414- URIST, MACDONALD, MOSS, AND SKOOG 



as in nonosteoporotic subjects of comparable age. Judging from 

 the extraordinary decrease in the thickness of the cortical bone, and 

 assuming an equal loss of cancellous bone, it was apparent that both 

 patients had lost approximately half of the mass of the skeleton. But 

 in the face of this great deficiency, balance studies demonstrated 

 that osteoporotics were able to turn over almost as much calcium 



TABLE II. Summary oi Observations 



Abbreviations: C*, average of subjects with no osteoporosis. Ca intake, average 

 daily intake of calcium in the diet in grams. Ca balance, daily gain or loss of calcium 

 in diet in milligrams per daj'. Sr*''0, time in minutes during which cortical bone of 

 the tibia produced an increase in the counting rate of the .scintillation probe-photomulti- 

 plier tube-ratemeter system. A, accretion rate in grams of calciiun per day, estimated 

 by method of Bauer et al. (1961); Sr*^ was used in all but Case No. 8, where Ca''^ was 

 used. Cx, percentage of the thickness of the cortex of the tibia of nonosteoporotic sub- 

 jects of comparable age. SM, bone weight in kilograms estimated on the basis of normal 

 bone mass = 16 per cent of body weight. TC, total uptake of tetracycline in y per 

 gram of weight of fresh cortical bone obtained at liiopsy. 



and phosphorus as patients with 100 per cent of their bone tissue. 

 Tetracycline labeling of osteoporotic bone correlated with chemical 

 balance and radioisotope studies revealed that this activity was ac- 

 complished chiefly by the exchangeable, labile, reactive, or metabolic 

 bone tissue — which constitutes only approximately 1 per cent of 

 the total bone mass in normal subjects and perhaps not more than 

 2 to 3 per cent in osteoporotics. Thus, kinetic studies were unable 

 to provide a measurement of the activity of either the total bone 

 mass or the relatively nonreactive structural bone, which is where 

 the deficiency exists in patients with osteoporosis. 



