The femur (French: le fémur) or thigh bone is the largest and longest bone in the skeleton, and transmits the entire weight of the trunk from the hip to the tibia. In the erect posture, it inclines from above downward and medially, approaching at the lower extremity its fellow of the opposite side, but separated from it above by the width of the true pelvis. It presents for examination a superior extremity, including the head, neck, and two trochanters, an inferior extremity, expanded laterally into two condyles, and a shaft.

The upper extremity of the femur

The upper extremity is surmounted by a smooth, globular portion called the head, forming more than half a sphere, directed upward and medially for articulation with the acetabulum. With the exception of a small rough depression, the fovea, for the ligamentum teres, a little below and behind the center of the head, . its surface is covered with cartilage in the recent state. The head is connected with the shaft by the neck, a stout rectangular column of bone which forms with the shaft, in the adult, an angle of about 125°. Its anterior surface is in the same plane with the front aspect of the shaft, but is marked off from it by a ridge to which the capsule of the hip-joint is attached. The ridge, which commences at the great trochanter in a small prominence, or tubercle, extends obliquely down- ward, and winding to the back of the femur, passes by the lesser trochanter and becomes continuous with the medial lip of the linea aspera, on the posterior aspect of the shaft. This ridge forms the intertrochanteric line or spiral line of the femur. The intertrochanteric line receives the bands of the ilio-femoral thickening of the capsule of the hip-joint. The posterior surface of the neck is smooth and concave and its medial two-thirds is enclosed in the capsule of the hip-joint. The superior border of the neck, perforated by large nutrient foramina, is short and thick, and runs downward to the great trochanter. The inferior border, longer and narrower than the superior, curves downward to terminate at the lesser trochanter.

The trochanters are the prominences which afford attachment to the rotator muscles of the thigh; they are two in number - great and lesser.

The great trochanter is a thick, quadrilateral process surmounting the junction of the neck with the shaft, and presents for examination two surfaces and four borders. The lateral surface is broad, rough, and continuous with the lateral surface of the shaft. It is marked by a diagonal ridge running from the postero-superior to the antero-inferior angle, which receives the insertion of the gluteus medius. The ridge divides the surface into two triangular areas: an upper, covered by the gluteus medius, and occasionally separated from it by a bursa, and a lower, covered by a bursa to permit the free gliding of the tendon of the gluteus maximus. Of the medial surface the lower and anterior portion is joined with the rest of the bone; the upper and posterior portion is free, concave, and presents a deep depression, the trochanteric or digital fossa, which receives the tendon of the obturator externus. The fore part of the surface is marked by an impression for the insertion of the obturator internus and two gemelli.

The left femur (anterior view.)

Of the four borders, the superior, thick and free, presents near the center an oval mark for the insertion of the -piriformis; the anterior border, broad and irregular, receives the gluteus minimus; the posterior border, thick and rounded, is continuous with the intertrochanteric crest, the prominent ridge uniting the two trochanters behind. Above the middle of this line is an elevation, termed the tubercle of the quadratus, for the attachment of the upper part of the quadralus femoris. The inferior border corresponds with the line of junction of the base of the trochanter with the shaft; it is marked by a prominent ridge for the origin of the upper part of the vastus lateralis.

The lesser trochanter is a conical eminence projecting medially from the posterior and medial aspect of the bone, where the neck is continuous with the shaft. Its summit is rough and gives attachment to the tendon of the ilio-psoas. The fibers of the iliacus extend beyond the trochanter and are inserted into the surface of the shaft immediately below.

The body or shaft of the femur

The body or shaft of the femur is almost cylindrical, but is slightly flattened in front and strengthened behind by a projecting longitudinal ridge, the linea aspera, for the origin and insertion of muscles. The linea aspera extends along the middle third of the shaft and presents a medial lip and a lateral lip separated by a narrow interval. When followed into the upper third of the shaft, the three parts diverge. The lateral lip becomes continuous with the gluteal tuberosity and ends at the base of the great trochanter. The ridge affords insertion to the gluteus maximus, and when very prominent is termed the third trochanter. The medial lip curves medialward below the lesser trochanter, where it becomes continuous with the intertrochanteric line; the intervening portion bifurcates and is continued upward as two lines, one of which ends at the small trochanter, and receives some fibers of the iliacus, whilst the other is the linea pectinea and marks the insertion of the pectineus muscle.

The left femur (posterior view.)

Toward the lower third of the shaft the medial and lateral lips of the linea aspera again diverge, and are prolonged to the condyles by the medial and lateral supra-condylar lines, enclosing between them a triangular surface of bone, the popliteal surface [planum popliteum] of the femur, which forms the upper part of the floor of the popliteal space. The lateral line is the more prominent and terminates below in the lateral epicondyle. The medial one is interrupted above, where the femoral vessels are in relation with the bone, better marked below, where it terminates in the adductor tubercle, a small sharp projection at the summit of the medial epicondyle, which affords attachment to the tendon of the adductor magnus.

Near the center of the linea aspera is the foramen for the medullary artery, directed upward toward the head of the bone.

A diagram to show the pressure and tension curves of the femur (After Wagstaffe.)

From the medial lip of the linea aspera and the lower part of the intertrochanteric line arises the vastus medialis (internus), and from the lateral lip and the side of the gluteal ridge arises the vastus lateralis (externus). The adductor magnus is inserted into the medial lip of the linea aspera, from the medial side of the gluteal tuberosity above, and the medial supra- condylar line below. Between the adductor magnus and vastus medialis (internus) four muscles are attached: the pectineus and iliacus above, then the adductor brevis, and lowest of all, the adductor longus. Above, in the interval between the adductor magnus and the vastus lateralis (externus), the gluteus maximus is inserted; in the interval, lower down is the short head of the biceps, taking origin from the lower two-thirds of the lateral] lip of the linea aspera and the upper two-thirds of the lateral supra-condylar line. On the popliteal surface of the bone, just above the condyles, are two rough areas from which fibers of the two heads of the gastrocnemius take origin. Above the area for the lateral head of the gastrocnemius is a slight roughness for the plantaris.

For purposes of description it is convenient to regard the shaft of the femur as presenting anterior, medial, and lateral surfaces, although definite borders separating the surfaces from one another do not exist. All three surfaces are smooth and the anterior is not separated from the lateral by ridges of any kind. In the middle third of the shaft the medial and lateral surfaces approach one another behind, being separated by the linea aspera.

The shaft is overlapped on its medial side by the vastus medialis (internus), and on its lateral side by the vastus lateralis (externus). The upper three-fourths of the anterior and lateral surfaces afford origin to the vastus intermedius (crureus), and the lower fourth of the anterior surface, to the articularis genu (sub-crureus). The medial surface is free from muscular attachment.

trensverse section of the shaft of femur to show the medullary cavity.

Section of the upper end of the femur to show the calcar femorale.

The lower extremity of the femur

The lower extremity presents two cartilage-covered eminences or condyles, separated behind by the intercondyloid fossa. The lateral condyle is wider than its fellow and more prominent anteriorly; the medial condyle is narrower, more prominent, and longer, to compensate for the obliquity of the shaft. When the femur is in the natural position, the inferior surfaces of the condyles are on the same plane, and almost parallel, for articulation with the upper surfaces on the head of the tibia. The two condyles are continuous in front, forming a smooth trochlear surface [facies patellaris] for articulation with the patella. This surface presents a median vertical groove and two convexities, the lateral of which is wider, more prominent, and prolonged farther upward. The patellar surface is faintly marked off from the tibial articular surfaces by two irregular grooves, best seen while the lower end is still coated with cartilage. The lateral groove commences on the medial margin of the lateral condyle near the front of the intercondylar fossa, and extends obliquely forward to the lateral margin of the bone. The general direction of the medial groove is from front to back, turning medially in front and extending backward as a faint ridge which marks off from the rest of the medial condyle a narrow semilunar facet for articulation with the medial perpendicular facet of the patella in extreme flexion. The grooves receive the semilunar menisci in the extended position of the joint. The tibial surfaces are almost parallel except in front, where the medial turns laterally to become continuous with the patellar surface.

The femur at birth.

The opposed surfaces of the two condyles form the boundaries of the inter-condylar fossa and give attachment to the crucial ligaments which are lodged within it. The posterior crucial ligament is attached to the fore part of the lateral surface of the medial condyle and the anterior crucial ligament to the back part of the medial surface of the lateral condyle. The two remaining surfaces of the condyles are broad and convex, and each presents an epicondyle (tuberosity) for the attachment of lateral ligaments. The medial epicondyle, the larger of the two, is surmounted by the adductor tubercle, behind which is an impression for the medial head of the gastrocnemius on the upper aspect of the condyle; below and behind the lateral epicondyle is a deep groove which receives the tendon of the popliteus muscle when the knee is flexed, and its anterior end terminates in a pit from which the tendon takes origin. Above the lateral epicondyle is a rough impression for the lateral head of the gastrocnemius.

The interior of the shaft of the femur

The interior of the shaft of the femur is hollowed out by a large medullary canal, and the extremities are composed of cancellated tissue invested by a thin compact layer. The arrangement of the cancelli in the upper end of the bone forms a good illustration of the effect produced by the mechanical conditions to which bones are subject. In the upper end of the bone the cancellous tissue is arranged in divergent curves. One system springs from the lower part of the neck and upper end of the shaft medially and spreads into the great trochanter ('pressure lamellae'). A second system springs from the lateral part of the shaft and arches upward into the neck and head ('tension lamelae'), crossing the former almost at right angles. A second set of pressure lamellae springs from the lower thick wall of the neck, and extends into the upper part of the head to end perpendicularly in the articular surface mainly along the lines of greatest pressure. A nearly vertical plate of compact tissue (calcar femorale) projects into the neck of the bone from the inferior cervical tubercle toward the great trochanter. This is placed in the line through which the weight of the body falls, and adds to the stability of the neck of the bone; it is said to be liable to absorption in old age. In the lower end of the bone the vertical and horizontal fibers are so disposed as to form a rectangular meshwork.

The left femur at the twentieth year (posterior view.)

The figure shows the relgations of the epiphysial and capsular lines.

Blood-supply of the femur

The head and neck of the femur receive branches from the inferior gluteal, obturator, and circumflex arteries, and the trochanters from the circumflex arteries. The nutrient vessel of the shaft is derived from either the second or third perforating artery, or there may be two nutrient vessels arising usually from the first and third perforating. The vessels of the inferior extremity arise from the articular branches of the popliteal and the anastomotic branch of the femoral (supremagenu).

Ossification

The femur is ossified from one primary center for the shaft and from four epiphysial centers. The shaft begins to ossify in the seventh week of intra-uterine life. Early in the ninth month a nucleus appears for the lower extremity. During the first year, the nucleus for the head of the bone is visible, and in the fourth year that for the trochanter major. The center for the lesser trochanter appears about the thirteenth or fourteenth year. The lesser trochanter joins the shaft at the seventeenth, the great trochanter at the eighteenth, the head about the nineteenth, and the lower extremity at the twentieth year.

Articulation

The neck of the femur is an apophysis, or outgrowth from the shaft. The line of fusion of the condylar epiphysis with the shaft passes through the adductor tubercle.

The morphological relation of the patellar facet to the tibial portions of the condyles is worthy of notice. In a few mammals, such as the ox, this facet remains separated from the condyles by a furrow of rough bone,

The angle which the neck of the femur forms with the shaft at birth measures, on an average, 160°. In the adult, it varies from 110° to 140°; hence the angle decreases greatly during the period of growth. When once growth is completed, the angle, as a rule, remains fixed. (Humphry.)