Femoral Neck

The femoral neck is a flattened, pyramidal segment of bone that connects the femoral head to the shaft of the femur. It is one of the most clinically important parts of the proximal femur due to its role in transmitting forces from the hip joint to the femoral shaft and its susceptibility to fracture. The femoral neck plays a vital biomechanical role in supporting the body’s weight and facilitating lower limb mobility while maintaining a balance between strength and range of motion.

Structure

The femoral neck is cylindrical and measures approximately 3–5 cm in length in adults. It forms an oblique angle with the femoral shaft, known as the angle of inclination. The neck is slightly flattened anteroposteriorly and is narrower than the femoral head and shaft. It extends medially, superiorly, and slightly anteriorly from the shaft to the head of the femur.

Surfaces

• Anterior surface: Broad and flat, with the capsular attachment of the hip joint covering most of it.
• Posterior surface: Narrower and more concave; it lies deeper and has no capsular attachment on its posteroinferior aspect.
• Superior border: Short and thick; merges into the base of the femoral head.
• Inferior border: Longer and more slender, continuous with the intertrochanteric line and crest.

Key Angles

• Angle of inclination: Formed between the femoral shaft and neck, typically 125° in adults. Variations include:
  • Coxa vara: Decreased angle (<120°), leading to increased stress on the femoral neck.
  • Coxa valga: Increased angle (>135°), reducing the lever arm of hip abductors.
• Angle of anteversion: The anterior angulation of the femoral neck relative to the condylar plane; typically 10–20°.

Location

The femoral neck lies between the femoral head medially and the greater and lesser trochanters laterally. It is located within the capsule of the hip joint and is entirely intracapsular. The anterior surface is mostly covered by the joint capsule, while the posterior-inferior aspect remains extracapsular. The neck helps position the femoral head deeply within the acetabulum, ensuring both articulation and stability.

Function

• Force transmission: Transfers loads from the femoral head to the femoral shaft during standing, walking, and running.
• Joint positioning: Maintains the head of the femur in optimal alignment with the acetabulum for efficient motion.
• Mobility and leverage: Provides a mechanical advantage for the muscles acting around the hip, especially the abductors.

Relations

• Medial: Femoral head
• Lateral: Greater and lesser trochanters
• Anterior: Iliopsoas tendon, hip joint capsule
• Posterior: Piriformis and other short lateral rotator muscles

Blood Supply

The blood supply to the femoral neck is of critical importance due to its association with avascular necrosis following fractures. It receives blood from the following vessels:

• Retinacular arteries: Branches of the medial and lateral circumflex femoral arteries travel in the retinacula of the joint capsule and supply most of the femoral neck and head.
• Artery of the ligamentum teres: A branch of the obturator artery, enters via the fovea capitis but contributes minimally in adults.

Disruption of these arteries—especially the retinacular branches of the medial circumflex femoral artery—during a fracture can result in avascular necrosis of the femoral head.

Nerve Supply

Innervation of the femoral neck is primarily from articular branches of the femoral nerve, obturator nerve, and sciatic nerve. These nerves provide proprioceptive and pain sensation to the joint capsule and periosteum surrounding the neck.

Ossification

The femoral neck develops from the primary ossification center of the femoral shaft and the secondary ossification center of the femoral head. The neck forms as part of the growing metaphysis between the head and shaft. Fusion of the femoral head epiphysis with the neck occurs during adolescence, typically between ages 14 to 18 years.

Capsular Attachments

The capsule of the hip joint attaches around the margin of the acetabulum proximally and to the intertrochanteric line anteriorly and midway along the neck posteriorly. Thus:

• Anterior femoral neck: Entirely intracapsular.
• Posterior femoral neck: Only the proximal half is intracapsular; the distal half lies outside the capsule.

Trabecular Architecture

Internally, the femoral neck has a lattice of trabecular bone that aligns along stress lines:

• Primary compressive trabeculae: Extend from the medial cortex to the inferior part of the femoral head.
• Primary tensile trabeculae: Cross obliquely from the lateral cortex to the superior femoral head.
• Ward’s triangle: An area of relative radiolucency where trabeculae are sparse, often associated with osteoporotic fracture.

Fracture Classification

Femoral neck fractures are commonly classified based on their location and degree of displacement:

Type	Description	Clinical Relevance
Subcapital	Just below the femoral head	High risk of disrupting blood supply
Transcervical	Through the mid-neck	Common fracture site; often displaced
Basicervical	Junction of neck and intertrochanteric region	Better blood supply, lower AVN risk

Clinical Significance

• Femoral neck fractures: Common in the elderly, particularly those with osteoporosis. Displaced fractures often disrupt blood flow and require surgical fixation or arthroplasty.
• Avascular necrosis: Complication of femoral neck fractures due to loss of blood supply to the femoral head.
• Hip joint instability: Abnormal angles of the femoral neck can alter biomechanics, causing functional and postural problems (e.g., coxa vara or valga).
• Slipped capital femoral epiphysis: Though technically involving the physis, it leads to deformity of the neck orientation and gait abnormalities.
• Stress fractures: Can occur in athletes due to repetitive load; may present as groin pain with no history of trauma.

Imaging

The femoral neck is visualized using multiple imaging modalities:

• X-rays: AP pelvis and lateral hip views are standard for evaluating fractures, alignment, and angle measurements.
• MRI: Sensitive for early detection of stress fractures or avascular necrosis.
• CT scans: Helpful for evaluating fracture complexity and preoperative planning.

Radiographic evaluation often includes measurement of the neck-shaft angle, assessment for cortical thinning, and visualization of trabecular patterns to identify risk factors for fracture.