The femur (also called the thigh bone) is the longest, heaviest, and strongest bone in the human body. It articulates proximally with the acetabulum of the pelvis (hip joint) and distally with the tibia and patella (knee joint).

1. Brief Anatomy

The femur features distinct regional anatomical structures that must be systematically evaluated on radiographs:

• Proximal End: Head, neck, greater trochanter, lesser trochanter, intertrochanteric crest/line.
• Shaft (Body): Slightly bowed anteriorly; contains the linea aspera (posterior ridge).
• Distal End: Medial condyle, lateral condyle, intercondylar fossa (notch), patellar surface, and adductor tubercle.

2. Indications

Femur radiography is performed for a variety of clinical presentations:

• Trauma: Suspected fracture, dislocation, or pathological fracture (metastatic disease).
• Pain: Unexplained thigh pain, limp, or localized swelling.
• Infection: Osteomyelitis.
• Tumors: Primary bone tumors (e.g., osteosarcoma) or metastatic lesions.
• Post-operative: Evaluation of intramedullary rod placement, plate/screw fixation, or joint replacement (hip/knee stems).
• Metabolic Disease: Evaluation of metabolic disease e.g., Paget’s disease or osteoporosis.

3. Patient Preparation

A. Psychological Preparation

• Explain the Procedure: Inform the patient that 2–3 images will be taken and that the cassette/detector will be placed alongside or under the leg.
• Pain Management: Warn the patient that positioning (especially rolling for the lateral) may cause discomfort; reassure them that you will move slowly and stop if they experience severe pain.
• Immobilization: Explain the use of sandbags or sponges to hold the leg in position to prevent motion blur.
• Trauma Protocol: If a fracture is suspected, clearly state that you will not force the leg into any position; you will rotate the entire patient or use a horizontal beam if necessary.

B. Physical Preparation

• Draping/Gown: Provide a gown and remove all radiopaque items (zippers, snaps, thick jeans, belts, and metal objects from pockets).
• Radiation Protection: Shield the gonads with a lead apron (provided it does not obscure the anatomy of interest). For a trauma AP hip, shield the contralateral gonad.
• Immobilization: Use non-slip sponges to secure the foot in a neutral position (toes up) for the AP view.
• Rotation: For the AP view, internally rotate the leg 15–20 degrees (if no trauma) to bring the femoral neck into profile. For trauma, do not rotate; keep the leg as is.

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4. Basic Projections (Routine)

Two basic projections are taken routinely, preferably with both the knee and hip joint included on the image. If this is impossible to achieve, then the joint nearest to the site of injury should be included.

Projection 1: Anteroposterior (AP) Femur

This projection demonstrates the entire femoral shaft, proximal metaphysis, and distal metaphysis in the coronal plane.

• Patient Position: Supine on the X-ray table with both lengths extended. Leg extended with toes pointing superiorly.
• Part Position: Internally rotate the limb 15°–20° (unless trauma) to place the femoral neck parallel to the IR. Center the midpoint of the femur (palpate the ASIS and the patella; the midpoint is roughly halfway between). Sandbags are placed below the knee to help maintain stability. The cassette is positioned in the bucky tray immediately under the limb, adjacent to the posterior aspect of the thigh to include both hip and knee joints.
• IR/Cassette: 35 x 43 cm (14 x 17 inches) or 35 x 90 cm (if using a long-length cassette).
• Central Ray (Centering Point): Perpendicular to the midpoint of the femur.
• SID: 100–110 cm (40–44 inches).
• Technical Factors (Digital):
  • kVp: 70–80
  • mAs: 5–15 (Adjust based on patient thickness; use AEC if available—center cell over the shaft).

Projection 2: Lateral Femur

This projection demonstrates the femur in the sagittal plane; it is crucial for detecting subtle fractures and assessing anterior/posterior displacement.

• Patient Position:
  • Non-trauma: Patient lies on the affected side (recumbent lateral). The unaffected leg is flexed and placed behind the affected leg.
  • Trauma/Unable to Roll: Perform a Horizontal Beam (Cross-Table) Lateral with the patient supine, using a grid and a vertical IR.
• Part Position: Flex the knee of the affected side slightly (about 30°) if possible. Ensure the epicondyles are perpendicular to the IR to achieve a true lateral.
• IR: 35 x 43 cm (collimate to the femur shaft, not the entire leg).
• Central Ray: Perpendicular to the midpoint of the femur.
• Technical Factors:
  • kVp: 75–85 (Increase by 8–10 kVp from AP due to increased tissue thickness in the lateral position).
  • mAs: 10–20
  • Grid: Required.

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5. Alternative (Special) Projections

A. Trauma Supine Lateral (Cross-Table)

Performed when there is a suspected fracture of the femoral shaft or hip in a patient who cannot be log-rolled.

• Position: Patient remains supine. Place the IR vertically against the lateral aspect of the thigh, supported by sandbags.
• CR: Directed horizontally, perpendicular to the IR and the long axis of the femur, entering the medial aspect of the thigh.
• Technical Factors: Increase mAs by 50% due to the increased OID (Object-to-Image Distance).

B. Proximal Femur (Hip) AP & Lateral (Frog-leg)

Performed when there is isolated proximal femoral/neck pathology (not for trauma cases).

• Position: AP as described above. For the “Frog-leg” lateral, flex the knee, abduct the hip, and externally rotate the leg until the sole of the foot rests against the medial side of the opposite knee.
• CR: Perpendicular to the femoral neck (2.5 cm distal to the mid-inguinal point).

C. Distal Femur (Sunrise/Skyline View - Modified)

Performed to evaluate the patellofemoral joint and intercondylar notch. This is often done as part of a knee series, but can be included to visualize the distal femoral condyles.

• Position: Patient prone or supine with knee flexed 40–45°.
• CR: Directed caudally at 45° to the long axis of the tibia, entering the popliteal space.

D. Long-Leg (Full-Length) Weight-Bearing Femur

Performed in pre-operative planning for total knee replacement or limb-length discrepancy.

• Requires: A 35 x 90 cm cassette or digital stitching (DDR). Uses a vertical grid.
• Position: Patient standing. AP and Lateral views are obtained with the CR centered at the knee, but collimation remains open from the hip to the ankle.
• Technical Factors: High kVp (80–90) and high mAs to penetrate the hip and knee joints simultaneously.

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6. Image Critique (Evaluation Criteria)

AP Projection

• Inclusion: Entire femur from the hip joint (superior to the greater trochanter) to the knee joint (inferior to the tibial plateau).
• Rotation: The femoral neck is in profile (foreshortened minimally). The lesser trochanter is not visible or only slightly visible on the medial border.
• Condyles: The distal condyles are symmetric, with the intercondylar notch visible centrally.
• Collimation: Tight collimation to skin edges; gonadal shielding should be visible (if it does not obscure the anatomy).
• Exposure: Adequate penetration to visualize trabecular bone in the neck and shaft. Soft tissue margins are distinct with no motion blur.

Lateral Projection

• Inclusion: Entire femur from the hip to the knee.
• Rotation: The distal femoral condyles are stacked (one directly on top of the other). The patella is in profile anteriorly.
• Shaft: The linea aspera (posterior cortex) is seen as a dense, sharp line. There should be no superimposition of the medial and lateral epicondyles.
• Position: The hip and knee are in the same lateral plane (no tilt).
• Exposure: Adequate penetration through the thick muscular thigh; bony trabeculae must be visible in the metaphysis.