hip (slide carefully using a detector cover)
- Use grid technique when possible (portable grid or Bucky)
- Higher mA, shorter exposure time to minimize motion
- AP hip only — cross-table lateral can be attempted with the portable unit if clinical need is high
Geriatric Considerations
Elderly patients with hip fractures often present with:
- Osteoporotic bone — reduces contrast; use lower kVp (70–80) to maintain subject contrast
- Limited cooperation due to pain, dementia, or delirium
- Thin soft tissues — may require lower mAs to prevent overexposure
- Skin fragility — handle patients gently to avoid skin tears
Key Clinical Fact — Hip Fracture Mortality
Hip fractures are a major cause of morbidity and mortality in the elderly. Approximately 300,000 hip fractures occur annually in the United States, and the majority require surgical repair. The AP pelvis (or AP hip) is the initial imaging study in most cases, followed by cross-table lateral or CT if the X-ray is negative but clinical suspicion remains high. The radiograph you produce guides the orthopedic surgeon's decision to operate.
Pediatric Hip Considerations
Pediatric hip imaging requires special attention to technique, positioning adaptations, and radiation protection.
Developmental Dysplasia of the Hip (DDH)
- Neonates and infants < 6 months: Ultrasound (Graf method) is the gold standard — the femoral head is cartilaginous and not visible on X-ray
- Infants > 6 months and children: AP pelvis with both hips in neutral position + Lowenstein lateral (or frog-leg bilateral) for evaluation of femoral head position and acetabular development
Slipped Capital Femoral Epiphysis (SCFE)
SCFE is the most common adolescent hip disorder, typically affecting children aged 11–15 years. The Dunn method (or frog-leg lateral) is essential for diagnosis because the AP view often appears normal in early SCFE.
Radiation Protection
- Use pediatric technique charts (kVp 60–70, low mAs)
- Gonadal shielding is especially important — ensure shields are correctly placed without obscuring anatomy
- Collimate closely — limit exposure to the area of interest only
ARRT Practice Questions
Test Your Knowledge
Try these ARRT-style multiple choice questions based on this article. Click an option to check your answer — correct answers turn green, wrong ones turn red.
The cross-table lateral (Danelius-Miller method) is the correct choice for any patient with a suspected hip fracture. It uses a horizontal beam with the patient supine, requiring zero movement of the affected hip. The frog-leg lateral is contraindicated because flexing and abducting the hip can displace a nondisplaced fracture or worsen a dislocation. The Dunn method requires 90° hip flexion — also contraindicated in acute trauma.
With proper 15–20° internal rotation of the femur, the lesser trochanter faces posteriorly and should not be visible on the AP projection. The presence of a visible lesser trochanter indicates external rotation (or insufficient internal rotation), which foreshortens the femoral neck and can obscure an occult fracture.
The centering point for a unilateral AP hip is the midpoint between the ASIS and the superior border of the pubic symphysis on the affected side. This targets the femoral head and neck at the hip joint.
Legg-Calvé-Perthes disease (avascular necrosis of the femoral head in children) is best evaluated with a frog-leg lateral or Dunn method projection of the affected hip. These lateral views demonstrate the femoral head in profile and can reveal early subchondral fractures (the crescent sign) that may not be visible on the AP view.
The cross-table lateral (Danelius-Miller method) requires the patient to remain supine with the affected hip in a neutral position. The unaffected hip is flexed and elevated out of the beam path. A grid cassette is placed vertically against the lateral aspect of the affected hip, and the central ray is directed horizontally (cross-table) from the medial aspect, entering at the hip joint.