THE FEMUR 



253 



"Fig. 249 shows a longitudinal frontal section through the left femur, which is the mate of the 

 right femur on which the mathematical analysis was made. In this midsection the system of 

 tensile trabeculse, which rises from the lateral (outer) part of the shaft and crosses over the central 

 area to end in the medial portion of the shaft, neck and head, is clearly shown. This figure also 

 shows the compressive system of trabecula? which rises on the medial portion of the shaft and 

 crosses the central area to end in the head, neck and greater trochanter. By comparing the posi- 

 1 ion of these two systems of trabeculse shown in Fig. 249 with the lines of maximum and minimum 

 stresses shown in Figs. 248 and 250 it is seen that the tensile system of trabeculse corresponds 

 exactly with the position of the lines of maximum and minimum tensile stresses which were 

 determined by mathematical analysis. In a similar manner, the compressive system of trabecula> 

 in Fig. 249 corresponds exactly with the lines of maximum and minimum compressive stresses 

 computed by mathematical analysis. 



"The amount of vertical shear varies almost uniformly from a maximum of 90 pounds (90 per 

 cent, of the load on the femur-head) midway between sections 4 and 6, to a minimum of 5.7 

 pounds at section 18" (Fig. 251). There is a gradual diminution of the spongy bone from section 

 <> to section 18 parallel with the diminished intensities of the vertical shear. 



1. The trabeculae of the upper femur, as shown in frontal sections, are arranged in two general 

 systems, compressive and tensile, which correspond in position with the lines of maximum and 

 minimum stresses in the femur determined by the mathematical analysis of the femur as a mechan- 

 ical structure. 



2. The thickness and spacing of the trabeculse vary with the intensity of the maximum stresses 

 nt various points in the upper femur, being thickest and most closely spaced in the regions where 

 the greatest stresses occur. 



3. The amount of bony material in the spongy bone of the upper femur varies in proportion to 

 1 he intensity of the shearing force at the various sections. 



4. The arrangement of the trabeculse in the positions of maximum stresses is such that the 

 greatest strength is secured with a minimum of material. 



Significance of the Inner Architecture of the Shaft. 1 . Economy for resisting shear. The shearing 

 stresses are at a minimum in the shaft. "It is clear that a minimum amount of material will be 

 required to resist the shearing stresses." As horizontal and vertical shearing stresses are most 

 efficiently resisted by material placed near the neutral plane, in this region a minimum amount 

 of material will be needed near the neutral axis. In the shaft there is very little if any material 

 in the central space, practically the only material near the neutral plane being in the compact 

 >one, but lying at a distance from the neutral axis. This conforms to the requirement of mechanics 

 or economy, as a minimum of material is provided for resisting shearing stresses where these 

 stresses are a minimum. 



2. Economy for resisting bending moment. "The bending moment increases from a minimum 

 it section 4 to a maximum between sections 16 and 18, then gradually decreases almost uniformly 

 o near section 75." "To resist bending moment stresses most effectively the material should 

 >e as far from the neutral axis as possible." It is evident that the hollow shaft of the femur is 

 in efficient structure for resisting bending moment stresses, all of the material in the shaft being 

 elatively at a considerable distance from the neutral axis. It is evident that the hollow shaft 

 provides efficiently for resisting bending moment not only due to the load on the femur-head, but 

 'rom any other loads tending to produce bending in other planes. 



3. Economy for resisting axial stress. 



The inner architecture of the shaft is adapted to resist in the most efficient manner the com- 

 bined action of the minimal shearing forces and the axial and maximum bending stresses. 



The structure of the shaft is such as to secure great strength with a relatively small amount of 

 naterial . 



The Distal Portion of the Femur. In frontal section (Fig. 249) in the distal 6 inches of the 

 emur "there are to be seen two main systems of trabeculse, a longitudinal and a transverse 

 system. The trabeculse of the former rise from the inner wall of the shaft and continue in per- 

 'ectly straight lines parallel to the axis of the shaft and proceed to the epiphyseal line, whence 

 'hey continue in more or less curved lines to meet the articular surface of the knee-joint at right 

 .ingles at every point. Near the center there are a few thin, delicate, longitudinal trabeculaD 

 ,vhich spring from the longitudinal trabeculse just described, to which they are joined by fine 

 ransverse filaments that lie in planes parallel to the sagittal plane. 



"The trabeculse of the transverse system are somewhat lighter in structure than those of the 

 longitudinal system, and consist of numerous trabeculse at right angles to the latter. 



"As the distal end of the femur is approached the shaft gradually becomes thinner until the 

 articular surface is reached, where there remains only a thin shell of compact bone. With the 

 radual thinning of the compact bone of the shaft, there is a simultaneous increase in the amount 

 of the spongy bone, and a gradual flaring of the femur which gives this portion of the bone a 



idually increasing gross area of cross-section. 



"There is a marked thickening of the shell of bone in the region of the intercondyloid fossa 

 vhere the anterior and posterior crucial ligaments are attached. This thickened area is about 





