27() 



JOHN C. KOCH 



III tlio feiuiu's studied, the body weight being 200 pounds, the 

 load carried by the loaded femur under various conditions dis- 

 cussed is as follows: 



CONDITION 



Standing 

 Walking. 

 Running. 

 Jumping. 



Falls 



LO.\D, IN TERMS OP 

 BODY WEIGHT 



0.30 



0.80 

 1.60 



pounds 



60 

 160 

 320 



Variable, but greater than for 



running 

 Variable, but greater than for 



running 



This furnishes a basis for an approximation of the factor of 

 safety in the femur. By referring to figure 17 it is seen that 

 the greatest unit-stresses in both tension and compression oc- 

 cur at section 8, which is therefore the weakest section of the 

 entire femur, when carrying a load on the femur-head in the 

 normal manner. The maximum stresses at section 8 for a load 

 of 100 pounds on the femur-head are 1310 and 974 pounds per 

 square inch, in compression and tension, respectively. The 

 maximum stresses, under normal conditions, are those due to 

 running and are equal to those produced by a static load of 320 

 pounds on the femur-head. The maximum stresses at section 8 

 due to the load of 320 pounds will be 3.2 times as great as for the 

 100-pound load; or 4192 and 3117 pounds per square inch in 

 compression and tension, respectively The excellent physical 

 condition and comparative youth of the subject from whom the 

 femurs analyzed were taken, warrants using the higher values 

 of the ultimate strength of compact bone, as determined by ac- 

 tual tests by Rauber and other investigators. For this reason 

 in the determination of the factor of safety of the femurs analyzed 

 the values 24,000 and 17,700 pounds per square inch in compres- 

 sion and tension, respectively, will be used. The factor of safety 

 at section 8 will be the smaller of the values found by dividing 

 the ultimate strengths of compact bone by the maximum stresses 

 at this section. The factor of safety in compression is thus found 

 to be 5.71, and for tension it is 5.68; therefore the factor of safety 



