Femur X-Ray Positioning: AP, Lateral, and Horizontal Beam Lateral Views
The femur — your thighbone — is the longest, largest, and strongest bone in the human body. Imaging it presents a unique challenge: you need to capture the entire shaft from the hip joint down to the knee joint on a single image (or two), while managing trauma patients who often can't move the injured leg. Getting these views right the first time is critical — femur fractures are painful, and repeat exposures are the last thing your patient needs.
In this guide, you'll learn the three essential projections for femur imaging: the AP full-length femur, the lateral femur, and the horizontal beam lateral (trauma) view. We'll cover Clark's-recommended centering points, technical factors, evaluation criteria, and the most common positioning errors — plus ARRT exam tips to help you lock in these concepts for the registry.
Anatomy You Need to Know
The femur has three main regions that matter for positioning:
- Proximal femur — includes the femoral head, neck, greater trochanter, and lesser trochanter. This region articulates with the acetabulum to form the hip joint.
- Femoral shaft (diaphysis) — the long, slightly bowed tubular body. It has a gentle medial bow (curves inward toward the midline). The linea aspera (a ridge of muscle attachments) runs along the posterior surface.
- Distal femur — includes the medial and lateral condyles, intercondylar notch, and patellar surface. This region articulates with the tibia and patella to form the knee joint.
📝 ARRT Exam Tip — The Medial Femoral Bow
The femur has a natural medial bow — it curves inward toward the midline. On a properly positioned AP femur, this bow should be visible without rotation. If the femur appears straight or laterally bowed, suspect patient rotation. This is a frequently tested concept on the ARRT registry: the medial bow is a normal finding, not a deformity.
Key landmarks for positioning:
- ASIS (anterior superior iliac spine) — palpable landmark for proximal centering reference
- Greater trochanter — palpable on the lateral thigh, approximately at the level of the pubic symphysis
- Superior pole of the patella — palpable distal landmark
- Mid-thigh — the midpoint between the hip joint (greater trochanter level) and knee joint (superior patella)
Projection 1: AP Femur (Clark's)
The AP femur is the primary projection for evaluating femoral shaft fractures, bone tumors, and leg-length discrepancies. The goal is to image the entire femur from the hip joint to the knee joint on a single image.
Patient Position
- Supine with arms at sides or across the chest
- Both legs extended, with the affected leg positioned straight
- Internally rotate the foot 15–20° to place the femoral neck in profile (same as AP hip positioning)
- Midsagittal plane centered to the grid
- Ensure the pelvis is not rotated — both ASIS should be equidistant from the table
Central Ray (Clark's)
- Perpendicular (0°) to the IR
- Centering point: Midway between the ASIS (or greater trochanter) and the superior pole of the patella — this places the CR at approximately mid-thigh
- The centering should be at the midpoint of the femoral shaft, not the anatomical mid-point of the thigh
Technical Factors
| Parameter | Value |
|---|---|
| IR size | 35 × 43 cm (14 × 17 in), lengthwise |
| IR orientation | Portrait (lengthwise along the femur) |
| SID | 100–110 cm (40 in) |
| Grid | Yes |
| kVp | 70–80 |
| mAs | 8–16 |
| Breathing | Suspend respiration |
| Shielding | Gonadal shielding (when not obscuring the area of interest) |
⚠️ Full-length consideration: In tall patients, the femur may exceed the length of a single 35 × 43 cm (14 × 17 in) IR. In this case, use two IRs taped together (or two exposures) overlapped by approximately 2.5 cm (1 in) at the mid-shaft. Mark the second IR clearly to avoid confusion.
Evaluation Criteria
- Both hip and knee joints included on the image — the most critical check
- Entire femoral shaft visible from the femoral head to the distal condyles
- No rotation — femoral condyles should be symmetric; the lesser trochanter should be partially visible (not fully profiled medially)
- Medial bow of the femoral shaft appears natural (not exaggerated or straightened)
- Adequate penetration to visualize the medullary cavity and the cortical margins
- Collimation tight to the femur with soft tissue visible
- No motion blur
Projection 2: Lateral Femur (Clark's)
The lateral femur provides a perpendicular view of the femoral shaft, essential for evaluating anterior-posterior displacement in fractures and assessing the femoral neck in the lateral plane.
Patient Position
- Lateral recumbent on the affected side (affected side down against the table)
- Affected leg straight, knee slightly flexed (approximately 45°) for comfort
- The unaffected leg is brought behind the affected leg, with a supportive pad between the knees
- Ensure a true lateral of the femur — the femur should be parallel to the IR plane
- A small pad under the waist may be needed to align the femur horizontally
- Arms folded in front or raised for comfort
Central Ray (Clark's)
- Perpendicular (0°) to the IR
- Centering point: Midway between the hip joint (greater trochanter) and the knee joint (lateral femoral condyle) — mid-thigh
- The CR enters the medial aspect of the thigh and exits the lateral aspect
Technical Factors
| Parameter | Value |
|---|---|
| IR size | 35 × 43 cm (14 × 17 in), lengthwise |
| IR orientation | Portrait |
| SID | 100 cm (40 in) |
| Grid | Yes |
| kVp | 75–85 (increase 5–10 kVp over AP due to increased tissue thickness) |
| mAs | 10–20 |
| Breathing | Suspend respiration |
Evaluation Criteria
- Femoral condyles superimposed — the single most important check for a true lateral. If the condyles appear as separate structures, the femur is rotated.
- Greater trochanter superimposed by the femoral shaft (partially or completely)
- Both joints included — hip and knee — if possible on the image
- Anterior and posterior cortical margins sharply defined along the full shaft
- No rotation — the femoral head and neck should not be foreshortened
- Adequate penetration through the thickest part of the thigh
Trauma Alternative: Horizontal Beam Lateral
When the patient cannot be moved into a lateral recumbent position (due to suspected fracture, spinal injury, or severe pain):
- Patient remains supine
- The unaffected leg is elevated and supported with a positioning pad or sling
- The IR is placed vertically against the lateral aspect of the affected thigh
- CR is horizontal (cross-table), perpendicular to the mid-thigh
- SID: 100–110 cm (40 in)
- kVp: 75–85 (higher due to increased OID from the horizontal beam)
- Grid: Yes (if possible); air-gap technique may substitute in portable settings
The horizontal beam lateral is the safest way to obtain a lateral femur in trauma. Never rotate or manipulate a fractured femur — you risk displacing bone fragments and damaging the femoral artery.
🚨 Clinical Pearl — When the Femur Is Too Long for One Image
The adult femur is approximately 40–50 cm (16–20 in) in a tall patient. If you're using a standard 35 × 43 cm (14 × 17 in) IR and the anatomy doesn't fit, you have two options: (1) Two-exposure technique — shoot proximal and distal halves, overlapping by 2.5 cm at mid-shaft. Label the second exposure clearly. (2) Single exposure at higher SID — increase SID to 110–120 cm (48 in) to reduce magnification and include more anatomy on one IR. This increases geometric unsharpness slightly, so it's a trade-off. For leg-length studies, always use the two-exposure technique with a measuring ruler placed alongside the thigh.
Positioning Errors: Quick Reference
| Error | Likely Cause | Correction |
|---|---|---|
| Hip or knee joint cut off | CR centered too high or too low | Center exactly midway between hip and knee |
| Femur appears straight (no medial bow) | Patient externally rotated | Internally rotate the foot 15–20° |
| Femoral condyles not superimposed (lateral) | Patient not in true lateral | Check the patella is perpendicular to the IR |
| Lesser trochanter fully profiled medially | Excessive internal rotation | Reduce internal rotation — only 15–20° needed |
| Femoral shaft underexposed | kVp too low for the tissue thickness | Increase to 75–80 kVp for lateral |
| Motion blur (AP) | Patient unstable or breathing | Ensure patient comfort, use exposure foot switch |
| Magnification / distortion (trauma lateral) | Increased OID with horizontal beam | Use grid to reduce scatter; accept some magnification |
| Two-image series doesn't match | Overlap not marked or insufficient | Overlap exposures by ~2.5 cm; label clearly |
📝 ARRT Exam Tip — The Cephalad vs Caudad Trap for Femur Imaging
Unlike the sacrum and coccyx, the AP femur uses a 0° (perpendicular) CR — no angle is needed because the femoral shaft is straight. The AP sacrum requires 15° cephalad and the AP coccyx requires 10° caudad, but the femur is straightforward: perpendicular CR, centered midway between the hip and knee joints. The ARRT often tests this distinction between spine and extremity positioning. For more lower extremity positioning practice, check out our hip X-ray positioning guide and knee X-ray positioning guide.
ARRT Exam Prep: Femur Positioning Quick Review
The ARRT registry typically includes 2–4 questions on femur positioning, often integrated with lower extremity questions. Here are the highest-yield facts:
- AP femur centering — midway between the hip joint and the knee joint (not the anatomical midpoint of the thigh)
- IR orientation — 35 × 43 cm (14 × 17 in) placed lengthwise (portrait) to include the full femur
- Internal rotation — 15–20° of the foot to place the femoral neck in profile
- Lateral femur evaluation — femoral condyles superimposed = true lateral
- Trauma lateral — horizontal beam (cross-table) — patient remains supine, never rotate a fractured femur
- Full-length technique — may require two IRs overlapped by 2.5 cm for tall patients
- kVp comparison — lateral requires 5–10 kVp higher than AP due to greater tissue thickness
- SID — standard 100 cm (40 in); increase to 110–120 cm for full-length or leg-length studies
- Grid — always use a grid for adult femur (thickness > 10 cm)
- The medial femoral bow — this is normal anatomy, not pathology. The femur curves slightly toward the midline.
Clinical Pearls: Tips from the Techs
- Palpate the greater trochanter and patella — these are your most reliable landmarks for centering. Don't guess the mid-thigh point; take the extra 5 seconds to feel for these bony prominences.
- Support the unaffected leg on trauma laterals — elevate it on a radiolucent step stool or positioning pads. A leg sling suspended from an IV pole works in a pinch. The key is to clear the CR path from the unaffected thigh.
- Watch out for grid cutoff on horizontal beam laterals — if the CR isn't perfectly perpendicular to the grid lines (especially with a focused grid), you'll get uneven density across the image. Use a grid with a wider focus range or switch to air-gap technique.
- Document the two-exposure technique — if you use two IRs for a full-length femur, mark them clearly (e.g., "Femur AP proximal" and "Femur AP distal"). The radiologist needs to know which is which.
- Immobilize, immobilize, immobilize — femur fractures are exquisitely painful. Even slight patient movement will blur the image. Use sandbags, positioning sponges, and gentle reassurance. Consider increasing mA and decreasing exposure time to freeze motion.
- When in doubt, do a full-length scout first — an AP scout with wide collimation gives you the information you need to plan your detailed views. It also reassures the trauma team that the femur alignment is maintained.
Summary
| Projection | Patient Position | CR Angle | Centering Point | Primary Purpose |
|---|---|---|---|---|
| AP Femur | Supine, leg extended, foot internally rotated 15–20° | 0° (perpendicular) | Midway between greater trochanter and superior patella | Full-length femur evaluation, fracture, tumor, leg length |
| Lateral Femur | Lateral recumbent on affected side, knee flexed 45° | 0° (perpendicular) | Mid-thigh between hip and knee joints | AP displacement, fracture alignment |
| Horizontal Beam Lateral | Supine (trauma) — unaffected leg elevated | 0° (horizontal) | Mid-thigh at level of femoral shaft | Trauma femur without patient movement |
📝 Key Takeaway — Three Views, Same Centering Principle
All three femur projections use the same centering principle: midway between the hip and knee joints. Master this centering and adjust only the patient position and CR orientation for each view. The most common ARRT question is the AP femur centering — remember midway between hip and knee, not mid-shaft. For more lower extremity positioning practice, check out our hip X-ray positioning guide and knee X-ray positioning guide.