248 JOHN C. KOCH 



tlu>A' may be closely approximated by calculating the amounts 

 antl directions of these maximum stresses at a number of points 

 in each of the sections comprising the upper femur. 



The directions and amounts of these maxinmm and minimum 

 internal unit-stresses have been computed in each section at 

 interA'als of yq inch from the neutral axis. In figure 18 the paths 

 of these maximum and minimum stresses are plotted to show their 

 relation to the whole femur. Figure 19 is drawn to full scale 

 to show the position of these paths of resultant stress in the 

 upper femur. In both figures it will be noted that these lines 

 have the following characteristics: 



1. All intersections of lines from opposite sides are at right 

 angles. 



2. The intersection of all lines with the neutral axis is at an 

 angle of 45 degrees. 



3. The lines are in two systems: one, compressive rising on 

 the lateral or outer side of the femur perpendicular to the sur- 

 face of the femur and crossing the neutral axis and gradually 

 fusing in the medial portion of the shaft; the other, tensile, ris- 

 ing on the medial side of the femur perpendicular to the surface 

 and crossing the neutral axis and fusing gradually below the 

 greater trochanter in the lateral portion of the shaft. 



In figure 19 a is shown approximately the intensity of the maxi- 

 mum and minimum tensile and compressive stresses calculated 

 for a load of 100 pounds on the femur head. It will be noted 

 that the minimum compressive stresses in all the lines of com- 

 pression are lateral to the neutral axis, and the maximum com- 

 pressive stresses are medial to the neutral axis. The minimum 

 tensile stresses in any line of tensile stress are medial to the 

 neutral axis and the maximum stresses are lateral to the neutral 

 axis. The maximum stresses in either system are, in general, 

 found as the direction of the line of stress becomes parallel to 

 the neutral axis. As these lines leave the neutral axis and 

 gradually approach a direction perpendicular to the neutral axis 

 they approach a value of zero. 



