The femur is a long bone, the largest and strongest one in the human body. It falls into the same structural category as the tibia, humerus, and radius. Long bones are defined not just by their length but by their shape: a hollow shaft connecting two wider, rounded ends. The femur fits this definition perfectly, averaging about 50 cm (roughly 20 inches) in adults and making up about 25% of a person’s total height.
What Makes the Femur a Long Bone
Bones are classified into five types based on their shape: long, short, flat, irregular, and sesamoid. Long bones are longer than they are wide and have three distinct regions. The shaft (called the diaphysis) is the straight, cylindrical middle section. The two ends (called epiphyses) are the bulbous, wider portions that form joints with neighboring bones. In the femur, the upper end forms the ball of the hip joint, and the lower end helps form the knee joint.
What sets long bones apart from other types is that hollow shaft. The interior of the femur’s shaft contains a central cavity filled with yellow bone marrow in adults, which is mostly fat tissue. The wider ends, by contrast, are packed with a honeycomb-like lattice of spongy bone. Some of the spaces within that spongy bone contain red bone marrow, where your body produces blood cells. This combination of a hollow middle and spongy ends gives long bones their signature blend of strength and relatively light weight.
How the Femur Is Built
The outer layer of the femur’s shaft is dense, solid bone tissue known as cortical (or compact) bone. This is the hard, white material most people picture when they think of bone. It provides the rigidity and load-bearing capacity the thighbone needs to support your entire body weight with every step. The shaft walls are thick, especially along the back of the bone where mechanical stress is greatest.
At the femoral neck, the short bridge connecting the ball of the hip to the main shaft, the distribution of dense bone changes dramatically. The lower half of the neck has cortical bone roughly 6 mm thick, while the upper half thins to just 0.3 mm. This uneven distribution is one reason the femoral neck is a common fracture site, particularly in older adults with reduced bone density.
Inside the rounded ends, the spongy bone isn’t randomly arranged. Its tiny struts and plates align along the directions of force the bone regularly experiences, creating an internal architecture that efficiently transfers weight from the hip joint down through the shaft and into the knee.
What the Femur Does
The femur’s primary job is weight-bearing. It supports the full weight of your upper body whenever you stand, walk, run, or jump. Beyond just holding weight, it acts as a lever for movement. The muscles of your thigh, hip, and buttocks attach to various ridges and bumps along its surface, and when they contract, the femur transmits that force into motion at the hip and knee joints. It also serves as an anchor point for the ligaments and tendons that stabilize both joints.
Because the femur angles inward from the hip to the knee rather than running straight down, it brings your knees closer to your center of gravity. This inward angle is slightly greater in women than in men, a difference related to the wider pelvis that accommodates childbirth.
How the Femur Grows
The femur begins forming remarkably early. Bone tissue first appears in the shaft around day 43 of embryonic development, making it one of the earliest bones to start hardening. The lower end begins turning from cartilage to bone shortly before birth, which is why the presence of this bone center on imaging is used as a marker of fetal maturity.
After birth, additional growth centers appear on a predictable schedule: the femoral head during the first year of life, the large bump near the top of the shaft (the greater trochanter) around age 3 to 4, and the smaller bump (the lesser trochanter) around age 13 to 14. Growth in length happens at plates of cartilage near each end of the bone. These growth plates gradually close and harden between ages 18 and 24, at which point the femur reaches its final adult length. This is why the femur, like other long bones, stops growing in your mid-twenties.
When the Femur Breaks
Breaking the femur typically requires significant force in younger, healthy adults, such as a car accident or a high-impact fall. In older adults with weakened bones, a simple fall from standing height can fracture the femoral neck or the area just below it. These hip fractures are among the most common and serious injuries in people over 65.
Fractures along the main shaft generally require surgery, usually involving a metal rod placed inside the bone’s hollow cavity to hold the pieces in alignment while they heal. After surgery, most people stay in the hospital for a few days. Healing of the shaft typically shows clear progress on X-rays by three months, with a “fluffy” new bone formation visible at the fracture site. Full healing and return to normal activity often take longer. In some cases, the bone fails to heal properly, a complication known as nonunion, which may require additional procedures.
Fractures near the hip or knee involve the spongy bone at the ends rather than the dense shaft, and they heal differently. The blood supply to the femoral head is particularly vulnerable, so fractures through the neck of the femur sometimes cut off circulation to the ball of the hip joint, which can lead to the bone tissue dying if not treated promptly.