Relationship between local and total bone mass in osteoporosis

The relationship between total body calcium (TBC) and local bone calcium mass (BCM) was studied in a group of osteoporotic patients (12 females and two males) with a wide range of body size. Two methods were used to estimate BCM: photon absorptiometry and radiographic morphometry. TBC was estimated by total-body neutron-activation analysis. Since 99% of TBC is located in the skeleton, it was assumed that TBC was essentially a measure of total skeletal (calcium) mass. TBC ranged from 482 to 1076 g. High correlations with r values from 0.84 to 0.94, p < 0.001, were found between TBC and BCM measured by absorptiometry at six different sites of radius, ulna, and humerus. Additionally, high correlations with r values from 0.89 to 0.95, p < 0.001, were found between BCM at the distal tenth of the radius and BCM at the five other sites. A high correlation was also found between body height and TBC, but partial correlations indicated that this accounted for very little of the correlation between TBC and BCM. In contrast to the above, correlations between BCM determined by radiographic morphometry and TBC were weak or nonsignificant. When TBC and BCM were expressed as per cent of their mean value, the slopes of the estimating equations, describing the relationships between TBC and BCM, were essentially the same, but significantly less than one, which is the value of the slope expected if TBC and BCM had changed at the same relative rate. From these relationships, we conclude (1) that the rate of change in BCM was similar in the six sites examined, and (2) that the rate of change in these six sites was relatively more rapid than the change in total body calcium. Whether the change in rate of BCM was related to rate of increase during attainment of maximum BCM or subsequent rate of loss of BCM or both remains to be determined.