Anatomical Features of Femoral Neck Fractures

The key elements in the clinical management of femoral neck fractures primarily encompass three aspects: bony anatomy, vascular anatomy, and capsular anatomy.

Bony Anatomy

The femoral neck connects the femoral head to the intertrochanteric region. The calcar femorale is a part of the medial femoral neck, composed of longitudinal dense bone that originates from the posteromedial region of the femoral shaft just below the lesser trochanter and radiates laterally toward the greater trochanter, thereby reinforcing the posteroinferior aspect of the femoral neck. Compared with the tension side of the superior femoral neck, the trabeculae in the calcar region extend superiorly under compressive stress toward the weight‑bearing dome of the femoral head (Fig. 1). In young patients, the medial femoral neck cortex is typically 4–6 mm thick. A thick anteromedial bone column supports the femoral head–neck junction, which may explain why most fracture comminution occurs posteriorly.

Fig. 1 – Red lines: medial compressive stress system and secondary medial compressive stress system; black lines: lateral tensile stress system; blue lines: secondary tensile stress system. The triangular area is the weak zone.

There is a specific spatial relationship between the femoral neck and the rest of the femur, the most accepted parameter being the femoral neck–shaft angle. In the coronal plane, the femoral neck forms an oblique angle with the femoral shaft, averaging 130° ± 7°. However, the neck–shaft angle and femoral neck length vary by sex and individual, but are generally consistent bilaterally in the same patient. In the coronal plane, the center of the femoral neck is constantly located at the level of the tip of the greater trochanter. In the horizontal plane, the femoral neck has an anteversion angle of approximately 10° ± 7° relative to the posterior plane of the femoral condyles (Fig. 2). This angle is distinct from the femoral neck torsion angle obtained by rotating the femoral neck around its own axis.

Fig. 2 – Representative computed tomography scout view for measuring the femoral rotation angle. The angle between the central axis of the femoral neck and the horizontal plane is measured, as well as the angle between the tangent line along the posterior femoral condyles and the horizontal plane. The femoral rotation angle is obtained by adding these two angles; the example shows anteversion.

Capsular Anatomy

Most (but not all) of the femoral neck is intracapsular and therefore bathed in synovial fluid, a feature that may influence fracture healing potential. On the medial side of the capsule, there is a bare area devoid of vascular covering, where vertical fracture lines are most commonly seen. The synovial fold vessels are usually located posterior to this bare area and extend cephalad along the femoral neck within a thin soft‑tissue layer, penetrating the cortex near the femoral head. The annular fibers of the zona orbicularis form a sling‑like structure in the posteroinferior part of the capsule surrounding the femoral neck; the longitudinal fibers constitute the iliofemoral, ischiofemoral, and pubofemoral ligaments. The hip capsule is taut in extension and internal rotation, while it becomes lax in mild flexion and external rotation (Fig. 3).

Fig. 3 – Cadaveric left hip model (neutral position) demonstrating capsular ligament anatomy.

Vascular Anatomy

The blood supply pattern of the femoral neck is complex, with the dominant source varying across different life stages (Fig. 4). In adults, the main blood supply to the femoral neck comes from the superoposterior and inferoposterior branches of the medial circumflex femoral artery (MFCA). The MFCA arises from the deep femoral artery, runs posterolaterally along the superior border of the quadratus femoris muscle toward the trochanteric fossa, and its deep branch courses along the posterior surface of the obturator externus muscle, passes beneath the conjoint tendon, and penetrates the joint capsule.

Fig. 4 – (Description not provided in original text; please refer to original figure caption if available.)