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The femur is the largest and longest bone of the body. Its shaft forms a long cylindrical tube with a forward bow. It contains thick walls of compact bone surrounding a hollow medullary cavity. The strong wall is thickest near the narrow center of the shaft where the medullary cavity is also most spacious. As the shaft becomes progressively wider toward each end, the compact wall of bone becomes thinner and the medullary cavity accumulates spongy bone. The proximal end consists of a short cantilevered neck capped by a smooth, round articular head. Rounded projections of bone form at the base of the cantilevered neck. The distal end, consisting of two large, knuckle-like processes separated by an intermediate groove, is larger than the proximal end. In the standing position the femoral shaft angles obliquely from lateral to medial.


The term femur, which now refers to the bone of the thigh, originally designated the entire thigh anatomy. Its linguistic origins are from the Latin fero, which means to bear. Interestingly, the term did not originally refer to bearing the weight of the body, but to being fruitful or fertile. This probably references the thigh's role in childbearing.


The femur articulates with three bones: the os coxa, patella, and tibia. The greater circumference of the head forms a bearing-like articular surface that receives the concave acetabulum of the os coxa. Its anterodistal face forms a shallow grooved articular surface for articulation with the posterior aspect of the patella. This articular surface extends further proximally on the anterolateral condylar aspect, being less extensive on the medial side. The distal ends of the two femoral condyles are transversely convex in all directions. They are longer in the anterior to posterior axis and shorter in the medial to lateral axis. These articular surfaces curve upward onto the posterior aspect of the condyles.


The femur ossifies from five centers. Other than the clavicle, it is the first long bone to begin to ossify. The first center emerges in the middle of the femoral shaft during the seventh embryonic week. This center progresses toward both extremities forming an ossified shaft at birth. The second center to emerge is that of the distal epiphysis. This forms the entire condylar end of the femur, first appearing during the ninth fetal month. This center is the last to fuse with the diaphysis. It fuses during the sixteenth year in females and the eighteenth year in males. The growth plate of this epiphysis crosses through the adductor tubercle, the site of attachment for the adductor magnus muscle. A third center begins to ossify in the head of the femur. This ossification is initiated at the end of the first postnatal year. The ossification center forms a cap-like epiphysis with the neck. It fuses with the neck between the fourteenth and seventeenth year, fusing earlier in females. The fourth center to emerge is that of the greater trochanter that begins to ossify during the fourth year. It joins the shaft approximately one year after puberty. Another proximal epiphysis, the lesser trochanter, is the final center to ossify and the first center to fuse, doing so at the onset of puberty. Notice that the order of appearance of the epiphyses is just opposite to their order of fusion.


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