The femur, or thigh bone, is the longest, heaviest, and strongest bone in the entire human body. All of the body’s weight is supported by the femurs during many activities, such as running, jumping, walking, and standing. Extreme forces also act upon the femur thanks to the strength of the muscles of the hip and thigh that act on the femur to move the leg.
On its proximal end, the femur forms a smooth, spherical process known as the head of the femur. The head of the femur forms the ball-and-socket hip joint with the cup-shaped acetabulum of the coxal (hip) bone. The rounded shape of the head allows the femur to move in almost any direction at the hip, including circumduction as well as rotation around its axis. Just distal from the head, the femur narrows considerably to form the neck of the femur. The neck of the femur extends laterally and distally from the head to provide extra room for the leg to move at the hip joint, but the thinness of the neck provides a region that is susceptible to fractures.
At the end of the neck, the femur turns about 45 degrees and continues distally and slightly medially toward the knee as the body of the femur. At the top of the body of the femur on the lateral and posterior side is a large, rough bony projection known as the greater trochanter. Just medial and distal to the greater trochanter is a smaller projection known as the lesser trochanter. The greater and lesser trochanters serve as a muscle attachment sites for the tendons of many powerful muscles of the hip and groin such as the iliopsoas group, gluteus medius, and adductor longus. The trochanters also widen and strengthen the femur in a critical region of high stresses due to external trauma and the force of muscle contractions.
On its distal end, the femur forms the knee joint with the tibia of the lower leg. The distal end of the body of the femur widens significantly above the knee to form the rounded, smooth medial and lateral condyles. The medial and lateral condyles of the femur meet with the medial and lateral condyles of the tibia to form the articular surfaces of the knee joint. Between the condyles is a depression called the intercondylar fossa that provides space for the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL), which stabilize the knee along its anterior/posterior axis.
The Body or Shaft: The body, almost cylindrical in form, is a little broader above than in the center, broadest and somewhat flattened from before backward below. It is slightly arched, so as to be convex in front, and concave behind, where it is strengthened by a prominent longitudinal ridge, the linea aspera. It presents for examination three borders, separating three surfaces. Of the borders, one, the linea aspera, is posterior, one is medial, and the other, lateral.
The Lower Extremity: The lower extremity, larger than the upper, is somewhat cuboid in form, but its transverse diameter is greater than its antero-posterior; it consists of two oblong eminences known as the condyles. In front, the condyles are but slightly prominent, and are separated from one another by a smooth shallow articular depression called the patellar surface; behind, they project considerably, and the interval between them forms a deep notch, the intercondyloid fossa. The lateral condyle is the more prominent and is the broader both in its antero-posterior and transverse diameters, the medial condyle is the longer and, when the femur is held with its body perpendicular, projects to a lower level. When, however, the femur is in its natural oblique position the lower surfaces of the two condyles lie practically in the same horizontal plane.
The Architecture of the Femur: The various parts of the femur taken together form a single mechanical structure wonderfully well-adapted for the efficient, economical transmission of the loads from the acetabulum to the tibia; a structure in which every element contributes its modicum of strength in the manner required by theoretical mechanics for maximum efficiency.” “The internal structure is everywhere so formed as to provide in an efficient manner for all the internal stresses which occur due to the load on the femur-head. Throughout the femur, with the load on the femur-head, the bony material is arranged in the paths of the maximum internal stresses, which are thereby resisted with the greatest efficiency, and hence with maximum economy of material.”
The Inner Architecture of the Upper Femur: “The spongy bone of the upper femur (to the lower limit of the lesser trochanter) is composed of two distinct systems of trabeculæ arranged in curved paths: one, which has its origin in the medial (inner) side of the shaft and curving upward in a fan-like radiation to the opposite side of the bone; the other, having origin in the lateral (outer) portion of the shaft and arching upward and medially to end in the upper surface of the greater trochanter, neck and head. These two systems intersect each other at right angles