The femur, or thigh bone, is the longest and strongest bone in the human body, playing a central role in standing, walking, and lower body movement. The femoral neck is a relatively narrow segment of bone that connects the spherical head of the femur to its long shaft. This connecting piece functions as a strut, transmitting immense forces from the pelvis and upper body down to the leg, making it integral to human mobility.
Precise Anatomical Location and Structure
The femoral neck is located at the top (proximal end) of the femur, extending laterally and slightly upward from the main shaft. Its primary role is to connect the femoral head, which forms the ball of the hip’s ball-and-socket joint, to the rest of the femur. This connection point is situated just above the greater and lesser trochanters, which are large bony protuberances serving as attachment sites for powerful hip and thigh muscles.
The structure is angled in two distinct ways relative to the shaft. The Angle of Inclination (neck-shaft angle) is measured in the frontal plane, typically ranging between 115 and 140 degrees in adults. This angle allows the shaft of the femur to descend closer to the body’s midline, which aids stability during walking.
A second angulation, the Angle of Anteversion (or torsion angle), is measured in the horizontal plane and refers to the slight forward twist of the femoral neck relative to the knee joint. This angle is generally between 8 and 14 degrees in adults. Both the inclination and anteversion angles dictate the joint’s range of motion and mechanical alignment. The neck has a tubular shape, composed of dense cortical bone surrounding internal spongy, or trabecular, bone.
The Role in Movement and Weight Bearing
The femoral neck acts as a specialized lever arm, facilitating the wide range of motion at the hip joint. Its angle allows for the rotation and leverage necessary for the hip abductor muscles to balance the pelvis during single-limb stance, such as walking. The neck transmits the entire body weight from the pelvis to the shaft of the femur and down to the ground.
During low-speed walking, the neck manages complex biomechanical forces that can exceed three times the body weight. These forces create a bending moment, resulting in compressive forces on its lower (inferior) aspect and tensile forces on its upper (superior) aspect. The internal trabecular bone is aligned in specific patterns, known as Wolff’s law, to resist and absorb these multi-directional stresses.
The density and architecture of this internal bone system allow the femoral neck to manage forces generated by external impacts and internal muscle contractions. Muscle forces during movements like running or jumping can generate loads two to three times greater than body weight. The structural integrity of the femoral neck is paramount for absorbing these stresses and enabling smooth, powerful lower-body movement.
Why the Femoral Neck is Prone to Fracture
Despite its strength, the femoral neck is a common site for serious fractures because it is subjected to massive bending forces while being relatively narrow. The risk of fracture increases significantly with age, primarily due to the weakening of bone density known as osteoporosis. This condition causes an imbalance in bone remodeling, where the breakdown of bone tissue by osteoclasts outpaces the rebuilding by osteoblasts.
When bone density diminishes, the structural integrity of the internal trabecular network is compromised, making the bone tissue appear porous and fragile. The most common mechanism for fracture in older persons is a low-energy fall, often a sideways impact onto the greater trochanter. The loss of bone mass means the neck can no longer withstand the force of the fall, leading to a break.
Femoral neck fractures are classified based on their location, such as subcapital (just below the head) or transcervical (through the neck). The vulnerability of this area is complicated by its blood supply, which can be easily disrupted by a fracture, potentially leading to complications and prolonged recovery. The combination of high mechanical stress and age-related bone loss makes the femoral neck a uniquely susceptible point in the skeletal system.