458 



J. JOWSEY 



% inactive 



16 



Bone formation and resorption 

 14 



12 



10 



Normal 



n Formation 



o Resorption 



Osteoporotic 

 ■ Formation 

 • Resorption 



Cortical thickness 



% 

 60 



oNorma 

 •Osteop. 



_i 1 i 1 — I 



Porosity:<-3 closed 

 osteons 



Relation between inactive 

 surface and area 



/xxlO" /mm' 

 40- 



o Normal 

 • Osteop 

 _i I I I I I I 



o Normal 

 • Osteop. 

 _j I I I I I I 



20 40 60 80 20 40 60 80 20 40 60 80 



Age Age Age 



Fig. 4. Turnover data from the mid-shaft of the femur of normal and 

 osteoporotic individuals. See text for details. 



1960), and the average value is 1 micron per day. Resorption rates 

 cannot be directly measured, but since a mass of bone that is not 

 losing or gaining tissue, such as cortical bone in a normal 20- to 30- 

 year-old human, shows approximately equal lengths of surface in- 

 volved in resorption and in formation, it is probably not incorrect 

 to assume that the rate of resorption is approximately equal and 

 also averages 1 micron per dav. However, rates of resorption and 

 formation must vary from place to place and time to time. Fomia- 

 tion is somewhat more rapid when an osteon is just beginning to 

 be laid down, and the rate becomes slower as the osteon is com- 



