The femur (; : femurs or femora ), or thigh bone is the only bone in the thigh — the region of the lower limb between the hip and the knee. In many four-legged animals, the femur is the upper bone of the hindleg.
The top of the femur fits into a socket in the pelvis called the hip joint, and the bottom of the femur connects to the shinbone (tibia) and kneecap (patella) to form the knee. In humans the femur is the largest and thickest bone in the body.
Structure
The femur is the only bone in the upper leg and the longest bone in the human body. The two femurs converge medially toward the knees, where they articulate with the proximal ends of the tibiae. The angle at which the femora converge is an important factor in determining the femoral-tibial angle. In females, thicker pelvic bones cause the femora to converge more than in males.
In the condition genu valgum (knock knee), the femurs converge so much that the knees touch. The opposite condition, genu varum (bow-leggedness), occurs when the femurs diverge. In the general population without these conditions, the femoral-tibial angle is about 175 degrees.
The femur is the thickest bone in the human body. a ratio found in both men and women across most ethnic groups with minimal variation. This ratio is useful in anthropology, as it provides a reliable estimate of a person's height from an incomplete skeleton.
The femur is classified as a long bone, consisting of diaphysis (shaft or body) and two epiphyses (extremities) that articulate with the hip and knee bones.
Body
thumb|The femur details
The body of the femur (or shaft) is large, thick and almost cylindrical in form. It 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 which diverges proximally and distal as the medial and lateral ridge. Proximally the lateral ridge of the linea aspera becomes the gluteal tuberosity while the medial ridge continues as the pectineal line. Besides the linea aspera the shaft has two other bordes; a lateral and medial border. These three bordes separate the shaft into three surfaces: One anterior, one medial and one lateral. Due to the vast musculature of the thigh the shaft can not be palpated. A structure of minor importance in humans, the incidence of the third trochanter varies from 17–72% between ethnic groups and it is frequently reported as more common in females than in males.
Lower part
thumbnail|right|Lower extremity of right femur viewed from below.
thumbnail|right|Left [[knee joint from behind, showing interior ligaments.]]
The lower extremity of the femur (or distal extremity) is the thickest femoral extremity, the upper extremity is the shortest femoral extremity. It is somewhat cuboid in form, but its transverse diameter is greater than its antero-posterior (front to back). It consists of two oblong eminences known as the condyles.
By the sixth week of development, the first hyaline cartilage model of the femur is formed by chondrocytes. Endochondral ossification begins by the end of the embryonic period and primary ossification centers are present in all long bones of the limbs, including the femur, by the 12th week of development. The hindlimb development lags behind forelimb development by 1–2 days.
Function
As the femur is the only bone in the thigh, it serves as an attachment point for all the muscles that exert their force over the hip and knee joints. Some biarticular muscles – which cross two joints, like the gastrocnemius and plantaris muscles – also originate from the femur. In all, 23 individual muscles either originate from or insert onto the femur.
In cross-section, the thigh is divided up into three separate fascial compartments divided by fascia, each containing muscles. These compartments use the femur as an axis, and are separated by tough connective tissue membranes (or septa). Each of these compartments has its own blood and nerve supply, and contains a different group of muscles. These compartments are named the anterior, medial and posterior fascial compartments.
Muscle attachments
{|align=right
|-
| thumb|140px|Muscle attachments <br>(seen from the front) || thumb|140px|Muscle attachments <br>(seen from the back)
|}
{| class="sortable wikitable"
! Muscle || Direction || Attachment
|-
| Iliacus muscle || Insertion || Lesser trochanter
|-
| Psoas major muscle || Insertion || Lesser trochanter
|-
| Gluteus maximus muscle || Insertion || Gluteal tuberosity
|-
| Gluteus medius muscle || Insertion || Lateral surface of greater trochanter
|-
| Gluteus minimus muscle || Insertion || Forefront of greater trochanter
|-
| Piriformis muscle || Insertion || Superior boundary of greater trochanter
|-
| Gemellus superior muscle || Insertion || Upper edge of obturator internus's tendon (indirectly greater trochanter)
|-
| Obturator internus muscle || Insertion || Medial surface of greater trochanter
|-
| Gemellus inferior muscle || Insertion || Lower edge of obturator internus's tendon (indirectly greater trochanter)
|-
| Quadratus femoris muscle || Insertion || Intertrochanteric crest
|-
| Obturator externus muscle || Insertion || Trochanteric fossa
|-
| Pectineus muscle || Insertion || Pectineal line
|-
| Adductor longus muscle || Insertion || Medial ridge of linea aspera
|-
| Adductor brevis muscle || Insertion || Medial ridge of linea aspera
|-
| Adductor magnus muscle || Insertion || Medial ridge of linea aspera and the adductor tubercle
|-
| Vastus lateralis muscle || Origin || Greater trochanter and lateral ridge of linea aspera
|-
| Vastus intermedius muscle || Origin || Front and lateral surface of femur
|-
| Vastus medialis muscle || Origin || Distal part of intertrochanteric line and medial ridge of linea aspera
|-
| Short head of biceps femoris || Origin || Lateral ridge of linea aspera
|-
| Popliteus muscle || Origin || Under the lateral epicondyle
|-
| Articularis genu muscle || Origin || Lower 1/4 of anterior femur deep to vastus intermedius
|-
| Gastrocnemius muscle || Origin || Behind the adductor tubercle, over the lateral epicondyle and the popliteal facies
|-
| Plantaris muscle || Origin || Over the lateral condyle
|}
Clinical significance
Fractures
A femoral fracture that involves the femoral head, femoral neck or the shaft of the femur immediately below the lesser trochanter may be classified as a hip fracture, especially when associated with osteoporosis. Femur fractures can be managed in a pre-hospital setting with the use of a traction splint.
Cortical desmoid
Cortical desmoid (also known as a tug lesion or periosteal desmoid) is an irregularity of the distal femoral cortex commonly observed is adolescents.
Other animals
thumb|Femora of [[moa chicks.]]
In primitive tetrapods, the main points of muscle attachment along the femur are the internal trochanter and third trochanter, and a ridge along the ventral surface of the femoral shaft referred to as the adductor crest. The neck of the femur is generally minimal or absent in the most primitive forms, reflecting a simple attachment to the acetabulum. The greater trochanter was present in the extinct archosaurs, as well as in modern birds and mammals, being associated with the loss of the primitive sprawling gait. The lesser trochanter is a unique development of mammals, which lack both the internal and fourth trochanters. The adductor crest is also often absent in mammals or alternatively reduced to a series of creases along the surface of the bone. Structures analogous to the third trochanter are present in mammals, including some primates. snakes, and other non-walking vertebrates have vestigial femurs. In some snakes, the protruding end of a pelvic spur, a vestigial pelvis and femur remnant which is not connected to the rest of the skeleton, plays a role in mating. This role in mating is hypothesized to have possibly occurred in Basilosauridae, an extinct family of whales with well-defined femurs, lower legs and feet. Occasionally, the genes that code for longer extremities cause a modern whale to develop miniature legs (atavism).
One of the earliest known vertebrates to have a femur is the Eusthenopteron, a prehistoric lobe-finned fish from the Late Devonian period.
Viral metagenomics
A recent study has revealed that bone is a significantly richer source of persistent DNA viruses than previously thought. In addition to Parvovirus B19 and Hepatitis B virus, ten other viruses were discovered, including several members of the herpesvirus and polyomavirus families, as well as human papillomavirus type 31 and torque teno virus.
Invertebrates
In invertebrate zoology the name femur appears in arthropodology. The usage is not homologous with that of vertebrate anatomy; the term "femur" simply has been adopted by analogy and refers, where applicable, to the most proximal of (usually) the two longest jointed segments of the legs of the Arthropoda. The two basal segments preceding the femur are the coxa and trochanter. This convention is not followed in carcinology but it applies in arachnology and entomology. In myriapodology, another segment, the prefemur, connects the trochanter and femur.
Additional media
<gallery>
File:Human femur.stl|3D image
File:Slide2DADE.JPG|Muscles of thigh. Lateral view.
File:Slide2EA.JPG|Muscles of thigh. Cross section.
File:Blausen 0401 Femur DistributionofForces.png|Distribution forces of the femur
File:Femur Anatomy by Jason Christian.webm|Femur anatomy
</gallery>