10.1 Proximal Femur Anatomy, Landmarks, and Regions of Interest
Key Takeaways
- The proximal femur comprises the femoral head, femoral neck, greater trochanter, lesser trochanter, intertrochanteric region, and proximal femoral shaft, each characterized by distinct cortical and trabecular ratios.
- The femoral neck consists of mixed trabecular (~25%–30%) and cortical (~70%–75%) bone, whereas the trochanteric region is predominantly trabecular (~50%), the femoral shaft is predominantly cortical (>90%), and Ward's area forms an anatomical trigone of low trabecular density at the convergence of compressive and tensile arches.
- By clinical convention, the patient's left hip is scanned routinely unless contraindicated by previous fracture, total or hemi-arthroplasty prosthesis, severe degenerative osteoarthritis, or indwelling metallic orthopedic hardware.
- Standardized hip positioning requires 15° to 25° of femoral abduction to align the femoral shaft parallel to the longitudinal table axis, combined with 15° to 25° of internal rotation using a dedicated positioning fixture.
- Internal rotation of 15° to 25° counteracts natural femoral anteversion (10°–15°), brings the femoral neck parallel to the tabletop, elongates the neck to maximum true anatomical length, profiles the greater trochanter laterally, and conceals or minimizes the lesser trochanter; inadequate rotation foreshortens the neck and artificially elevates bone mineral density (BMD).
10.1 Proximal Femur Anatomy, Landmarks, and Regions of Interest
Quick Answer: The proximal femur comprises the femoral head, femoral neck, greater trochanter, lesser trochanter, intertrochanteric region, and proximal shaft. ISCD states that BMD may be measured at either hip; many facilities adopt a single default side for consistency, and whichever side is chosen becomes that patient’s side for all follow-up unless contraindicated by prior fracture, arthroplasty, or metallic hardware. Densitometry requires the patient supine, leg abducted 15°–25° to position the shaft parallel to the scan field, and internally rotated 15°–25° using a manufacturer foot brace. Internal rotation neutralizes 10°–15° natural femoral anteversion, aligns the neck parallel to the tabletop, maximizes projected neck length, profiles the greater trochanter, and minimizes the lesser trochanter. Inadequate internal rotation foreshortens the neck and profiles the lesser trochanter, artificially elevating bone mineral density (BMD).
Gross and Radiologic Anatomy of the Proximal Femur
The proximal femur is a primary weight-bearing central DXA site. Technologists must master its gross landmarks and radiographic projections:
- Femoral Head: Smooth, spherical articular surface articulating with the pelvic acetabulum. Excluded from diagnostic regions of interest (ROIs) because acetabular overlap and joint calcifications distort areal density.
- Femoral Neck: Constricted cylindrical bridge connecting the head to the shaft (~125°–130° neck-shaft angle; 10°–15° anterior anteversion). Histologically, it contains mixed trabecular (~25%–30%) and cortical (~70%–75%) bone, providing tensile and compressive strength.
- Greater Trochanter: Massive lateral prominence serving as the insertion for hip abductors (gluteus medius and minimus). Predominantly trabecular (~50% trabecular, 50% cortical).
- Lesser Trochanter: Conical posteromedial prominence where the iliopsoas tendon inserts. Radiographically, it is the key indicator of rotational alignment; it should appear as a tiny profile or be obscured completely.
- Intertrochanteric Region: Diagonal osseous expanse between greater and lesser trochanters, composed of mixed trabecular and cortical bone.
- Proximal Femoral Shaft: Diaphyseal segment distal to the lesser trochanter, composed predominantly of dense compact cortical bone (>90% cortical).
- Ward's Area (Ward's Triangle): A focal trigone of low trabecular density in the central femoral neck formed at the intersection of principal compressive and tensile trabecular arches.
| Anatomical Region | Predominant Bone Composition | Biomechanical Function | DXA Diagnostic Role |
|---|---|---|---|
| Femoral Head | Mixed (~30% trabecular, ~70% cortical) | Coxofemoral articulation | Excluded from analysis |
| Femoral Neck | ~25%–30% Trabecular, ~70%–75% Cortical | Resists bending/shear forces | Primary diagnostic ROI |
| Greater Trochanter | ~50% Trabecular, ~50% Cortical | Hip abductor attachment | Lateral boundary of Total Hip |
| Lesser Trochanter | Cortical shell, marrow core | Iliopsoas insertion | Marker of rotational positioning |
| Intertrochanteric | ~50% Trabecular, ~50% Cortical | Distributes axial loads | Component of Total Hip ROI |
| Femoral Shaft | >90% Cortical | Rigid weight-bearing pillar | Alignment reference for shaft axis |
| Ward's Area | Lowest trabecular density | Trabecular arch intersection | Mathematical nadir; excluded from diagnosis |
Patient Positioning Protocols
Central hip densitometry demands standardized patient setup with the patient supine and pelvis level:
Hip Side Selection
By convention, the left hip is scanned routinely unless contraindicated by:
- Prior Hip Fracture: Callus and structural deformity falsely alter BMD. Scan the contralateral hip.
- Total Hip Arthroplasty (THA) or Hemiarthroplasty: Prosthetic metal completely attenuates the beam. Scan the contralateral hip.
- Severe Local Osteoarthritis: Joint sclerosis and osteophytes falsely elevate BMD. Scan the opposite hip.
- Indwelling Hardware / Pinning: Orthopedic screws invalidate the operative side.
Critical Positioning Maneuvers
- Femoral Abduction (15° to 25°): The technologist abducts the leg 15° to 25° from the anatomical midline. This aligns the femoral shaft straight and parallel to the longitudinal edge of the table and scan field, preventing the femoral neck from crowding into the acetabulum.
- Internal Rotation (15° to 25°): The lower extremity must be internally rotated 15° to 25° from the hip joint and secured with a manufacturer-supplied positioning foot brace / abduction wedge.
- Biomechanical Purpose: Neutralizes natural 10°–15° femoral anteversion.
- Radiologic Benchmarks: Aligns the femoral neck parallel to the table surface and detector, elongates the neck to maximum true anatomical length, profiles the greater trochanter laterally, and conceals or minimizes the lesser trochanter.
- Positioning Error Consequence: Inadequate internal rotation leaves the hip externally rotated, projecting a foreshortened neck and a large, prominent lesser trochanter. This condenses bone mineral content into an artificially reduced projected area, falsely elevating calculated BMD.
Laser Centering and Scan Field
Center the positioning laser crosshair over the femoral shaft approximately 2 inches (5 cm) distal to the greater trochanter. The scan field must include the acetabular margin superiorly, ischium medially, greater trochanter laterally, and at least 3 inches of shaft inferiorly.
| Positioning Error | Visual Radiographic Appearance | Densitometric Impact (BMD g/cm²) |
|---|---|---|
| Inadequate Internal Rotation | Large lesser trochanter profile; foreshortened femoral neck | Artificially elevated BMD (false negative) |
| Excessive Internal Rotation (>30°) | Trochanter rotates anteriorly over neck; obscured lesser trochanter | Falsely elevated BMD or ROI error |
| Adduction of Femur | Femoral shaft tilted inward; neck superimposes acetabulum | Artificially elevated BMD from pelvic overlap |
| Excessive Abduction (>30°) | Shaft tilted outward; lateral trochanter clipped | Axis calculation failure; invalid scan |
| Pelvic Obliquity / Tilt | Asymmetric obturator foramen; rotated pelvis | Non-reproducible geometry and BMD |
What is the primary biomechanical rationale for internally rotating the patient's lower extremity 15° to 25° during proximal femur DXA positioning?
Regarding the histological and structural bone composition of the proximal femur, which statement accurately reflects the distribution of cortical and trabecular bone across anatomical sub-regions?
During quality review of a left hip DXA acquisition, the technologist observes a large, fully profiled lesser trochanter and a noticeably foreshortened femoral neck. What technical positioning error occurred, and how will it influence the calculated bone mineral density (BMD)?