RAREFYING DISEASE OF THE SKELETON 431 



extreme alteration in the structure of the bone tissue, the percentage 

 of the skeleton that was reactixe or exchangeable with tracer sub- 

 stances was proportional to the \ olume of the percentage of skeleton 

 that was relatively nonreactive or nonexchangeable. Our impression 

 was that in individuals in earlv stages of the disease, in which quan- 

 titative data would have been most valual^le for diagnosis and 

 treatment, it was not possible to distinguish normal from osteo- 

 porotic individuals. Further development of the mathematical treat- 

 ment of radioisotope kinetic data and the osteogram of MacDonald 

 (1960) may increase the value of measurements of skeletal turn- 

 over with Sr^^, Ca^', and other isotopes in patients with local, as 

 well as systemic, bone disease (Dow and Stanbury, 1960; Green- 

 berg et al., 1961; Bauer et al, 1961). 



Fractures 



We assumed in previous reports that physiologic aging or atrophv 

 of the spine did not produce gross deformities or spontaneous col- 

 lapse of vertebrae, and that radiographic evidence of multiple com- 

 pression fractures, ballooned discs, and thin cortex distinguished 

 osteoporosis as a disorder (Urist et al., 1959; Urist, 1960Z?; Urist 

 and Moon, 1961 ) . The degenerative changes in the bone tissue were 

 the same in many respects in physiologic aging and in osteoporosis 

 (Table V), and the only difference could have been in the rate of 

 decline of the bone mass. In both conditions, the ratio of weight 

 to volume or densitv declined from 0.3 to 0.2, but in osteoporosis 

 the decline occurred at an earlier age and a faster rate (Fig. 12). 

 Bone failure or spontaneous fractures of the spine occurred when 

 the density fell below 0.15, or 50 per cent of that of the normal adult 

 human skeleton at age 20. It was interesting in this connection that 

 many patients with osteoporosis reported sudden and recent weight 

 loss. 



The high incidence of fractures in the aged has been attributed 

 to lower bone density, but the correlation with osteoporosis is in- 

 complete. The incidence of severe osteoporosis is 26 per cent in 

 healthy aged women (with average age 85), but it is 76 per cent 

 in women, average age 73, with all types of fractures of the hip 

 (Urist et al, 1959; Urist, 1960Z7; Urist and Moon, 1961). Stevens 



